Mihai N Ducea
- Professor, Geosciences
No activities entered.
Isotope geology, tectonics, Geology
Petrology and tectonics
GeophysicsGEOS 596F (Spring 2021)
ResearchGEOS 900 (Spring 2021)
ThesisGEOS 910 (Spring 2021)
ResearchGEOS 900 (Fall 2020)
Independent StudyGEOS 599 (Spring 2020)
PetrologyGEOS 356 (Spring 2020)
ResearchGEOS 900 (Spring 2020)
Teaching GeosciencesGEOS 397A (Spring 2020)
Independent StudyGEOS 599 (Fall 2019)
ResearchGEOS 900 (Fall 2019)
ThesisGEOS 910 (Summer I 2019)
Mineral-Petrol-GeochemGEOS 596A (Spring 2019)
PetrologyGEOS 356 (Spring 2019)
ThesisGEOS 910 (Spring 2019)
Tectonic PetrologyGEOS 408 (Fall 2018)
Tectonic PetrologyGEOS 508 (Fall 2018)
ThesisGEOS 910 (Fall 2018)
ResearchGEOS 900 (Spring 2018)
ThesisGEOS 910 (Spring 2018)
ResearchGEOS 900 (Fall 2017)
ThesisGEOS 910 (Fall 2017)
ResearchGEOS 900 (Summer I 2017)
Directed ResearchGEOS 492 (Spring 2017)
DissertationGEOS 920 (Spring 2017)
Economic GeologyGEOS 596B (Spring 2017)
PetrologyGEOS 356 (Spring 2017)
ResearchGEOS 900 (Spring 2017)
Teaching GeosciencesGEOS 397A (Spring 2017)
ThesisGEOS 910 (Spring 2017)
Directed ResearchGEOS 492 (Fall 2016)
Independent StudyGEOS 499 (Fall 2016)
Independent StudyGEOS 599 (Fall 2016)
Physical GeologyGEOS 251 (Fall 2016)
ResearchGEOS 900 (Fall 2016)
Teaching GeosciencesGEOS 397A (Fall 2016)
ThesisGEOS 910 (Fall 2016)
Directed ResearchGEOS 492 (Spring 2016)
DissertationGEOS 920 (Spring 2016)
Geo & ThermochronologyGEOS 474A (Spring 2016)
Geo & ThermochronologyGEOS 574A (Spring 2016)
Honors ThesisGEOS 498H (Spring 2016)
PetrologyGEOS 356 (Spring 2016)
Structure-TectonicsGEOS 596E (Spring 2016)
Teaching GeosciencesGEOS 397A (Spring 2016)
- DeCelles, P. G., Ducea, M. N., Carrapa, B., & Kapp, P. A. (2014). Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile. Geological Society of America.
- DeCelles, P. G., Ducea, M. N., Carrapa, B., & Kapp, P. A. (2015). Preface and Acknowledgments. Geological Society of America Memoirs, 212, v--v.
- Becker, T. P., Summa, L. L., Ducea, M. N., & Karner, G. D. (2014). Temporal growth of the Puna Plateau and its bearing on the post--Salta Rift system subsidence of the Andean foreland basin at 25° 30′ S. Geological Society of America Memoirs, 212, MWR212--20.
- Chapman, A. D., Chapman, A. D., Ducea, M. N., Ducea, M. N., Kidder, S., Kidder, S., Petrescu, L., & Petrescu, L. (2014). Geochemical constraints on the petrogenesis of the Salinian arc, central California: Implications for the origin of intermediate magmas. LITHOS, 200, 126-141.
- Currie, C. A., Ducea, M. N., DeCelles, P. G., & Beaumont, C. (2015). Geodynamic models of Cordilleran orogens: Gravitational instability of magmatic arc roots. Geological Society of America Memoirs, 212, 1--22.
- DeCelles, P., Zandt, G., Beck, S., Currie, C., Ducea, M., Kapp, P., Gehrels, G., Carrapa, B., Quade, J., & Schoenbohm, L. (2014). Cyclical orogenic processes in the Cenozoic central Andes. Geological Society of America Memoirs, 212, MWR212--22.
- Ducea, M. N., Saleeby, J. B., & Bergantz, G. (2015). The Architecture, Chemistry, and Evolution of Continental Magmatic Arcs. Annual Review of Earth and Planetary Sciences, 43(1).
- Molofsky, L. J., Molofsky, L. J., Killick, D., Killick, D., Ducea, M. N., Ducea, M. N., Macovei, M., Macovei, M., Chesley, J. T., Chesley, J. T., Ruiz, J., Ruiz, J., Thibodeau, A., Thibodeau, A., Popescu, G. C., & Popescu, G. C. (2014). A novel approach to lead isotope provenance studies of tin and bronze: applications to South African, Botswanan and Romanian artifacts. JOURNAL OF ARCHAEOLOGICAL SCIENCE, 50, 440-450.
- Murray, K. E., Ducea, M. N., & Schoenbohm, L. (2015). Foundering-driven lithospheric melting: The source of central Andean mafic lavas on the Puna Plateau (22° S--27° S). Geological Society of America Memoirs, 212, 139--166.
- Putirka, K. D., Putirka, K. D., Canchola, J., Canchola, J., Rash, J., Rash, J., Smith, O., Smith, O., Torrez, G., Torrez, G., Paterson, S. R., Paterson, S. R., Ducea, M. N., & Ducea, M. N. (2014). Pluton assembly and the genesis of granitic magmas: Insights from the GIC pluton in cross section, Sierra Nevada Batholith, California. AMERICAN MINERALOGIST, 99(7), 1284-1303.
- Rossel, P., Rossel, P., Oliveros, V., Oliveros, V., Mescua, J., Mescua, J., Tapia, F., Tapia, F., Ducea, M. N., Ducea, M. N., Calderon, S., Calderon, S., Charrier, R., Charrier, R., Hoffman, D., & Hoffman, D. (2014). The Upper Jurassic volcanism of the Rio Damas-Tordillo Formation (33 degrees-35.5 degrees S): Insights on petrogenesis, chronology, provenance and tectonic implications. ANDEAN GEOLOGY, 41(3), 529-557.
- Zhang, L., Zhang, L., Ding, L., Ding, L., Pullen, A., Pullen, A., Xu, Q., Xu, Q., Liu, D., Liu, D., Cai, F., Cai, F., Yue, Y., Yue, Y., Lai, Q., Lai, Q., Shi, R., Shi, R., Ducea, M. N., , Ducea, M. N., et al. (2014). Age and geochemistry of western Hoh-Xil-Songpan-Ganzi granitoids, northern Tibet: Implications for the Mesozoic closure of the Paleo-Tethys ocean. LITHOS, 190, 328-348.
- Zhang, L., Zhang, L., Ducea, M. N., Ducea, M. N., Ding, L., Ding, L., Pullen, A., Pullen, A., Kapp, P., Kapp, P., Hoffman, D., & Hoffman, D. (2014). Southern Tibetan Oligocene-Miocene adakites: A record of Indian slab tearing. LITHOS, 210, 209-223.
- Balintoni, I., Balica, C., Ducea, M. N., & Hann, H. (2013). Peri-Gondwanan terranes in the Romanian Carpathians: A review of their spatial distribution, origin, provenance, and evolution. Geoscience Frontiers.More infoAbstract: The basement of the Romanian Carpathians is made of Neoproterozoic to early Paleozoic peri-Gondwanan terranes variably involved in the Variscan orogeny, similarly to other basement terrains of Europe. They were hardly dismembered during the Alpine orogeny and traditionally have their own names in the three Carpathian areas. The Danubian domain of the South Carpathians comprises the Drǎgşan and Lainici-Pǎiuş peri-Amazonian terranes. The Drǎgşan terrane originated within the ocean surrounding Rodinia and docked with Rodinia at ∼800 Ma. It does not contain Cadomian magmatism and consequently it is classified as an Avalonian extra-Cadomian terrane. The Lainici-Pǎiuş terrane is a Ganderian fragment strongly modified by Cadomian subduction-related magmatism. It is attached to the Moesia platform. The Tisoviţa terrane is an ophiolite that marks the boundary between Drǎgşan and Lainici-Pǎiuş terranes. The other basement terranes of the Romanian Carpathians originated close to the Ordovician North-African orogen, as a result of the eastern Rheic Ocean opening and closure. Except for the Sebeş-Lotru terrane that includes a lower metamorphic unit of Cadomian age, all the other terranes (Bretila, Tulgheş, Negrişoara and Rebra in the East Carpathians, Someş, Biharia and Baia de Arieş in the Apuseni mountains, Fagaraş, Leaota, Caraş and Padeş in the South Carpathians) represent late Cambrian-Ordovician rock assemblages. Their provenance, is probably within paleo-northeast Africa, close to the Arabian-Nubian shield. The late Cambrian-Ordovician terranes are defined here as Carpathian-type terranes. According to their lithostratigraphy and origin, some are of continental margin magmatic arc setting, whereas others formed in rift and back-arc environment and closed to passive continental margin settings. In a paleogeographic reconstruction, the continental margin magmatic arc terranes were first that drifted out, followed by the passive continental margin terranes with the back-arc terranes in their front. They accreted to Laurussia during the Variscan orogeny. Some of them (Sebeş-Lotru in South Carpathians and Baia de Arieş in Apuseni mountains) underwent eclogite-grade metamorphism. The Danubian terranes, the Bretila terrane and the Someş terrane were intruded by Variscan granitoids. © 2013 China University of Geosciences (Beijing) and Peking University.
- Conroy, J. L., Overpeck, J. T., Cole, J. E., Liu, K., Wang, L., & Ducea, M. N. (2013). Dust and temperature influences on glaciofluvial sediment deposition in southwestern Tibet during the last millennium. Global and Planetary Change, 107, 132-144.More infoAbstract: Dust over the Himalayas and southern Tibetan Plateau may influence high-elevation temperature and hydrology across Asia, but little is known about long-term dust variability and its relationship with regional climate over the last millennium. Here we have reconstructed a proxy record of past dust and temperature impacts on glacial streamflow and sediment deposition in Kiang Co, a small lake on the southwestern Tibetan Plateau. Terrigenous elemental variability in the Kiang Co sediment record, a proxy for glaciofluvial sediment deposition in the lake, covaries with the Dasuopu ice core dust record from the central Himalayas on centennial timescales. The relationship between the Kiang Co sediment record and Dasuopu indicates coherent dust forcing across the central Himalayas over much of the last millennium, and suggests that regional dustiness influences glacial streamflow and sediment transport on centennial timescales. In addition, the Kiang Co terrigenous elemental abundance record is positively correlated with May-September temperatures from the Tibetan Plateau to central India from 1870 to 2007. AD, highlighting the important influence of temperature on melting and streamflow in this region, as well as a relationship between dust and temperature on centennial timescales. These findings strengthen the likelihood of future dustiness with continued warming, along with further increases in melting and high-elevation streamflow in coming decades. © 2013 Elsevier B.V.
- Ducea, M. N., Ducea, M. N., Seclaman, A. C., Seclaman, A. C., Murray, K. E., Murray, K. E., Jianu, D., Jianu, D., Schoenbohm, L. M., & Schoenbohm, L. M. (2013). Mantle-drip magmatism beneath the Altiplano-Puna plateau, central Andes. GEOLOGY, 41(8), 915-918.
- Ducea, M. N., Seclaman, A. C., Murray, K. E., Jianu, D., & Schoenbohm, L. M. (2013). Mantle-drip magmatism beneath the Altiplano-Puna plateau, central Andes. Geology, 41(8), 915-918.More infoAbstract: Convective removal of continental lithospheric roots has been postulated to be the primary mechanism of recycling lithospheric mass into the asthenosphere under large plateaux such as the Altiplano-Puna in the central Andes. Convective instabilities are especially likely to develop where there is extensive intermediate arc-like magmatism in the upper plate, as the residual masses complementing these magmatic products are typically denser than the underlying mantle. Mafic volcanic rocks erupted on the central Andean Altiplano-Puna plateau during the past 25 m.y. contain evidence of this process. Here we use equilibration temperatures, age data, and geochemical constraints--primarily based on transition metals--to show that the most important source materials by mass for this mantle-derived magmatism are pyroxenites from the lower parts of the lithosphere, with only minor contributions from mantle peridotite. Pyroxenites are denser than typical upper mantle whether they are garnet bearing or not, and are therefore likely to contribute to destabilizing parts of the continental lithosphere. The pattern of melting is consistent with the process of foundering/dripping of small-scale (
- Kidder, S. B., Herman, F., Saleeby, J., Avouac, J., Ducea, M. N., & Chapman, A. (2013). Shear heating not a cause of inverted metamorphism. Geology, 41(8), 899-902.More infoAbstract: An archetypal example of inverted metamorphism purportedly resulting from shear heating is found in the Pelona Schist of southern California (United States). Recent studies demonstrate that the Pelona Schist was subducted and accreted at the onset of Laramide flat subduction under thermal and kinematic conditions not considered in earlier numerical models. To test the shear heating hypothesis under these conditions, we constructed a thermokinematic model of flat subduction initiation involving continuous accretion of the schist. A neighborhood algorithm inversion demonstrates that available metamorphic and thermochronologic constraints in the Sierra Pelona mountains are satisfied only if accretion rates were 0.2-3.6 km/m.y and shear heating was minimal (shear stress 0-19 MPa). Minimal shear heating is also consistent with an inversion of models constrained by thermochronology of the East Fork (of the San Gabriel River) exposure of the schist. Shear heating inhibits the formation of modeled inverted gradients during accretion and should not be considered an important factor in their generation. © 2013 Geological Society of America.
- Kidder, S. B., Kidder, S. B., Herman, F., Herman, F., Saleeby, J., Saleeby, J., Avouac, J., Avouac, J., Ducea, M. N., Ducea, M. N., Chapman, A., & Chapman, A. (2013). Shear heating not a cause of inverted metamorphism. GEOLOGY, 41(8), 899-902.
- Pearson, D. M., Pearson, D. M., Kapp, P., Kapp, P., DeCelles, P. G., DeCelles, P. G., Reiners, P. W., Reiners, P. W., Gehrels, G. E., Gehrels, G. E., Ducea, M. N., Ducea, M. N., Pullen, A., & Pullen, A. (2013). Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude: Northwestern Argentina. GEOSPHERE, 9(6), 1766-1782.
- Rossel, P., Oliveros, V., Ducea, M. N., Charrier, R., Scaillet, S., Retamal, L., & Figueroa, O. (2013). The Early Andean subduction system as an analog to island arcs: Evidence from across-arc geochemical variations in northern Chile. Lithos, 179, 211-230.More infoAbstract: The Upper Jurassic volcanic rocks of the Pre-Cordillera and High Andes of northern Chile (26-31°S) represent a back-arc magmatic chain formed during an earlier stage of Andean subduction. After the Callovian, the back-arc basin gradually changed from marine to continental conditions and was characterized by basaltic to rhyolitic rocks erupted along two belts, parallel to the coeval arc. The western belt comprises the Picudo and Algarrobal formations, whereas the eastern belt comprises the Lagunillas Formation and the Quebrada Vicuñita Beds. New major and trace element data, along with whole rock Sr, Nd and Pb isotopes are presented for these volcanic belts and compared to the geochemical features of the Jurassic and Early Cretaceous arc magmatism. Ar-Ar and U-Pb ages constrain the back arc volcanism to have evolved between 163.9. ±. 1.4 and 148.9. ±. 1.2. Ma.Rocks belonging to the western belt have steep multi-element patterns and low concentrations of HREE, suggesting the presence of garnet in the source, and a more radiogenic isotopic composition than the arc magmatism. Parental magmas of these back-arc lavas would have been generated through melting of a depleted mantle, although less depleted than the sub-arc mantle, and interacted with minor amounts of Paleozoic crust. The geochemical composition of the rocks belonging to the eastern belt is more heterogeneous and suggests involvement of different magmatic sources, including depleted mantle as well as an OIB-type mantle within the wedge. In spite the fact that the Jurassic Andean arc was built over a continental plate, the architecture of the volcanic chains and geochemical variations observed among the arc and back-arc rocks in northern Chile resemble those in modern island arcs, and thus support the hypothesis that early Andean subduction developed under extensional tectonic conditions. © 2013 Elsevier B.V.
- Rossel, P., Rossel, P., Oliveros, V., Oliveros, V., Ducea, M. N., Ducea, M. N., Charrier, R., Charrier, R., Scaillet, S., Scaillet, S., Retamal, L., Retamal, L., Figueroa, O., & Figueroa, O. (2013). The Early Andean subduction system as an analog to island arcs: Evidence from across-arc geochemical variations in northern Chile. LITHOS, 179, 211-230.
- Toljić, M., Matenco, L., Ducea, M. N., Stojadinović, U., Milivojević, J., & Derić, N. (2013). The evolution of a key segment in the europe-adria collision: The fruška gora of northern serbia. Global and Planetary Change, 103(1), 39-62.More infoAbstract: The large number of roll-back systems in Mediterranean orogens poses interesting questions concerning interacting extensional back-arc deformation driven by different slabs. One such area characterized by a critical lack of kinematic studies is the connection between the Carpathians and Dinarides, where the Fruška Gora is an isolated inselberg of basement and Mesozoic cover surrounded by Miocene sediments. This area recorded a complex evolution related to the Cretaceous-Paleogene collision between Europe- and Adria-derived tectonic units, the Miocene extension of the Pannonian Basin and its subsequent inversion. This evolution has been analysed in a kinematic study combined with biostratigraphic and Rb-Sr thermochronology of sediments and theirmetamorphism. Results demonstrate a poly-phase tectonic evolution and allowed the discrimination of deformation events and basement affinities. The protolith of the Fruška Gora metamorphic core contains a typical Triassic-Jurassic sequence of the distal Adriatic margin that is overlain by Upper Cretaceous-Paleogene sediments deposited in the Neotethys subduction zone. A part of this basement still records a Late Jurassic (~148 Ma) burial metamorphic event that is associatedwith the coeval structural emplacement of overlying oceanic crust. Three successive deformation eventswere associatedwith the Latest Cretaceous-Early Oligocene contraction. The subsequent exhumation of the Fruška Gora metamorphic core started at ~28 Ma in the footwall of a large extensional detachment and continued by normal faulting during Early-Middle Miocene times. The large-scale extension took place during the extension of the Pannonian Basin and was associated with coeval translations and clockwise rotations of the Fruška Gora. Its present-day antiformal geometry truncated by high-angle reverse faults with S-ward vergence was established during the inversion of the Pannonian Basin, an effect of the late stage Pliocene-Quaternary Adriatic indentation. © 2012 Elsevier B.V. All rights reserved.
- Chapman, A. D., Saleeby, J. B., Wood, D. J., Piasecki, A., Kidder, S., Ducea, M. N., & Farley, K. A. (2012). Late cretaceous gravitational collapse of the southern sierra nevada batholith, California. Geosphere, 8(2), 314-341.More infoAbstract: The Sierra Nevada batholith is an ~600-km-long, NNW-trending composite arc assemblage consisting of a myriad of plutons exhibiting a distinct transverse zonation in structural, petrologic, geochronologic, and isotopic patterns. This zonation is most clearly expressed by a west-to-east variation from mafi c to felsic plutonic assemblages. South of 35.5°N, the depth of exposure increases markedly, and fragments of shallow-level eastern Sierra Nevada batholith affi nity rocks overlie deeper-level western zone rocks and subjacent subduction accretion assemblages along a major Late Cretaceous detachment system. The magnitude of displacement along this detachment system is assessed here by palinspastic reconstruction of vertical piercing points provided by batholithic and metamorphic pendant structure and stratigraphy. Integration of new and published U-Pb zircon geochronologic, thermobarometric, (U-Th)/He thermochronometric, and geochemical data from plutonic and metamorphic framework assemblages in the southern Sierra Nevada batholith reveal seven potential correlations between dispersed crustal fragments and the Sierra Nevada batholith autochthon. Each correlation suggests at least 50 km of south- to southwest-directed transport and tectonic excision of ~5-10 km of crust along the Late Cretaceous detachment system. The timing and pattern of regional dispersion of crustal fragments in the southern Sierra Nevada batholith is most consistent with Late Cretaceous collapse above the underplated accretionary complex. We infer, from data presented herein (1) a high degree of coupling between the shallow and deep crust during extension, and (2) that the development of modern landscape in southern California was greatly preconditioned by Late Cretaceous tectonics. © 2012 Geological Society of America.
- Cristofolini, E. A., Otamendi, J. E., Ducea, M. N., Pearson, D. M., Tibaldi, A. M., & Baliani, I. (2012). Detrital zircon U-Pb ages of metasedimentary rocks from Sierra de Valle Fértil: Entrapment of Middle and Late Cambrian marine successions in the deep roots of the Early Ordovician Famatinian arc. Journal of South American Earth Sciences, 37, 77-94.More infoAbstract: We obtained detrital zircon ages from three metasedimentary rocks with siliciclastic protoliths that were buried to paleodepths of 20-27 km during Early Ordovician Famatinian arc magmatism. In all three samples, U-Pb zircon age distributions are polymodal but the dominant peaks make well-defined clusters, matching the ages of orogenic systems that characterize West Gondwana. The most prominent peaks of constituent age populations at 520 Ma, 600 Ma, and 1050 Ma reflect derivation from sources that are characteristic of the West Gondwana margin. Moreover, grains with Meso- and Paleo-Proterozoic ages appear as small fractions (
- Economos, R. C., Paterson, S. R., Said, L. O., Ducea, M. N., Anderson, J. L., & Padilla, A. J. (2012). Gobi-Tianshan connections: Field observations and isotopes from an early Permian arc complex in southern mongolia. Bulletin of the Geological Society of America, 124(11-12), 1688-1701.More infoAbstract: The Tianshan orogen, in the southern Central Asian orogenic belt, consists of continental fragments stitched together during mid- to late Paleozoic arc magmatism generated by the closure of the paleo-Asian Ocean. Controversy persists regarding the timing of final structural amalgamation of the region and therefore whether Permian magmatism was generated in a subduction or intraplate environment. Based on new field mapping, zircon U-Pb geochronology, and isotope data from the 295-290 Ma Gobi-Tianshan intrusive complex in southwestern Mongolia, we argue that this complex is a voluminous intermediate batholith generated by subduction and that it is related to plutons of similar age and character in Tianshan tectono-magmatic belts to the west. In the study area, as well as in Carboniferous and Permian plutons across the Tianshan, mantle isotopic signatures remain consistently primitive. Permian plutons show an increase in radiogenic Sr with no concurrent decrease in radiogenic Nd, which may be due to the influence of subducting continental sediment in the early Permian. This model explains the transitional nature of magmatic compositions and structures in the Gobi-Tianshan intrusive complex. © 2012 Geological Society of America.
- Otamendi, J. E., Ducea, M. N., & Bergantz, G. W. (2012). Geological, petrological and geochemical evidence for progressive construction of an Arc crustal section, sierra de valle fértil, famatinian Arc, Argentina. Journal of Petrology, 53(4), 761-800.More infoAbstract: The petrogenesis of calc-alkaline magmatism in the Famatinian arc is investigated in the central Sierra Valle Fértil, a major, lower to middle crustal section of the Early Ordovician active margin of West Gondwana. Large-scale field relationships show a gradual and continuous compositional variation of the plutonic sequence, ranging from olivine-bearing gabbronorites to hornblende- and biotite-bearing granodiorites. Distinctive lithostratigraphic units are, however, discernible as one compositional type of plutonic rock dominates over mappable areas. These results allow us to identify a continuous plutonic arc stratigraphy that progressively exposes shallower paleo-depths towards the east. At all the exposed levels, calc-alkaline plutonic rocks are volumetrically dominant, interrupted only by granulite-facies migmatites and leucogranites. The migmatites are interpreted to be refractory remnants of supracrustal sedimentary successions, whereas the peraluminous leucogranites have field relationships and chemical and isotopic compositions suggesting that they were produced via anatexis of metasedimentary packages. Mass-balance calculations predict that a parental gabbroic magma after progressive closed-system fractionation would crystallize about 80% of the original mass to yield a granodioritic daughter. Because the crystallizing mineral assemblage comprises hornblende and plagioclase, mass balance suggests a volume of residual amphibole-rich gabbroic rocks much larger than that observed, suggesting that differentiation is significantly driven by open-system processes. Indeed, the combination of field and petrographic observations with bulk-rock geochemistry and petrogenetic modeling demonstrates that most dioritic and tonalitic rocks are hybrids formed by either (1) bulk assimilation of metasedimentary materials into gabbroic magmas, or (2) multi-stage and complex interactions between gabbroic rocks and metasedimentary-derived leucogranitic melts. The source region of the granodioritic magmas is located at the transition zone between a tonalite-dominated intermediate unit and a granodiorite-dominated silicic unit. Typical granodiorites have a hornblende-bearing mineralogy, metaluminous chemical signature and isotopic compositions [ 87Sr/ 86Sr(T) = 0·7075-0·7100 and ε Nd(T) ~ -5·0] broadly overlapping those of the tonalites of the intermediate rock unit. These major compositional features of the granodiorites can be best explained if three end-member components contribute to their generation. As field observational data suggest, primitive gabbroic rocks, metaluminous intermediate magmas and anatectic leucogranitic melts mixed to produce the calc-alkaline granodiorites; however, the exact petrological process generating the granodioritic magmas is unclear because the mafic end-member may have been incorporated as mafic inclusions in the intermediate magmas or as syn-magmatic dikes, or both. The polygenetic nature of the intermediate to silicic plutonic rocks, along with the preponderance of parental gabbroic rocks at the inferred base of the plutonic column, suggests an upward growth of the intermediate to silicic crust that involved the complete reconstitution of the pre-existing crustal configuration. The main implication of this study is that intermediate and silicic plutonic rocks in the Valle Fértil section formed within a crustal column in which the mass transfer and heat input of mantle-derived magmas promoted fusion of fertile metasedimentary rocks and favored mixing of gabbroic or dioritic magmas with crustal granitic melts. Our results lend support to models asserting that the thermal and material budget of arc magmatism is primarily governed by the rate at which mafic magmas ascend from their mantle sources and intrude repeatedly into the crust. © The Author 2012. Published by Oxford University Press. All rights reserved.
- Robinson, A. C., Ducea, M., & Lapen, T. J. (2012). Detrital zircon and isotopic constraints on the crustal architecture and tectonic evolution of the northeastern Pamir. Tectonics, 31(2).More infoAbstract: New detrital zircon and isotopic (Nd and Sr) analyses from the eastern Pamir provide information on the depositional age and tectonic terrane affiliation of regional metamorphic terranes. Our results show the following. First, detrital zircon analyses from metasedimentary units along the Kongur Shan extensional system dominantly yield Triassic maximum depositional ages, with a similar age distribution to the Tibetan Songpan-Ganzi terrane. Further, zircon analyses from quartzofeldspathic gneisses in the core of the Muztaghata massif show the protoliths are Triassic granites. These units are interpreted to be part of the Permian-Triassic Karakul-Mazar arc-accretionary complex terrane. Second, εNd (0) compositions of Triassic granites overlap with other metasedimentary units not analyzed for detrital zircons and are also interpreted to be part of the Karakul-Mazar terrane. Third, schists in the Sares-Muztaghata gneiss dome structurally above Triassic orthogneisses yield an Ordovician maximum depositional age with a distinct detrital age distribution similar to the Tibetan Qiangtang terrane and are interpreted to be part of the Central Pamir terrane. Finally, Triassic and Ordovician schists along the Muztaghata massif record an Early Jurassic metamorphic event interpreted to date south-directed subduction of the Karakul-Mazar terrane beneath the Central Pamir during final closure of the Paleo-Tethys. These results, integrated with previously published results and field relations, reveal a complex Mesozoic to Cenozoic interleaving of tectonic terranes in the eastern Pamir with emplacement of the Karakul Mazar terrane both above and below the Kunlun and Central Pamir terranes to the north and south, respectively.
- Balintoni, I., Balica, C., Ducea, M. N., & Stremţan, C. (2011). Peri-Amazonian, Avalonian-type and Ganderian-type terranes in the South Carpathians, Romania: The Danubian domain basement. Gondwana Research, 19(4), 945-957.More infoAbstract: The Danubian domain basement of the South Carpathians, Romania, comprises two Neoproterozoic continental crustal fragments, the Drǎgşan and Lainici-Pǎiuş terranes, which were sutured by the closure of an intervening oceanic domain, the Tişoviţa terrane. Magmatic and detrital zircons extracted from an orthogneiss, four granitoid plutons, two metasedimentary units, and a Liassic sandstone were dated by zircon U/Pb LA-ICP-MS. The Fǎgeţel augen gneiss from the Drǎgşan terrane basement yielded an age of 803.2 ± 4.4. Ma, the oldest well-constrained crystallization age reported from the Romanian Carpathians basement. The Tismana, Şuşiţa, Novaci and Olteţ granitoid plutons, which intrude the Lainici-Pǎiuş terrane basement, yielded ages of 600.5 ± 4.4, 591.0 ± 3.5, 592.7 ± 4.9, and 588 ± 2.9. Ma, respectively. The Tismana granitoid age of 600. Ma and the youngest detrital zircon ages of 637-622. Ma from a metaquartzite within the Lainici-Paiuş terrane, constrain the deposition of the metaquartzite protolith to ca. 620-600. Ma. The 803. Ma age represents an old Pan-African age, whereas the younger Neoproterozoic ages suggest Pan-African/Cadomian thermotectonic events. Detrital and inherited zircon ages within the Drǎgşan and Lainici-Paiuş terranes attest to a peri-Amazonian, Avalonian-type provenance for the Drǎgşan terrane and possibly a Ganderian-type provenance for the Lainici-Pǎiuş terrane. The Lainici-Pǎiuş terrane rifted off Gondwana before the Drǎgşan terrane. Both terranes were attached to Moesia during the Early Paleozoic. © 2010 International Association for Gondwana Research.
- Balintoni, I., Balica, C., Seghedi, A., & Ducea, M. (2011). Peri-Amazonian provenance of the Central Dobrogea terrane (Romania) attested by U/Pb detrital zircon age patterns. Geologica Carpathica, 62(4), 299-307.More infoAbstract: The Central Dobrogea Shield is a part of the Moesia, a Paleozoic composite terrane located southward of the North Dobrogea Alpine orogen. The two geological units are separated from each other by a trans-lithospheric discontinuity, the Peceneaga-Camena transform fault. Along this fault, remnants of a Variscan orogen (i.e. North Dobrogea), recycled during the Alpine orogeny come in contact with two lithological entities of the Central Dobrogea Shield, unaffected by the Phanerozoic orogenic events: the Histria Formation, a flysch-like sequence of Ediacaran age very low-grade metamorphosed and its basement, the medium-grade metamorphosed Altîn Tepe sequence. Southward, along the reverse hidden Palazu fault, the Histria Formation meets South Dobrogea, formed of quite different geological formations. Detrital zircon from the Histria Formation yielded U/Pb LA ICP MS ages that show provenance patterns typical of peri-Amazonian terranes. Such terranes were sourced by orogens ranging from Paleoarchean to Neoproterozoic. The ages between 750 and 600 Ma differentiate the Amazonian sources from the Baltican and Laurentian sources, since they are lacking from the last ones. The youngest ages of 587 and 584 Ma suggest for the Histria Formation a maximum late Ediacaran deposition age. At the same time, the continuity of the Ordovician sediments over the Palazu fault revealed by drill-cores favours a Cambrian junction between Central and South Dobrogea.
- Cecil, M. R., & Ducea, M. N. (2011). K-Ca ages of authigenic sediments: Examples from Paleozoic glauconite and applications to low-temperature thermochronometry. International Journal of Earth Sciences, 100(8), 1783-1790.More infoAbstract: K-Ca ages of Cambrian glauconites from the Llano uplift, central Texas, were determined in order to re-evaluate the ability of the K-Ca system to constrain the timing of deposition of sedimentary packages. All of the K-Ca ages presented here were found to be younger than their stratigraphic ages. In addition to being too young, the K-Ca ages are also highly variable, ranging in age from Silurian to Permian. The oldest subset of glauconite ages are in agreement with previously published Rb-Sr ages from the same outcrop and provide further evidence for there having been a post-depositional thermal or recrystallization event that reset both the Rb-Sr and K-Ca systems. The range of younger glauconite K-Ca ages is similar to the distribution of available apatite fission track ages for the Llano basement. K-Ca ages are interpreted as thermochronologic data reflecting partial retention of Ca in thermally fluctuating basin conditions. Estimates of the closure temperature of Ca in glauconite are found to be 60-90°C for cooling rates of ~0.5-1°C/My. The K-Ca system is potentially useful as a low-temperature thermochronometer with closure temperatures
- Ducea, M. N. (2011). Fingerprinting orogenic delamination. Geology, 39(2), 191-192.
- González-León, C. M., Solari, L., Solé, J., Ducea, M. N., Lawton, T. F., Bernal, J. P., Becuar, E. G., Gray, F., Martínez, M. L., & Santacruz, R. L. (2011). Stratigraphy, geochronology, and geochemistry of the Laramide magmatic arc in north-central Sonora, Mexico. Geosphere, 7(6), 1392-1418.More infoAbstract: The Laramide magmatic arc in the Arizpe-Mazocahui quadrangle of northcentral Sonora, Mexico, is composed of volcanic rocks assigned to the Tarahumara Formation and several granitic plutons that intrude it. The arc was built over juxtaposed crustal basements of the Caborca and Mazatzal provinces. A basal conglomerate of the >4-km-thick Tarahumara Formation overlies deformed Proterozoic igneous rocks and Neoproterozoic to Early Cretaceous strata, thus constraining the age of a contractional tectonic event that occurred between Cenomanian and early Campanian time. The lower part of the Tarahumara Formation is composed of rhyolitic ignimbrite and ash-fall tuffs, andesite flows, and interbedded volcani clastic strata, and its upper part consists of rhyolitic to dacitic ignimbrites, ash-fall tuffs, and volcani clastic rocks. The Tarahumara Formation shows marked lateral facies change within the study area, and further to the north it grades into the coeval fluvial and lacustrine Cabullona Group. The age of the Tarahumara Formation is between ca. 79 and 59 Ma; the monzonitic to granitic plutons have ages of ca. 71-50 Ma. The informally named El Babizo and Huépac granites, La Aurora and La Alamedita tonalities, and the Puerta del Sol granodiorite compose the El Jaralito batholith in the southern part of the area. Major and trace element composition of the Laramide igneous rocks shows calc-alkaline differentiation trends typical of continental magmatic arcs, and the isotope geochemistry indicates strong contribution from a mature continental crust. Initial 87Sr/86Sr values range from 0.70589 to 0.71369, and εNd values range from -6.2 to -13.6, except for the El Gueriguito quartz monzonite value, -0.5. The Nd, Sr, and Pb isotopic values of the studied Laramide rocks permit comparison with the previously defined Laramide isotopic provinces of Sonora and Arizona. The El Gueriguito pluton and Bella Esperanza granodiorite in the northeastern part of the study area along with plutons and mineralization of neighboring northern Sonora have isotopic values that correspond with those of the southeastern Arizona province formed over the Mazatzal basement (Lang and Titley, 1998; Bouse et al., 1999). Isotopic values of the other Laramide rocks throughout the study area are similar to values of provinces A and B of Sonora (Housh and McDowell, 2005) and to those of the Laramide Pb boundary zone of western Arizona, while the Rancho Vaquería and La Cubana plutons in the northernmost part of the area have the isotopic composition of the Proterozoic Mojave province of the southwestern United States. These data permit us to infer that a covered crustal boundary, between the Caborca block with a basement of the Mojave or boundary zone and the Mazatzal province, crosses through the northeastern part of the area. The boundary may be placed between outcrops of the El Gueriguito and Rancho Vaquería plutons, probably following a reactivated Cretaceous thrust fault located north of the hypothesized Mojave-Sonora megashear, proposed to cross through the central part of the area. © 2011 Geological Society of America.
- Wetmore, P. H., & Ducea, M. N. (2011). Geochemical evidence of a near-surface history for source rocks of the central Coast Mountains Batholith, British Columbia. International Geology Review, 53(2), 230-260.More infoAbstract: Major and trace elemental concentrations as well as Sr and Pb isotopic data, obtained for 41 plutonic samples from the Coast Mountains Batholith ranging in age from ∼108 to ∼50Ma, indicate that the source regions for these rocks were relatively uniform and typical of Cordilleran arcs. The studied rocks are mineralogically and chemically metaluminous to weakly peraluminous and are mainly calc-alkaline. Initial whole-rock 87Sr/86Sr ratios range from 0.7035 up to 0.7053, whereas lead isotopic data range from 18.586 to 19.078 for 206Pb/204Pb, 15.545 to 15.634 for 207Pb/204Pb, and 37.115 to 38.661 for 208Pb/ 204Pb. In contrast to these relatively primitive isotopic data, δ 18O values for quartz separates determined for 19 of the samples range from 6.8 up to 10.0. These δ 18O values preclude the possibility that these melts were exclusively generated from the Mesozoic mantle wedge of this continental arc, just as the Sr and Pb data preclude significant involvement of an old (Precambrian) crustal/mantle lithospheric source. We interpret the high δ 18O component to represent materials that had a multi-stage crustal evolution. They were originally mafic rocks derived from a circum-Pacific juvenile mantle wedge that experienced a period of near-surface residence after initial crystallization. During this interval, these primitive rocks interacted with meteoric waters at low temperatures, as indicated by the high δ 18O values. Subsequently, these materials were buried to lower crustal depths where they remelted to form the high δ 18O component of the Coast Mountains Batholith. This component makes up at least 40% (mass) of the Cretaceous through Eocene batholith in the studied area. The remainder of the source materials comprising the Coast Mountains Batholith had to be new additions from the mantle wedge. A prolonged period of contractional deformation beginning with the Early Cretaceous collisional accretion of the Insular superterrane is inferred to have been responsible for underthrusting the high 18O component into the lower crust. We suggest that mafic rocks of the Insular superterrane (e.g. Alexander-Wrangellia) are of appropriate composition, and were accreted to and overthrust by what would become the Coast Mountains Batholith just prior to initiation of magmatism in the region. © 2011 Taylor & Francis.
- Balintoni, I., Balica, C., Ducea, M. N., Hann, H. P., & Şabliovschi, V. (2010). The anatomy of a Gondwanan terrane: The Neoproterozoic-Ordovician basement of the pre-Alpine Sebeş-Lotru composite terrane (South Carpathians, Romania). Gondwana Research, 17(2-3), 561-572.More infoAbstract: The Sebeş-Lotru terrane in the South Carpathians mountain range comprises a lower, Neoproterozoic metamorphic unit (Lotru) and an upper, Ordovician metamorphic unit (Cumpǎna) that were juxtaposed during the Variscan orogeny. Two orthogneisses from the Lotru metamorphic unit yield U/Pb LA-ICP-MS zircon crystallization ages of 549.3 ± 3.8 Ma and 587.5 ± 3 Ma, respectively. Two orthogneisses from the Cumpǎna metamorphic unit yield zircon crystallization ages of 458.9 ± 3.5 Ma and 466.0 ± 4.2 Ma, respectively. High U zircons from two other orthogneisses from the Cumpǎna metamorphic unit have ages ranging from 400 Ma to 320 Ma, which are interpreted to reflect protracted zircon recrystallization during the regionally significant Variscan collisional event. Detrital zircons from a metasedimentary gneiss in the Cumpǎna metamorphic unit have ages ranging from ~ 0.5 Ga to 2.8 Ga. The 0.5 Ga age constrains the maximum sediment deposition age to be late Cambrian. The source most compatible with the range of Precambrian detrital ages in the Sebeş-Lotru terrane is northeastern Gondwana. The Sebeş-Lotru terrane was part of a continental subduction/collision system as a lower plate after about 400 Ma and reached peak metamorphic conditions between 350 and 320 Ma (e.g. Medaris et al., 2003). A cross-cutting granite vein has a zircon U-Pb crystallization age of 321.5 ± 3.1 Ma, which constrains minimum age of ductile deformation during the Variscan collision in this region. The trace of the Rheic suture within the South Carpathians is located between the Ordovician upper part of the Sebeş-Lotru terrane and the Drǎgşan pre-Alpine terrane of the Danubian domain. © 2009 International Association for Gondwana Research.
- Balintoni, I., Balica, C., Ducea, M. N., Zaharia, L., Chen, F., Cliveti, M., Hann, H. P., Li, L., & Ghergari, L. (2010). Late cambrian-ordovician northeastern gondwanan terranes in the basement of the Apuseni Mountains, Romania. Journal of the Geological Society, 167(6), 1131-1145.More infoAbstract: The pre-Alpine basement of the Apuseni Mountains (Romanian Carpathians) comprises three terranes: Someş, Baia de Arieş and Biharia. Metaigneous rocks within these terranes record Late Cambrian Ordovician U-Pb zircon crystallization ages. Two samples from an orthogneiss layer in the Someş terrane yielded an age of 459.8 ± 2.7 Ma. In the Baia de Arieş terrane, an augen gneiss yielded an age of 470.8 ± 3.8 Ma, a porphyroid yielded an age of 467.8 ± 3.8 Ma and two metagranites yielded ages of 467.8 ± 4.7 Ma and 467.1 ± 3.9 Ma, respectively. Three samples from a metagranitoid outcrop located in the lower part of the Biharia terrane yielded an age of 495.0 ± 2.1 Ma. Three other samples from a metabasite layer situated lithostratigraphically several hundred metres above the metagranitoid yielded an age of 477.8 ± 3.2 Ma. Detrital U/Pb zircon ages from a paragneiss interlayered within the Baia de Arieş terrane scatter between 548 and 3518 Ma. Detrital zircon ages together with the inherited ages recorded by magmatic zircon populations suggest a northeastern Gondwanan provenance for the Someş, Baia de Arieş and Biharia terranes, adjacent to the Arabian-Nubian Shield. Lead loss in some zircons is interpreted to have taken place during the Variscan orogeny. © 2010 Geological Society of London.
- Balintoni, I., Balica, C., Seghedi, A., & Ducea, M. N. (2010). Avalonian and Cadomian terranes in North Dobrogea, Romania. Precambrian Research, 182(3), 217-229.More infoAbstract: The North Dobrogea orogen is a collage of dismembered terrane fragments between the Moesian platform and East European craton (Baltica). It records Alpine and Variscan deformation, magmatism and metamorphism. Its basement comprises three metamorphic complexes (Boclugea, Megina and Orliga) that are separated by tectonic boundaries. Detrital zircon U/Pb ages suggest the Boclugea and Orliga complexes represent two peri-Gondwanan terranes of Avalonian and Cadomian affinities, respectively. The new data clarify the original relationships between the North Dobrogea terranes, and Baltica and Moesia platform. © 2010 Elsevier B.V.
- Chapman, A. D., Kidder, S., Saleeby, J. B., & Ducea, M. N. (2010). Role of extrusion of the Rand and Sierra de Salinas schists in Late Cretaceous extension and rotation of the southern Sierra Nevada and vicinity. Tectonics, 29(5).More infoAbstract: The Rand and Sierra de Salinas schists of southern California were underplated beneath the southern Sierra Nevada batholith and adjacent Mojave-Salinia region along a shallow segment of the subducting Farallon plate in Late Cretaceous time. Various mechanisms, including return flow, isostatically driven uplift, upper plate normal faulting, erosion, or some combination thereof, have been proposed for the exhumation of the schist. We supplement existing kinematic data with new vorticity and strain analysis to characterize deformation in the Rand and Sierra de Salinas schists. These data indicate that the schist was transported to the SSW from deep to shallow crustal levels along a mylonitic contact (the Rand fault and Salinas shear zone) with upper plate assemblages. Crystallographic preferred orientation patterns in deformed quartzites reveal a decreasing simple shear component with increasing structural depth, suggesting a pure shear dominated westward flow within the subduction channel and localized simple shear along the upper channel boundary. The resulting flow type within the channel is that of general shear extrusion. Integration of these observations with published geochronologic, thermochronometric, thermobarometric, and paleomagnetic studies reveals a temporal relationship between schist unroofing and upper crustal extension and rotation. We present a model whereby trench-directed channelized extrusion of the underplated schist triggered gravitational collapse and clockwise rotation of the upper plate. © 2010 by the American Geophysical Union.
- A., G., Reiners, P. W., & Ducea, M. N. (2009). Unroofing history of Alabama and Poverty Hills basement blocks, Owens Valley, California, from apatite (U-Th)/He thermochronology. International Geology Review, 51(9-11), 1034-1050.More infoAbstract: Most of the 150km-long Owens Valley of east-central California, the westernmost graben of the Basin and Range Province and location of the active right-lateral Owens Valley Fault Zone, is filled with more than 2-3km of Neogene sediments. Two prominent but structurally enigmatic basement blocks, the Alabama Hills and Poverty Hills, rise from the floor of the southern part of the valley. The late Cenozoic tectonic origin of these basement blocks is not known, but previously published hypotheses include: (1) transpressional uplifts; (2) down-dropped normal fault blocks; and (3) giant landslides from adjacent ranges. We measured apatite (U-Th)/He ages on 15 samples from the Alabama and Poverty Hills to understand the history of shallow crustal exhumation of these structures, and to potentially correlate them to rocks from adjacent ranges. Apatite He ages for the Alabama and Poverty Hills range from ∼63 to 52 and ∼61 to 39Ma, respectively, with one sample from the Alabama Hills yielding an older weighted mean age of 79.3 ± 2.2Ma (2σ). These ages are similar to those measured at elevations 1.5-3.0km higher in both the adjacent Sierra Nevada and White/Inyo Mountains. Together with a lack of evidence for transpressional structures and the presence of extensive weathering of an ancient inherited surface on top of the Alabama Hills, these results are most consistent with an origin as a down-dropped normal fault block. A structural reconstruction using tilt-corrected southern Sierra Nevada age-elevation correlations requires 2.6km of vertical and 1.5km of eastward motion for the Alabama Hills along the Sierra Nevada Frontal Fault. Although we cannot conclusively rule out a landslide origin, our preferred explanation for the Poverty Hills data is that it is a down-dropped normal fault block that was transported from the Inyo Mountains along a right-lateral fault possessing a significant normal component.
- Balintoni, I., Balica, C., Ducea, M. N., Chen, F., Hann, H. P., & Şabliovschi, V. (2009). Late Cambrian-Early Ordovician Gondwanan terranes in the Romanian Carpathians: A zircon U-Pb provenance study. Gondwana Research, 16(1), 119-133.More infoAbstract: The basement of the Romanian Carpathians is comprised of pre-Variscan metasedimentary and metaigneous units. Age patterns from corresponding detrital zircons show similarities to those from the eastern Mediterranean region. Consistently, these patterns suggest a northern Gondwanan origin for the Carpathian terranes with the most common detrital sources representing fragments of the Pan-African Orogen, Arabian-Nubian Shield, Kibaran Orogen, and West African Craton (Eburnean Orogen and Liberian-Leonian Orogens), as well as the Saharan metacraton. Some contributions from the Indian Craton are also possible. The youngest detrital zircon ages constrain the maximum age of the sedimentary rock protoliths to Middle Cambrian. On the other hand, the U/Pb zircon ages of the metaigneous protoliths indicate predominantly Early Ordovician ages and bracket most magmatism in the Carpathian pre-Variscan basement units between Middle Cambrian and Late Ordovician. An exception is the Neoproterozoic lower part of the Sebes-Lotru terrane, whose igneous ages are equivalent to the Danubian Domain in the South Carpathians. The Carpathian pre-Variscan terranes were originally located along the eastern extension of the Galatian superterrane, and thus within a Late Cambrian to Ordovician extensional tectonic setting. © 2009 International Association for Gondwana Research.
- Ducea, M. N., Kidder, S., Chesley, J. T., & Saleeby, J. B. (2009). Tectonic underplating of trench sediments beneath magmatic arcs: The central California example. International Geology Review, 51(1), 1-26.More infoAbstract: We summarize the post Late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system. The Monterey terrane of North American origin was laterally transferred to the Pacific plate via the San Andreas fault. The Monterey terrane is an assembly of three tectonic units, Salinia, Nacimiento and Sierra de Salinas blocks, two of which have been previously identified as separate terranes. These blocks are separated by two regionally important thrust faults: the Sur fault as well as the Salinas shear zone. Based on thermobarometric and thermochronologic constraints and the existence of a common younger cover sequence, these blocks were juxtaposed together after the latest Cretaceous. The Salinian assemblage represents a crustal section through the continental interior side of the Mesozoic California arc and formed during the Late Cretaceous, primarily during a regionally significant magmatic flare-up between 95 and 80 Ma. In the Santa Lucia Range, parts of the arc are exposed to palaeo-depths in excess of 30 km. The Nacimiento and Sierra de Salinas assemblages comprise basement rocks representing Late Cretaceous variants of the Franciscan Complex and are interpreted to be correlative. They represent the lower plate of a regionally important thrust system; the upper plate is the Salinian assemblage, whereas the Sur and Salinas faults are local exposures of the structure. We concur with previous estimates of 150 to 180km of shortening during a brief time span (3 cm/yr. This fault system corresponds to the megathrust of the Farallon subduction beneath North America during the early stages of the regionally extensive episode of shallow subduction (Laramide orogeny). As a result, trench sediment was thrust under North America and tectonically underplated to the lower crust of North America. The Salinas shear zone, in particular, is a ductile expression of shallow subduction; thermobarometry in the upper plate, lower plate and the shear zone itself indicate that this is the fossil subduction megathrust originating at depths of ∼35 km. The entire system collapsed extensionally soon after the trench sediment was underthrust, possibly because of the lack of strength of the lower plate. Arc magmatism in the upper plate ceased at the onset of underplating. This regional example illustrates the significance of tectonic underplating in shallow subduction systems. Accretion-related trench sediment was shuffled from the trench to the sub-arc region of the upper plate, but not recycled into the mantle. This process requires that the subduction megathrust be located solely within the North American crust. This geometry requires a sudden migration of the subduction interface toward the arc and may apply to other regional examples, including the modern shallow subduction of the Cocos plate beneath southern Mexico. The tectonically underplated trench sediment undergoes regional, Barrovian metamorphism, after initially following a high-pressure/low-temperature path. Moreover, the shear zone marking the fossil intracrustal megathrust was subject to granulite-facies metamorphism and limited partial melting. © 2009 Taylor & Francis.
- Luffi, P., Saleeby, J. B., Lee, C. A., & Ducea, M. N. (2009). Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California. Journal of Geophysical Research B: Solid Earth, 114(3).More infoAbstract: Low-angle subduction of oceanic lithosphere may be an important process in modifying continental lithosphere. A classic example is the underthrusting of the Farallon plate beneath North America during the Laramide orogeny. To assess the relevance of this process to the evolution and composition of continental lithosphere, the mantle stratigraphy beneath the Mojave Desert was constrained using ultramafic xenoliths hosted in Plio-Pleistocene cinder cones. Whole-rock chemistry, clinopyroxene trace element and Nd isotope data, in combination with geothermometry and surface heat flow, indicate kilometer-scale compositional layering. The shallow parts are depleted in radiogenic Nd (εNd = -13 to -6.4) and are interpreted to be ancient continental mantle that escaped tectonic erosion by low-angle subduction. The deeper samples are enriched in radiogenic Nd (εNd = +5.7 to +16.1) and reveal two superposed mantle slices of recent origin. Within each slice, compositions range from fertile lherzolites at the top to harzburgites at the bottom: the latter formed by 25-28% low-pressure melt depletion and the former formed by refertilization of harzburgites by mid-ocean-ridge-basalt-like liquids. The superposition and internal compositional zonation of the slices preclude recent fertilization by Cenozoic extension-related magmas. The above observations imply that the lower Mojavian lithosphere represents tectonically subcreted and imbricated lithosphere having an oceanic protolith. If so, the lherzolitic domains may be related to melting and refertilization beneath mid-ocean ridges. The present Mojavian lithosphere is thus a composite of a shallow section of the original North American lithosphere underlain by Farallon oceanic lithosphere accreted during low-angle subduction. Copyright 2009 by the American Geophysical Union.
- Otamendi, J. E., Ducea, M. N., Tibaldi, A. M., Bergantz, G. W., D., J., & Vujovich, G. I. (2009). Generation of tonalitic and dioritic magmas by coupled partial melting of gabbroic and metasedimentary rocks within the deep crust of the Famatinian magmatic arc, Argentina. Journal of Petrology, 50(5), 841-873.More infoAbstract: The source regions of dioritic and tonalitic magmas have been identified in a deep crustal section of the Famatinian arc (Sierras Pampeanas of western Argentina). The source zones of intermediate igneous rocks are located at the transition between a gabbro-dominated mafic unit and a tonalite-dominated intermediate unit. In the upper levels of the mafic unit mafic magmas intruded into metasedimentary wall-rocks, crystallized mainly as amphibole gabbronorite and caused the partial melting of the surrounding metasediments. In turn, the leucogranitic melts sourced from the metasedimentary rocks intruded into the newly crystallized but still hot mafic layers and catalysed the process of partial melting of the gabbroic plutonic rocks. The gabbroic rocks became mafic migmatites comprising amphibole-rich pyroxene-bearing mesosomes and leucotonalitic veins. Significantly, most of the mafic migmatites have isotopic compositions [87Sr/86Sr(T) < 0.7063 and εNd(T) = -0.94 to +2.24] similar to those of the gabbroic rocks and distinct from those of their complementary leucotonalitic veins [87Sr/86Sr(T) = 0.7075-0.7126 and εNd(T) < -2.65], providing evidence for the idea that melting of the mafic rocks was triggered by the intrusion of leucogranitic anatectic melts [87Sr/86Sr(T) = 0.715 and εNd(T) = -6.21]. Mass-balance calculations show that the model reaction plagioclase + amphibole + leucogranitic melt → leucotonalitic melt + clinopyroxene ± orthopyroxene can better explain the partial melting of the gabbroic rocks. Based on field observations, we argue that the coalescence of leucotonalitic veins in the mafic migmatites led to breakdown of the solid matrix to form melt-dominated leucotonalitic pools. However, the leucotonalitic veins that crystallized before leaving behind the mafic migmatitic rock are chemically (elemental and isotopic) more evolved than the dioritic and tonalitic rocks. We envisage that once detached from their source region the leucotonalitic magmas were able to react, commingle and mix with entrained fragments of both mafic and metasedimentary rocks. This process gave rise to melts that became tonalitic and dioritic magmas. This study concludes that the generation of intermediate magmas is a multistage process with three critical steps: (1) influx and emplacement of hydrous mafic magmas into a deep crust containing metasedimentary country rocks; (2) physically and chemically coupled melting of mafic and metasedimentary rocks, leading to the formation of a leucotonalitic vein and dyke system that coalesces to form leucotonalitic or tonalitic magma bodies; (3) retrogression of the leucotonalitic magmas by partially assimilating entrained fragments of their mafic and metasedimentary precursors. The dimensions of the source zone seem to be insufficient to generate crustal-scale volumes of intermediate igneous rocks. However, the Famatinian paleo-arc crust would expose only those magma source zones that were still active during the tectonic closure of the arc. Ultimately, a time-integrated perspective indicates that early active source zones were cannibalized during the downward expansion of the plutonic bodies already dominated by intermediate plutonic rocks. © The Author 2009. Published by Oxford University Press. All rights reserved.
- Blondes, M. S., Reiners, P. W., Ducea, M. N., Singer, B. S., & Chesley, J. (2008). Temporal-compositional trends over short and long time-scales in basalts of the Big Pine Volcanic Field, California. Earth and Planetary Science Letters, 269(1-2), 140-154.More infoAbstract: Primitive basaltic single eruptions in the Big Pine Volcanic Field (BPVF) of Owens Valley, California show systematic temporal-compositional variation that cannot be described by simple models of fractional crystallization, partial melting of a single source, or crustal contamination. We targeted five monogenetic eruption sequences in the BPVF for detailed chemical and isotopic measurements and 40Ar/39Ar dating, focusing primarily on the Papoose Canyon sequence. The vent of the primitive (Mg# = 69) Papoose Canyon sequence (760.8 ± 22.8 ka) produced magmas with systematically decreasing (up to a factor of two) incompatible element concentrations, at roughly constant MgO (9.8 ± 0.3 (1σ) wt.%) and Na2O. SiO2 and compatible elements (Cr and Ni) show systematic increases, while 87Sr/86Sr systematically decreases (0.7063-0.7055) and εNd increases (- 3.4 to - 1.1). 187Os/188Os is highly radiogenic (0.20-0.31), but variations among four samples do not correlate with other chemical or isotopic indices, are not systematic with respect to eruption order, and thus the Os system appears to be decoupled from the dominant trends. The single eruption trends likely result from coupled melting and mixing of two isotopically distinct sources, either through melt-rock interaction or melting of a lithologically heterogeneous source. The other four sequences, Jalopy Cone (469.4 ± 9.2 ka), Quarry Cone (90.5 ±17.6 ka), Volcanic Bomb Cone (61.6 ± 23.4 ka), and Goodale Bee Cone (31.8 ± 12.1 ka) show similar systematic temporal decreases in incompatible elements. Monogenetic volcanic fields are often used to decipher tectonic changes on the order of 105-106 yr through long-term changes in lava chemistry. However, the systematic variation found in Papoose Canyon (100-102 yr) nearly spans that of the entire volcanic field, and straddles cutoffs for models of changing tectonic regime over much longer time-scales. Moreover, ten new 40Ar/39Ar ages combined with chemistry from all BPVF single eruption sequences show the long-term trend of BPVF evolution comprises the overlapping, temporal-compositional trends of the monogenetic vents. This suggests that the single eruption sequences contain the bulk of the systematic chemical variation, whereas their aggregate compositions define the long-term trend of volcanic field evolution. © 2008 Elsevier B.V. All rights reserved.
- Saleeby, J. B., Ducea, M. N., Busby, C. J., Nadin, E. S., & Wetmore, P. H. (2008). Chronology of pluton emplacement and regional deformation in the southern Sierra Nevada batholith, California. Special Paper of the Geological Society of America, 438, 397-427.More infoAbstract: Cretaceous plutonic rocks of the southern Sierra Nevada batholith between latitudes 35.5°N and 36°N lie in a strategic position that physically links shallow, subvolcanic levels of the batholith to lower-crustal (∼35 km deep) batholithic rocks. This region preserves an oblique crustal section through the southern Sierra Nevada batholith. Prior studies have produced large U/Pb zircon data sets for an aerially extensive region of the batholith to the north of this area and for the lower-crustal rocks of the Tehachapi complex to the south. We present a large set of new U/Pb zircon age data that ties together the temporal relations of pluton emplacement and intra-arc ductile deformation for the region. We define five informal intrusive suites in the area based on petrography, structural setting, U/Pb zircon ages, and patterns in initial 87Sr/86Sr (Sri). Two regionally extensive intrusive suites, the 105-98 Ma Bear Valley suite and 95-84 Ma Domelands suite, underlie the entire southwestern and eastern regions of the study area, respectively, and extend beyond the limits of the study area. A third regionally extensive suite (101-95 Ma Needles suite) cuts out the northern end of the Bear Valley suite and extends for an unknown distance to the north of the study area. The Bear Valley and Needles suites are tectonically separated from the Domelands suite by the proto-Kern Canyon fault, which is a regional Late Cretaceous ductile shear zone that runs along the axis of the southern Sierra Nevada batholith. The 105-102 Ma Kern River suite also lies west of the proto-Kern Canyon fault and constitutes the subvolcanic plutonic complex for the 105-102 Ma Erskine Canyon sequence, an ∼2-km-thick silicic ignimbrite hypabyssal complex. The 100-94 Ma South Fork suite lies east of the proto-Kern Canyon fault. It records temporal and structural relations of high-magnitude ductile strain and migmatization in its host metamorphic pendant rocks commensurate with magmatic emplacement. Integration of the U/Pb age data with structural and isotopic data provides insights into a number of fundamental issues concerning composite batholith primary structure, pluton emplacement mechanisms, compositional variations in plutons, and the chronology and kinematics of regional intra-arc ductile deformation. Most fundamentally, the popular view that Sierran batholithic plutons rise to mid- crustal levels (∼20-15 km) and spread out above a high-grade metamorphic substrate is rendered obsolete. Age and structural data of the study area and the Tehachapi complex to the south, corroborated by seismic studies across the shallow-level Sierra Nevada batholith to the north, indicate that felsic batholithic rocks are continuous down to at least ∼35 km paleodepths and that the shallower-level plutons, when and if they spread out, do so above steeply dipping primary structures of deeper- level batholith. This steep structure reflects incremental assembly of the lower crust by multiple magma pulses. Smaller pulses at deeper structural levels appear to be more susceptible to having source isotopic and compositional signatures modified by assimilation of partial melt products from metamorphic framework rocks as well as previously plated-out intrusives. Higher-volume magma pulses appear to rise to higher crustal levels without substantial compositional modifications and are more likely to reflect source regime characteristics. There are abundant age, petrographic, and structural data to indicate that the more areally extensive intrusive suites of the study area were assembled incrementally over 5-10 m.y. time scales. Incremental assembly involved the emplacement of several large magma batches in each (∼50 km2-scale) of the larger plutons, and progressively greater numbers of smaller batches down to a myriad of meter-scale plutons, and smaller dikes. The total flux of batholithic magma emplaced in the study area during the Late Cretaceous is about four times that modeled for oceanic-island arcs. Integration of the U/Pb zircon age data with detailed structural and strati- graphic studies along the proto-Kern Canyon fault indicates that east-side-up reverse-sense ductile shear along the zone was operating by ca. 95 Ma. Dextralsense ductile shear, including a small reverse component, commenced at ca. 90 Ma and was in its waning phases by ca. 83 Ma. Because ∼50% of the southern Sierra Nevada batholith was magmatically emplaced during this time interval, primarily within the east wall of the proto-Kern Canyon fault, the total displacement history of the shear zone is poorly constrained. Stratigraphic relations of the Erskine Canyon sequence and its relationship with the proto-Kern Canyon fault suggest that it was ponded within a 102-105 Ma volcano-tectonic depression that formed along the early traces of the shear zone. Such structures are common in active arcs above zones of oblique convergence. If such is the case for the Erskine Canyon sequence, this window into the early history of the "proto-Kern Canyon fault" could preserve a remnant or branch of the Mojave-Snow Lake fault, a heretofore cryptic hypothetical fault that is thought to have undergone large-magnitude dextral slip in Early Cretaceous time. The changing kinematic patterns of the proto-Kern Canyon fault are consistent with age and deformational relations of ductile shear zones present within the shallow-level central Sierra Nevada batholith, and with those of the deep-level exposures in the Tehachapi complex. This deformational regime correlates with flat-slab segment subduction beneath the southern California region batholithic belt and resultant tilting and unroofing of the southern Sierra Nevada batholith oblique crustal section. These events may be correlated to the earliest phases of the Laramide orogeny. © 2008 Geological Society of America.
- Ducea, M. N., Kidder, S., & Chesley, J. T. (2007). A geologic window into a subduction megathrust. Eos, 88(27), 277-.
- Horodyskyj, U. N., Lee, C. A., & Ducea, M. N. (2007). Similarities between Archean high MgO eclogites and Phanerozoic arc-eclogite cumulates and the role of arcs in Archean continent formation. Earth and Planetary Science Letters, 256(3-4), 510-520.More infoAbstract: Some insights into the origin of cratonic mantle can be gained from "eclogite" (loosely defined here as an assemblage containing garnet and any pyroxene) xenoliths hosted in kimberlites erupted through Archean (~ 2.5-3.5 Gy) cratons. One subset of Archean eclogite xenoliths, the low MgO Archean xenoliths, is presently believed to represent metamorphosed fragments of ancient altered oceanic crust, leading to the suggestion that Archean cratons were built, at least in part, by the accretion of oceanic lithospheric segments. However, another Archean subset, the high MgO Archean eclogite xenoliths, have major and compatible trace-element (Ni and Cr) systematics similar to high MgO arc-eclogite xenoliths originating from the lithospheric root underlying the Sierra Nevada batholith in California, an example of a Phanerozoic arc. The Sierran high MgO arc-eclogites represent cumulates from hydrous basaltic magmas beneath a thick continental arc. The compositional similarities between the Archean and Sierran high MgO eclogites suggest that not only might the Archean high MgO eclogites have a cumulate origin, as has previously been suggested, but they may be arc-related. If so, Archean high MgO eclogites provide evidence from within the mantle roots of cratons that some form of arc magmatism contributed to the formation and evolution of Archean continents. © 2007 Elsevier B.V. All rights reserved.
- Johnston, S., Hacker, B. R., & Ducea, M. N. (2007). Exhumation of ultrahigh-pressure rocks beneath the Hornelen segment of the Nordfjord-Sogn Detachment Zone, western Norway. Bulletin of the Geological Society of America, 119(9-10), 1232-1248.More infoAbstract: The Nordfjord-Sogn Detachment Zone of western Norway represents an archetype for crustal-scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahigh-pressure (UHP) terranes. In this paper, we investigate the role of normal-sense shear zones with respect to UHP exhumation using structural geology, thermobarometry, and geochronology of the Hornelen segment of the Nordfjord-Sogn Detachment Zone. The Hornelen segment of the zone is a 2-6 km thick, top-W shear zone, primarily developed within amphibolite-grade allochthonous rocks, that juxtaposes the UHP rocks of the Western Gneiss Complex in its footwall with lower-grade allochthons and Carboniferous-Devonian Basins in its hanging wall. New thermobarometry and Sm/Nd garnet geochronology show that these top-W fabrics were initiated at lower crustal depths of 30-40 km between 410 Ma and 400 Ma. Structural geology and quartz petrofabrics indicate that top-W shear was initially relatively evenly distributed across the shear zone, and then overprinted by discrete ductile-brittle detachment faults at slower strain rates during progressive deformation and exhumation. These results require a three-stage modal for UHP exhumation in which normal-sense shear zones exhumed UHP rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile-brittle structures. © 2007 Geological Society of America.
- Brady, R. J., Ducea, M. N., Kidder, S. B., & Saleeby, J. B. (2006). The distribution of radiogenic heat production as a function of depth in the Sierra Nevada Batholith, California. Lithos, 86(3-4), 229-244.More infoAbstract: Geochemical analyses and geobarometric determinations have been combined to create a depth vs. radiogenic heat production database for the Sierra Nevada batholith, California. This database shows that mean heat production values first increase, then decrease, with increasing depth. Heat production is ∼2 μW/m3 within the ∼3-km-thick volcanic pile at the top of the batholith, below which it increases to an average value of ∼3.5 μW/m3 at ∼5.5 km depth, then decreases to ∼0.5-1 μW/m3 at ∼15 km depth and remains at these values through the entire crust below 15 km. Below the crust, from depths of ∼40-125 km, the batholith's root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of ∼0.14 μW/m3. This is higher than the rates from most published xenolith studies, but reasonable given the presence of crustal components in the arc root assemblages. The pattern of radiogenic heat production interpreted from the depth vs. heat production database is not consistent with the downward-decreasing exponential distribution predicted from modeling of surface heat flow data. The interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the Sierra Nevada could be generated within the ∼35 km thick crust. This requires a very low heat flux from the mantle, which is consistent with a model of cessation of Sierran magmatism during Laramide flat-slab subduction, followed by conductive cooling of the upper mantle for ∼70 m.y. The heat production variation with depth is principally due to large variations in uranium and thorium concentration; potassium is less variable in concentration within the Sierran crust, and produces relatively little of the heat in high heat production rocks. Because silica content is relatively constant through the upper ∼30 km of the Sierran batholith, while U, Th, and K concentrations are highly variable, radiogenic heat production does not vary directly with silica content. © 2005 Elsevier B.V. All rights reserved.
- Cecil, M. R., Ducea, M. N., Reiners, P. W., & Chase, C. G. (2006). Cenozoic exhumation of the northern Sierra Nevada, California, from (U-Th)/He thermochronology. Bulletin of the Geological Society of America, 118(11-12), 1481-1488.More infoAbstract: Apatite and zircon (U-Th)/He ages from a 100-km-long range-perpendicular transect in the northern Sierra Nevada, California, are used to constrain the exhumation history of the range since ca. 90 Ma. (U-Th)/He ages in apatite decrease from 80 Ma along the low western range flanks to 46 Ma in the higher elevations to the east. (U-Th)/He ages in zircon also show a weak inverse correlation with elevation, decreasing from 91 Main the west to 66 Ma in the east. Rocks near the range crest, sampled at elevations of 2200-2500 m, yield the youngest apatite helium ages (46-55 Ma), whereas zircon helium ages are more uniform across the divide. These data reveal relatively rapid cooling rates between ca. 90 and 60 Ma, which are consistent with relatively rapid exhumation rates of 0.2-0.8 km/m.y., followed by a long period of slower exhumation (0.02-0.04 km/m.y.) from the early Paleogene to today. This is reflected in the low-relief morphology of the northern Sierra Nevada, where an Eocene erosional surface has long been identified. A long period of slow exhumation is also consistent with well-documented, widespread lateritic paleosols at the base of Eocene depositional units. Laterites preserved in the northern Sierra Nevada are the product of intense weathering in a subtropical environment and suggest an enduring, soil-mantled topography. We interpret this exhumation history as recording a Late Cretaceous to early Cenozoic period of relatively rapid uplift and unroofing followed by tectonic quiescence and erosional smoothing of Sierran topography through the Neogene. Well-documented recent incision appears to have had little effect on (U-Th)/He ages, suggesting that less than ∼3 km has been eroded from the Sierra Nevada since the early Cenozoic. © 2006 Geological Society of America.
- Kidder, S., & Ducea, M. N. (2006). High temperatures and inverted metamorphism in the schist of Sierra de Salinas, California. Earth and Planetary Science Letters, 241(3-4), 422-437.More infoAbstract: New field and thermobarometric work in the Californian Salinian block clarifies current and pre-Tertiary relationships between the schist of Sierra de Salinas and Cretaceous arc-related granitic rocks. The contact is variably preserved as a brittle fault and high-temperature mylonite zone, the Salinas shear zone, which represents the contact between North America and sediments accreted above the Farallon slab between ∼76 Ma and ∼70 Ma. Near granulite facies, prograde replacement of hornblende with clinopyroxene is associated with deformation of plutonic rocks at the base of the upper plate. In the lower plate, the schist of Sierra de Salinas, garnet-biotite thermometry indicates decreasing temperatures down-section from at least 714 °C to∼575 °C over an exposed thickness of ∼2.5 km, consistent with petrologic evidence of an inverted metamorphic gradient. The measured temperatures are significantly higher than observed at shallow levels above subducting slabs or predicted by 2D computational models assuming low shear stresses. Previous workers have called upon shear heating to explain similar observations in the correlative Pelona schist, an unlikely scenario given the results of recent rock deformation experiments which predict that feldspar-quartz-mica aggregates are far too weak to withstand stresses of ∼70 MPa required by the shear heating hypothesis. As an alternative, we propose that high temperatures resulted from conductive heating while the leading edge of the schist traveled ∼150 km beneath the recently active Salinian continental arc during the initiation of shallow subduction. Weakening of the schist due to high temperatures helped facilitate the collapse of the Salinian arc as the schist was emplaced. Schist emplacement coincided with loss of lower, mafic portions of the arc, and therefore evolution of the Southern California crust towards a more felsic composition. © 2005 Elsevier B.V. All rights reserved.
- Dickinson, W. R., Ducea, M., Rosenberg, L. I., Greene, H. G., Graham, S. A., Clark, J. C., Weber, G. E., Kidder, S., Brabb, E. E., & Ernst, W. G. (2005). Net dextral slip, Neogene San Gregorio-Hosgri fault zone, coastal California: Geologic evidence and tectonic implications. Special Paper of the Geological Society of America, 391, 1-43.More infoAbstract: Reinterpretation of onshore and offshore geologic mapping, examination of a key offshore well core, and revision of cross-fault ties indicate Neogene dextral strike slip of 156 ± 4 km along the San Gregorio-Hosgri fault zone, a major strand of the San Andreas transform system in coastal California. Delineating the full course of the fault, defining net slip across it, and showing its relationship to other major tectonic features of central California helps clarify the evolution of the San Andreas system. San Gregorio-Hosgri slip rates over time are not well constrained, but were greater than at present during early phases of strike slip following fault initiation in late Miocene time. Strike slip took place southward along the California coast from the western flank of the San Francisco Peninsula to the Hosgri fault in the offshore Santa Maria basin without significant reduction by transfer of strike slip into the central California Coast Ranges. Onshore coastal segments of the San Gregorio-Hosgri fault include the Seal Cove and San Gregorio faults on the San Francisco Peninsula, and the Sur and San Simeon fault zones along the flank of the Santa Lucia Range. Key cross-fault ties include porphyritic granodiorite and overlying Eocene strata exposed at Point Reyes and at Point Lobos, the Nacimiento fault contact between Salin- ian basement rocks and the Franciscan Complex offshore within the outer Santa Cruz basin and near Esalen on the flank of the Santa Lucia Range, Upper Cretaceous (Cam- panian) turbidites of the Pigeon Point Formation on the San Francisco Peninsula and the Atascadero Formation in the southern Santa Lucia Range, assemblages of Franciscan rocks exposed at Point Sur and at Point San Luis, and a lithic assemblage of Meso- zoic rocks and their Tertiary cover exposed near Point San Simeon and at Point Sal, as restored for intrabasinal deformation within the onshore Santa Maria basin. Slivering of the Salinian block by San Gregorio-Hosgri displacements elongated its northern end and offset its western margin delineated by the older Nacimiento fault, a sinistral strike-slip fault of latest Cretaceous to Paleocene age. North of its juncture with the San Andreas fault, dextral slip along the San Gregorio-Hosgri fault augments net San Andreas displacement. Alternate restorations of the Gualala block imply that nearly half the net San Gregorio-Hosgri slip was accommodated along the offshore Gualala fault strand lying west of the Gualala block, which is bounded on the east by the current master trace of the San Andreas fault. With San Andreas and San Gregorio-Hosgri slip restored, there remains an unresolved proto-San Andreas mismatch of ~100 km between the offset northern end of the Salinian block and the southern end of the Sierran-Tehachapi block. On the south, San Gregorio-Hosgri strike slip is transposed into crustal shortening associated with vertical-axis tectonic rotation of fault-bounded crustal panels that form the western Transverse Ranges, and with kinematically linked deformation within the adjacent Santa Maria basin. The San Gregorio-Hosgri fault serves as the principal link between transrotation in the western Transverse Ranges and strike slip within the San Andreas transform system of central California. © 2005 Geological Society of America.
- Ducea, M. N., Saleeby, J., Morrison, J., & Valencia, V. A. (2005). Subducted carbonates, metasomatism of mantle wedges, and possible connections to diamond formation: An example from California. American Mineralogist, 90(5-6), 864-870.More infoAbstract: We investigated calcite globules and veins in two spinel-garnet peridotite xenoliths from the sub-Sierra Nevada mantle. The studied xenoliths were entrained in a Miocene (11 Ma) volcanic plug. These carbonates are associated spatially with silicate glass inclusions, suggesting that they are primary inclusions - inclusions that formed at high temperature in the mantle and not at or close to the Earth's surface. The host peridotites represent samples of the lithospheric mantle wedge beneath the Mesozoic California magmatic arc, as indicated by radiogenic isotopic ratios measured on clinopyroxene separates [87Sr/86Sr(11 Ma) = 0.7058-0.7061, ε Nd (11 Ma) = -1.9 to -0.7]. Mineral chemistry of the peridotite major phases is typical of a mantle section that was depleted of melt. The δ18O values of olivine and orthopyroxene from the two samples are also typical of mantle rocks (δ 18O = 6-6.5‰). In contrast, calcite veins have δ18O of 18-20‰ and δ13C of -14‰, arguing for a subducted sedimentary origin for these carbonates. Presumably, the carbonates were expelled from the downgoing slab and fluxed into the overlying mantle wedge as CO2- or CO2-H2O-rich fluids or melts. The trace-element patterns of two analyzed calcite veins are typical of the arc signatures (e.g., depletions in high-field-strength elements) seen in calc-alkaline magmatic rocks worldwide. However, the cores of peridotite clinopyroxenes do not show that pattern, suggesting that the arc-like trace element signature was introduced via the recycled carbonate agent. A connection between mantle wedge carbonation and diamond formation in a subduction environment is proposed based on these observations.
- Hacker, B., Lufffi, P., Lutkov, V., Minaev, V., Ratschbacher, L., Plank, T., Ducea, M., Patiño-Douce, A., McWilliams, M., & Metcalf, J. (2005). Near-ultrahigh pressure processing of continental crust: Miocene crustal xenoliths from the Pamir. Journal of Petrology, 46(8), 1661-1687.More infoAbstract: Xenoliths of subducted crustal origin hosted by Miocene ultrapotassic igneous rocks in the southern Pamir provide important new information regarding the geological processes accompanying tectonism during the Indo-Eurasian collision. Four types have been studied: sanidine eclogites (omphacite, garnet, sanidine, quartz, biotite, kyanite), felsic granulites (garnet, quartz, sanidine and kyanite), basaltic eclogites (omphacite and garnet), and a glimmerite (biotite, clinopyroxene and sanidine). Apatite, rutile and carbonate are the most abundant minor phases. Hydrous phases (biotite and phengite in felsic granulites and basaltic eclogites, amphiboles in mafic and sanidine eclogites) and plagioclase form minor inclusions in garnet or kyanite. Solid-phase thermobarometry reveals recrystallization at mainly ultrahigh temperatures of 1000-1100°C and near-ultrahigh pressures of 2·5-2·8 GPa. Textures, parageneses and mineral compositions suggest derivation of the xenoliths from subducted basaltic, tonalitic and pelitic crust that experienced high-pressure dehydration melting, K-rich metasomatism, and solid-state re-equilibration. The timing of these processes is constrained by zircon ages from the xenoliths and 40Ar/39Ar ages of the host volcanic rocks to 57-11 Ma. These xenoliths reveal that deeply subducted crust may undergo extensive dehydration-driven partial melting, density-driven differentiation and disaggregation, and sequestration within the mantle. These processes may also contribute to the alkaline volcanism observed in continent-collision zones. © The Author 2005. Published by Oxford University Press. All rights reserved.
- Root, D. B., Hacker, B. R., Gans, P. B., Ducea, M. N., Eide, E. A., & Mosenfelder, J. L. (2005). Discrete ultrahigh-pressure domains in the Western Gneiss Region, Norway: Implications for formation and exhumation. Journal of Metamorphic Geology, 23(1), 45-61.More infoAbstract: New eclogite localities and new 40Ar/39Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh-pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km2 ; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. 40Ar/ 39Ar mica and K-feldspar ages show that this outcrop pattern is the result of gentle regional-scale folding younger than 380 Ma, and possibly 335 Ma.The UHP and intervening high-pressure (HP) domains are composed of eclogite-bearing orthogneiss basement overlain by eclogite-bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, cale-silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15-cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near-isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12-17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low-pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New 40Ar/39Ar hornblende ages of 402 Ma document that this decompression from eclogite-facies conditions at 410-405 Ma to mid-crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20-30 km. 40Ar/39Ar muscovite ages of 397-380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr-1), perhaps because of continued tectonic unroofing. © 2005 Blackwell Publishing Ltd.
- Valencia, V. A., Ruiz, J., Barra, F., Geherls, G., Ducea, M., Titley, S. R., & Ochoa-Landin, L. (2005). U-Pb zircon and Re-Os molybdenite geochronology from La Caridad porphyry copper deposit: Insights for the duration of magmatism and mineralization in the Nacozari District, Sonora, Mexico. Mineralium Deposita, 40(2), 175-191.More infoAbstract: Uranium-lead zircon (laser ablation multi-collector ICP-MS spot analysis) ages from La Caridad porphyry copper deposit in the Nacozari District, Northeastern Sonora, Mexico, suggest a short period of magmatism, between 55.5 and 52.6 Ma. Two U-Pb ages from the mineralized quartz monzonite unit, showing different textural characteristics, yielded indistinguishable crystallization ages (∼54 Ma), and indicate that the intrusion responsible for the mineralization occurred as a single large complex unit, instead of multiple pulses of magmatism. Some zircons analyzed also show inherited ages in cores recording dates of 112-124 Ma, 141-166 Ma and 1.4 Ga. The Re-Os molybdenite ages from the potassic and phyllic hydrothermal alteration veins yielded identical ages within error, 53.6 ±0.3 Ma and 53.8±0.3 Ma, respectively (weighted average of 53.7 ±0.21 Ma), supporting a restricted period for the mineralization. The geochronological data thus indicate a short-lived magmatic and hydrothermal system. The inherited zircons of Precambrian and Late Jurassic-Mid Cretaceous age found in the intrusive rocks of La Caridad deposit, can be explained considering two possible scenarios within the tectonic/magmatic evolution of the area. The first scenario considers the presence of a Precambrian anorogenic granitic basement that is intruded by Mesozoic (Jurassic-Cretaceous) units present beneath the La Caridad deposit. The second scenario suggests that the Mesozoic Glance Conglomerate Formation of Arizona underlies the Paleocene volcanic-igneous pile in the La Caridad area. © Springer-Verlag 2005.
- Zeng, L., Saleeby, J. B., & Ducea, M. (2005). Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada, California. Contributions to Mineralogy and Petrology, 150(4), 386-402.More infoAbstract: We provide data on the geochemical and isotopic consequences of nonmodal partial melting of a thick Jurassic pelite unit at mid-crustal levels that produced a migmatite complex in conjunction with the intrusion of part of the southern Sierra Nevada batholith at ca. 100 Ma. Field relations suggest that this pelitic migmatite formed and then abruptly solidified prior to substantial mobilization and escape of its melt products. Hence, this area yields insights into potential mid-crustal level contributions of crustal components into Cordilleran-type batholiths. Major and trace-element analyses in addition to field and petrographic data demonstrate that leucosomes are products of partial melting of the pelitic protolith host. Compared with the metapelites, leucosomes have higher Sr and lower Sm concentrations and lower Rb/Sr ratios. The initial 87Sr/86Sr ratios of leucosomes range from 0.7124 to 0.7247, similar to those of the metapelite protoliths (0.7125-0.7221). However, the leucosomes have a much wider range of initial εNd values, which range from -6.0 to -11.0, as compared to -8.7 to -11.3 for the metapelites. Sr and Nd isotopic compositions of the leucosomes, migmatites, and metapelites suggest disequilibrium partial melting of the metapelite protolith. Based on their Sr, Nd, and other trace-element characteristics, two groups of leucosomes have been identified. Group A leucosomes have relatively high Rb, Pb, Ba, and K2O contents, Rb/Sr ratios (0.15
- Ducea, M., House, M. A., & Kidder, S. (2003). Late cenozoic denudation and uplift rates in the Santa Lucia Mountains, California. Geology, 31(2), 139-142.More infoAbstract: Apatite (U-Th)/He ages from a vertical transect through the Santa Lucia Mountains, central California Coast Ranges, are used to reconstruct the history of exhumation and of bedrock and surface uplift in this region since ca. 6 Ma. We find a direct correlation between (U-Th)/He ages and elevation, which we interpret to correspond to denudation rates of ∼0.35 mm/yr between 6 and 2 Ma. The onset of bedrock uplift and exhumation ca. 6 Ma followed a change in plate motion ca. 8 Ma. After 2 Ma, denudation rates increased substantially (∼0.9 mm/yr). This is a rare instance in which long-term average bedrock (∼0.85 mm/yr) and surface (∼0.20 mm/yr) uplift can be calculated from denudation rates and stratigraphic data. The post-2 Ma denudation rate is about one order of magnitude higher than independently determined river erosion rates in the area. We suggest that this discrepancy indicates that exhumation of the steep western slopes of this segment of the Coast Ranges has been dominated by mass wasting via landslides, rather than fluvial erosion, at least since ca. 2 Ma. We also show that the bedrock uplift is predominantly tectonic, not isostatic.
- Medaris Jr., G., Ducea, M., Ghent, E., & Iancu, V. (2003). Conditions and timing of high-pressure Variscan metamorphism in the South Carpathians, Romania. Lithos, 70(3-4), 141-161.More infoAbstract: High-pressure (HP) metamorphic rocks, including garnet peridotite, eclogite, HP granulite, and HP amphibolite, are important constituents of several tectonostratigraphic units in the pre-Alpine nappe stack of the Getic-Supragetic (GS) basement in the South Carpathians. A Variscan age for HP metamorphism is firmly established by Sm-Nd mineral-whole-rock isochrons for garnet amphibolite, 358±10 Ma, two samples of eclogite, 341±8 and 344±7 Ma, and garnet peridotite, 316±4 Ma.A prograde history for many HP metamorphic rocks is documented by the presence of lower pressure mineral inclusions and compositional zoning in garnet. Application of commonly accepted thermobarometers to eclogite (grt+cpx±ky±phn±pg±zo) yields a range in "peak" pressures and temperatures of 10.8-22.3 kbar and 545-745 °C, depending on tectonostratigraphic unit and locality. Zoisite equilibria indicate that activity of H2O in some samples was substantially reduced, ca. 0.1-0.4. HP granulite (grt+cpx+hb+pl) and HP amphibolite (grt+hbl+pl) may have formed by retrogression of eclogites during high-temperature decompression. Two types of garnet peridotite have been recognized, one forming from spinel peridotite at ca. 1150-1300 °C, 25.8-29.0 kbar, and another from plagioclase peridotite at 560 °C, 16.1 kbar.The Variscan evolution of the pre-Mesozoic basement in the South Carpathians is similar to that in other segments of the European Variscides, including widespread HP metamorphism, in which P - T - t characteristics are specific to individual tectonostratigraphic units, the presence of diverse types of garnet peridotite, diachronous subduction and accretion, nappe assembly in pre-Westphalian time due to collision of Laurussia, Gondwana, and amalgamated terranes, and finally, rapid exhumation, cooling, and deposition of eroded debris in Westphalian to Permian sedimentary basins. © 2003 Elsevier B.V. All rights reserved.
- Park, S. K., & Ducea, M. N. (2003). Can in situ measurements of mantle electrical conductivity be used to infer properties of partial melts?. Journal of Geophysical Research B: Solid Earth, 108(5), EPM 14-1 - 14-12.More infoAbstract: Constraints on bulk conductivity from magnetotelluric measurements and petrological analyses of late Quaternary peridotite xenoliths from the southern Sierra Nevada allow evaluation of models commonly used to relate electrical conductivity to the physical and chemical state of the upper mantle. In these models, two conductive melts (basalt and sulfide) are embedded in a resistive matrix. Bounds on the amount of sulfide (0.06-0.4%) and the bulk conductivity (0.03-0.1 S/m) place constraints on the degree of interconnection between the melts. Because the sulfide melt is very conductive, even a small fraction of well-connected melt results in a bulk conductivity larger than 0.1 S/m. Similarly, completely disconnected melts result in bulk conductivities much less than 0.03 S/m. The only models which matched both the bulk conductivity and sulfide bounds consisted of a small fraction (
- Saleeby, J., Ducea, M., & Clemens-Knott, D. (2003). Production and loss of high-density batholithic root, southern Sierra Nevada, California. Tectonics, 22(6), 3-1 - 3-24.More infoAbstract: Eclogites are commonly believed to be highly susceptible to delamination and sinking into the mantle from lower crustal metamorphic environments. We discuss the production of a specific class of eclogitic rocks that formed in conjunction with the production of the Sierra Nevada batholith. These high-density eclogitic rocks, however, formed by crystal-liquid equilibria and thus contrast sharply in their petrogenesis and environment of formation from eclogite facies metamorphic rocks. Experimental studies show that when hydrous mafic to intermediate composition assemblages are melted in excess of 1 GPa, the derivative liquids are typical of Cordilleran-type batholith granitoids, and garnet + clinopyroxene, which is an eclogitic mineralogy, dominate the residue assemblage. Upper mantle-lower crustal xenolith suites that were entrained in mid-Miocene volcanic centers erupted through the central Sierra Nevada batholith are dominated by such garnet clinopyroxenites, which are shown further by geochemical data to be petrogenetically related to the overlying batholith as its residue assemblage. Petrogenetic data on garnet pyroxenite and associated peridotite and granulite xenoliths, in conjunction with a southward deepening oblique crustal section and seismic data, form the basis for the synthesis of a primary lithospheric column for the Sierra Nevada batholith. Critical aspects of this column are the dominance of felsic batholithic rocks to between 35 and 40 km depths, a thick (∼35 km) underlying garnet clinopyroxenite residue sequence, and interlayered spinel and underlying garnet peridotite extending to ∼125 km depths. The peridotites appear to be the remnants of the mantle wedge from beneath the Sierran arc. The principal source for the batholith was a polygenetic hydrous mafic to intermediate composition lower crust dominated by mantle wedge-derived mafic intrusions. Genesis of the composite batholith over an ∼50 m.y. time interval entailed the complete reconstitution of the Sierran lithosphere. Sierra Nevada batholith magmatism ended by ∼80 Ma in conjunction with the onset of the Laramide orogeny, and subsequently, its underlying mantle lithosphere cooled conductively. In the southernmost Sierra-northern Mojave Desert region the subbatholith mantle lithosphere was mechanically delaminated by a shallow segment of the Laramide slab and was replaced by underthrust subduction accretion assemblages. Despite these Laramide events, the mantle lithosphere of the greater Sierra Nevada for the most part remained intact throughout much of Cenozoic time. A pronounced change in xenolith suites sampled by Pliocene-Quaternary lavas to garnet absent, spinel and plagioclase peridotites, whose thermobarometry define an asthenosphere adiabat, as well as seismic data, indicate that much of the remaining sub-Sierran lithosphere was removed in Late Miocene to Pliocene time. Such removal is suggested to have arisen from a convective instability related to high-magnitude extension in the adjacent Basin and Range province and to have worked in conjunction with the recent phase of Sierran uplift and a change in regional volcanism to more primitive compositions. In both the Mio-Pliocene and Late Cretaceous lithosphere removal events the base of the felsic batholith was the preferred locus of separation. Copyright 2003 by the American Geophysical Union.
- Sen, G., Yang, H., & Ducea, M. (2003). Anomalous isotopes and trace element zoning in plagioclase peridotite xenoliths of Oahu (Hawaii): Implications for the Hawaiian plume. Earth and Planetary Science Letters, 207(1-4), 23-38.More infoAbstract: Survival of plagioclase in the residual melting column during melting can have a significant impact on the melting process beneath a mid-oceanic ridge [Asimow et al., Phil. Trans. R. Soc. London Ser. A 355 (1997) 255-281]. Here we investigate the origin of plagioclase that occurs in some rare mantle xenoliths from Oahu, Hawaii. The xenoliths are harzburgitic with less than 2 modal% clinopyroxene and are characterized by strong foliation and porphyroclastic texture. Olivine and orthopyroxene are common porphyroclasts; and only one xenolith (77PAII-9) contains a single large clinopyroxene porphyroclast with thick exsolved orthopyroxene lamellae. The strongly foliated groundmass shows well-developed triple-point junctions and is dominantly composed of olivine (ol85-90 opx7-14 cpx
- Ducea, M. N. (2002). Constraints on the bulk composition and root foundering rates of continental arcs: A California arc perspective. Journal of Geophysical Research B: Solid Earth, 107(11), ECV 15-1 - 15-13.More infoAbstract: Garnet pyroxenites are the most common deep lithospheric xenolith assemblages found in Miocene volcanic rocks that erupted through the central part of the Sierra Nevada batholith. Elemental concentrations and isotope ratios are used to argue that the Sierra Nevada granitoids and the pyroxenite xenoliths are the melts and the residues/cumulates, respectively, resulting from partial melting/fractional crystallization at depths exceeding 35-40 km. The estimated major element chemistry of the protolith resembles a basaltic andesite. Effectively, at more than about 40 km depth, batholith residua are eclogite facies rocks. Radiogenic and oxygen isotope ratios measured on pyroxenites document unambiguously the involvement of Precambrian lithosphere and at least 20-30% (mass) of crustal components. The mass of the residual assemblage was significant, one to two times the mass of the granitic batholith. Dense garnet pyroxenites are prone to foundering in the underlying mantle. An average removal rate of 25-40 km3/km Myr is estimated for this Cordilleran-type arc, although root loss could have taken place at least in part after the cessation of arc magmatism. This rate is matched by the average subcrustal magmatic addition of the arc (∼23-30 km3/km Myr), suggesting that the net crustal growth in this continental arc was close to zero. It is also suggested that in order to develop a convectively removable root, an arc must have a granitoid melt thickness of at least 20-25 km. Residues of thinner arcs should be mostly in the granulite facies; they are not gravitationally unstable with respect to the underlying mantle.
- Ducea, M. (2001). The California arc: Thick granitic batholiths, eclogitic residues, lithospheric-scale thrusting, and magmatic flare-ups. GSA Today, 11(11), 4-10.More infoAbstract: Recent geological and geophysical data show that a significant fraction of the crust (~33 km) in the central Sierra Nevada batholith is granitic, requiring that the batholith be underlain by a significant residual mass prior to Cenozoic extension. Although batholith residua are commonly though to be granulites, xenolith data indicate that eclogite facies residues were an important part of the California arc at depth. The arc was continuously active for >140 m.y., yet most surface and/or shallow crustal magmatism took place via two short-lived episodes: one in the Late Jurassic (160-150 Ma), and a second, more voluminous one in the Late Cretaceous (100-85 Ma). These magmatic flare-ups cannot be explained solely by increases in convergence rates and magmatic additions from the mantle. Isotopic data on xenoliths and midcrustal exposures suggest that North American lower crustal and lithospheric mantle was underthrusted beneath accreted rocks in the arc area. The Late Cretaceous flare-up is proposed to be the result of this major west dipping-lithospheric scale thrusting, an event that preceded flare-up by ~15-25 m.y. I suggest that the central part of the arc shut off at ~80 Ma because the source became melt-drained and not because of refrigeration from a shallowly subducting slab.
- Ducea, M. N., & Park, S. K. (2000). Enhanced mantle conductivity from sulfide minerals, southern Sierra Nevada, California. Geophysical Research Letters, 27(16), 2405-2408.More infoAbstract: Petrographic studies of peridotitic xenoliths entrained in late Quaternary basalts from beneath the southern Sierra Nevada have revealed the presence of accessory sulfide minerals along grain boundaries and fractures. Equilibration temperatures from the xenoliths are sufficiently high that the molten sulfides coexist with the basaltic melt. Sulfides are extremely conductive relative to the solid matrix or the basaltic melt, so a small fraction can increase the bulk conductivity of the mantle appreciably. Previous estimates of 2-5% partial melt from magnetotelluric measurements can be plausibly reduced to less than 1%. Such low melt percentages have longer residence times in the mantle and are more consistent with the volumetrically minor late Quaternary basalt flows and the primitive basalt compositions.
- Ducea, M. N., I., B., & Wyllie, P. J. (1999). Experimental determination of compositional dependence of hydrous silicate melts on sulfate solubility. European Journal of Mineralogy, 11(1), 33-43.More infoAbstract: We measured the solubility of SO3 in quartz-normative and nepheline-normative compositions in the system Na2O-Al2O3-SiO2-H2O at temperatures from 800-825°C and pressures between 1.5-1.7 kbar. We added 1.5,3, 10 or 15 wt% CaSO4 to two H2O-saturated synthetic compositions near the eutectics in quartz-albite-H2O and nepheline-albite-H2O. The low sulfate solubilities measured in the quartz-normative quenched glasses (0.06-0.12% SO3) are similar to previous determinations on quartz-normative melts from natural rock compositions. The sulfate contents of quenched glasses in the nepheline-normative system are up to 6 times higher (0.17-0.36 %SO3). Precipitation of anhydrite in the quartz-normative melt requires addition of about 6 wt% CaSO4 at 800°C, and 10 wt% CaSO4 at 820°C. In contrast,
- Ducea, M. N., & Saleeby, J. B. (1998). The age and origin of a thick mafic-ultramafic keel from beneath the Sierra Nevada batholith. Contributions to Mineralogy and Petrology, 133(1-2), 169-185.More infoAbstract: We present evidence for a thick (∼100 km) sequence of cogenetic rocks which make up the root of the Sierra Nevada batholith of California. The Sierran magmatism produced tonalitic and granodioritic magmas which reside in the Sierra Nevada upper- to mid-crust, as well as deep eclogite facies crust/upper mantle mafic-ultramafic cumulates. Samples of the mafic-ultramafic sequence are preserved as xenoliths in Miocene volcanic rocks which erupted through the central part of the batholith. We have performed Rb-Sr and Sm-Nd mineral geochronologic analyses on seven fresh, cumulate textured, olivine-free mafic-ultramafic xenoliths with large grainsize, one garnet peridotite, and one high pressure metasedimentary rock. The garnet peridotite, which equilibrated at ∼130 km beneath the batholith, yields a Miocene (10 Ma) Nd age, indicating that in this sample, the Nd isotopes were maintained in equilibrium up to the time of entrainment. All other samples equilibrated between ∼35 and 100 km beneath the batholith and yield Sm-Nd mineral ages between 80 and 120 Ma, broadly coincident with the previously established period of most voluminous batholithic magmatism in the Sierra Nevada. The Rb-Sr ages are generally consistent with the Sm-Nd ages, but are more scattered. The 87Sr/86Sr and 143Nd/144Nd intercepts of the igneous-textured xenoliths are similar to the ratios published for rocks outcroping in the central Sierra Nevada. We interpret the mafic/ultramafic xenoliths to be magmatically related to the upper- and mid-crustal granitoids as cumulates and/or restites. This more complete view of the vertical dimension in a batholith indicates that there is a large mass of mafic-ultramafic rocks at depth which complement the granitic batholiths, as predicted by mass balance calculations and experimental studies. The Sierran magmatism was a large scale process responsible for segregating a column of ∼30 km thick granitoids from at least ∼70 km of mainly olivine free mafic-ultramafic residues/cumulates. These rocks have resided under the batholith as granulite and eclogite facies rocks for at least 70 million years. The presence of this thick mafic-ultramafic keel also calls into question the existence of a "flat" (i.e., shallowly subducted) slab at Central California latitudes during Late Cretaceous-Early Cenozoic, in contrast to the southernmost Sierra Nevada and Mojave regions. © Springer-Verlag 1998.
- Ducea, M., & Saleeby, J. (1998). A case for delamination of the deep batholithic crust beneath the Sierra Nevada, California. International Geology Review, 40(1), 78-93.More infoAbstract: Surface exposures as well as deep-crustal and upper-mantle xenoliths constrain the composition of the lithospheric column beneath the Sierra Nevada mountain range (California) as it resulted from the generation of the Mesozoic Sierra Nevada batholith (SNB). After the cessation of magmatism at ∼80 Ma, the SNB consisted of a ∼30 to 35 km thick granitic crust underlain by a batholithic "root," a ∼70 km thick sequence of mafic-ultramafic, mainly eclogite-facies cumulate and residues. The deeper root assemblages consist largely of garnet and pyroxenes that precipitated as igneous cumulate phases during the SNB magmatism. The root assemblages were present beneath the SNB as recently as ∼8 to 12 Ma, when they were sampled as xenoliths in fast-ascending magmas erupted through the batholith. Several lines of evidence suggest that the eclogitic root may have disapeared from beneath the SNB since Miocene time, leading to a major change in the lithospheric column. There are no garnet-bearing xenoliths in the Pliocene and Quaternary volcanic rocks; instead, all xenolith lithologies found in the younger volcanic outcrops are peridotitic, have equilibrated at depths between 35 and 70 km, possess locked-in temperatures of ∼1150 to 1200°C, and display an asthenospheric-like adiabatic P-T trend. Some of the Pliocene uppermost-mantle peridotitic xenoliths contain exotic silica-rich glass inclusions that may have originated by partial melting of the eclogitic root. Geophysical evidence suggests that anomalously high seismic velocity may represent eclogitic bodies present at depths of 100 to 200 km beneath the SNB. All of these observations indicate that the "eclogitic" root may have detached and delaminated (sunk) into the underlying mantle, a process compensated by diapiric rise of asthenospheric peridotitic material to the base of the shallow (∼35 km) remnant crust. The delamination hypothesis is consistent with observations documenting the existence of a shallow Moho, a low-velocity, partially molten upper mantle observed today beneath the SNB, a gradual change in Miocene volcanism in the Sierra toward more primitive compositions, and significant late Miocene-Pliocene uplift in the area. If the magmatic arc has indeed lost its root, delamination is an important mechanism in the differentiation of the continental crust at Cordilleran-type margins. The present-day crustal column in the Sierra (the SNB) is a mass extracted from the Earth's mantle predominantly during the Phanerozoic, although not necessarily only during batholithic magmatism. The ∼35 km thick present-day crustal composition of the Sierra Nevada is similar to, or more evolved than, the average continental crust. Copyright © 1998 by V. H. Winston & Son, Inc. All rights reserved.
- Ducea, M., & Saleeby, J. (1998). Crustal recycling beneath continental arcs: Silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California. Earth and Planetary Science Letters, 156(1-2), 101-116.More infoAbstract: We describe silica-rich (up to ∼69.5% SiO2) glass inclusions trapped as grain boundary films and within-grain pockets in ultramafic xenoliths hosted by Pliocene basalts from the Sierra Nevada, California. The ultramafic xenoliths are lherzolites which equilibrated in the Sierra Nevada upper mantle at 1150-1180°C and ∼1.4-1.8 GPa. The glass inclusions have trachytic compositions, similar to previously described silicic melts from mantle xenoliths [1-9]. We have determined the Sr and Nd isotope compositions of the grain boundary films using a leaching technique, and calculated the glass isotopic compositions. The glass 87Sr/86Sr (0.7077-0.7085) and 143Nd/144Nd (≈0.51244) ratios are higher than in the ultramafic xenoliths and distinct from the host basalt ratios. Glasses are characterized by Nb depletions (Nbn/Nb5n ∼0.15), enrichment of light rare earth elements (Lan/Ybn ≈ 50), and the presence of negative Eu anomalies (Eun/Eu*n ≈0.7-0.86), indicating a crustal origin for the melt source. The Nd isotope ratios (εNd ≈ -4) are inconsistent with an oceanic crust as the source for these former melts. The source rocks must have been continental materials recycled in the mantle, either foundered lower crust or subducted sediment. Low Rb/Sr (0.036-0.077) and high Sr/Nd (> 35) ratios observed in the glasses are suggesting a lower crustal source. The Sierra Nevada lowermost crust (amphibole-bearing garnet pyroxenites and other dense Mesozoic cumulate mafic-ultramafic rocks), as defined by studies of older, Miocene xenolith-bearing volcanic rocks from the same area [M.N. Ducea, J. Saleehy, J. Geophys. Res. 101 (1996) 8229-8244], has isotopic compositions similar to the glass inclusions. Geologic [M.N. Ducea, J. Saleeby, J. Geophys. Res. 101 (1996) 8229-8244] and geophysical [G. Zandt, S. Ruppert, EOS Trans. AGU 77 (1990) 831] evidence indicate that the Sierra Nevada has lost its eclogitic arc root, probably by foundering in the mantle. We propose here that the silica-rich glasses were formed by low percent partial melting of the dense, cold Sierran batholithic lowermost crust during root delamination. Further tests need to be aimed at addressing the viability of the main alternative to our interpretation, i.e. derivation of glasses from melting subducted sediments. © 1998 Elsevier Science B.V. All rights reserved.
- Ducea, M. N., & Saleeby, J. B. (1996). Buoyancy sources for a large, unrooted mountain range, the Sierra Nevada, California: Evidence from xenolith thermobarometry. Journal of Geophysical Research B: Solid Earth, 101(4), 8229-8244.More infoAbstract: Xenoliths hosted by Cenozoic volcanic flows and plugs from the Central Sierra Nevada and Eastern Sierra Nevada, Owens Valley, and Inyo Mountains were studied for petrography and thermobarometry. The Central Sierra Nevada suite consists of abundant lower crustal feldspathic granulites, garnet clinopyroxenites, and mantle-derived peridotites and garnet websterites. Mafic crustal assemblages occur down to ∼65-70 km, although below 35-40 km, they are mainly in the eclogite facies. In contrast, the Eastern Sierra Region suites show peridotitic, pyroxenitic, and harzburgitic assemblages at depths of ≥35-40 km. They define an adiabat in PT space (T ∼ 1180-1250°C), suggesting the presence of the asthenospheric upper mantle close to the base of the crust. The ultramafic mantle rocks from the Central Sierra Nevada also define an adiabatic slope in PT space, possibly an artifact of side heating from the east. There is xenolith evidence that the Sierra Nevada lost about half of its original crust on the eastern side of the range. Regardless of the actual mechanism of crustal thinning, the loss of the eclogitic lowermost crust and replacement by peridotite in the eastern Sierra Nevada is a process accompanied by a substantial density decrease (>100 kg/m3). Overall, if the mechanism of eclogitic lowermost crust removal is viable, there are enough buoyancy sources to explain topographic differences between the Sierra Nevada and the adjacent Basin and Range, assuming isostatic equilibrium.
- Wernicke, B., Clayton, R., Ducea, M., Jones, C. H., Park, S., Ruppert, S., Saleeby, J., Snow, J. K., Squires, L., Fliedner, M., Jiracek, G., Keller, R., Klemperer, S., Luetgert, J., Malin, P., Miller, K., Mooney, W., Oliver, H., & Phinney, R. (1996). Origin of high mountains in the continents: The Southern Sierra Nevada. Science, 271(5246), 190-193.More infoAbstract: Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.
- Park, S. K., Clayton, R. W., Ducea, M. N., Wernicke, B., Jones, C. H., & Ruppert, S. D. (1995). Project combines seismic and magnetotelluric surveying to address the Sierran root question. Eos, 76(30), 297-298.More infoAbstract: Mt. Whitney, the highest peak in the contiguous US, lies less than 150 km from Death Valley, the nation's lowest point. Despite the nearness of these physiographic features, their geological settings vary markedly. What geologic processes lead to a high mountain range adjacent to a highly extending region? This is the Sierran root question; its answer depends on better understanding of the region's deep structure. To study the region's deep structure, the Southern Sierra Continental Dynamics (SSCD) project was organized. In September 1993 an SSCD multidisciplinary consortium employed both magnetotelluric soundings and closely spaced seismic recorders in the High Sierra for the first time to examine the crustal and upper mantle structure of the Sierra Nevada and the adjacent Basin and Range. -from Authors
- Ducea, M. N., McInnes, B. I., & Wyllie, P. J. (1994). Sulfur variations in glasses from volcanic rocks: effect of melt composition on sulfur solubility. International Geology Review, 36(8), 703-714.More infoAbstract: A worldwide data set of major element and sulfur analyses of undegassed lavas, pumices, and melt inclusions from 14 volcanic locations was selected to investigate the compositional effects of sulfur solubility in magmas. There is a strong correlation between chemical composition and the sulfur concentration: the less silicic and the more alkaline the rocks are, the more dissolved sulfur they can carry. Also, sulfur concentration is higher in rocks that represent less polymerized melts. Elemental correlations between FeO and S, well-defined for tholeiites, do not hold for alkaline melts. The compositional effects are at least as important as the better-known pressure, temperature, and f(O)2 dependencies. -from Authors