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  • 1
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    Linking thermodynamic modelling, Lu-Hf geochronology, and trace elements in garnet: New P-T-t paths from the Sevier hinterland (2015)
    Wiley
    Details
    Publication Date: 2015-07-15
    Description: Major-element, trace-element, and Lu-Hf geochronological data from amphibolite-facies pelitic schist in the Raft River and Albion Mountains of northwest Utah and southern Idaho indicate that garnet grew during increasing pressure, interpreted to be the result of tectonic burial and crustal thickening during Sevier orogenesis. Garnet growth was interrupted by hiatuses interpreted from discontinuities in major element zonation. Pressure-temperature paths were determined from the pre-hiatus portions of the garnet chemical zoning profiles and indicate an increase of ~2 kbar and ~50 °C in the western Raft River Mountains. Garnet Lu-Hf dates of 150 ± 1 Ma in the western Raft River Mountains and 138.7 ± 0.7 Ma and 132 ± 5 Ma in the southern Albion Mountains indicate the timing of garnet growth. Lutetium garnet zoning profiles indicate that the Lu-Hf ages are biased toward the post-hiatus or outer pre-hiatus segments, indicating that the determined ages likely post-date the recorded P-T path history or date the tail end of the paths. Crustal thickening associated with Sevier orogenesis in the western Raft River Mountains thus began slightly before 150 ± 1 Ma, in the Late Jurassic. This study shows that integrating P-T paths determined from garnet growth zoning with Lu-Hf garnet geochronology and in situ garnet trace element analyses is an effective approach for interpreting and dating deformation events in orogenic belts. This article is protected by copyright. All rights reserved.
    Print ISSN: 0263-4929
    Electronic ISSN: 1525-1314
    Topics: Geosciences
    Published by Wiley
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  • 2
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    Structural evolution of a composite middle to lower crustal section: The Sierra de Pie de Palo, northwest Argentina (2011)
    Wiley
    In: Tectonics
    Details
    Publication Date: 2011-01-27
    Description: The Sierra de Pie de Palo of northwest Argentina preserves middle to lower crustal metamorphic rocks that were penetratively deformed during Ordovician accretion of the Precordillera terrane to the Gondwana margin. New structural, petrologic, and geochronologic data from a 40 km structural transect reveals that the Sierra de Pie de Palo preserves a middle to lower crustal ductile thrust complex consisting of individual structural units and not an intact ophiolite and cover sequence. Top-to-the-west thrusting occurred intermittently on discrete ductile shear zones from ∼515 to ∼417 Ma and generally propagated toward the foreland with progressive deformation. Ordovician crustal shortening and peak metamorphic temperatures in the central portion of the Sierra de Pie de Palo were synchronous with retro-arc shortening and magmatic flare-up within the Famatina arc. Accretion of the Precordillera terrane resulted in the end of arc flare-up and the onset of synconvergent extension by ∼439 Ma. Continued synextensional to postextensional convergence was accommodated along progressively lower grade shear zones following terrane accretion and the establishment of a new plate margin west of the Precordillera terrane. The results support models of Cordilleran orogens that link voluminous arc magmatism to periods of regional shortening. The deformation, metamorphic, and magmatic history within the Sierra de Pie de Palo is consistent with models placing the region adjacent to the Famatina margin in the middle Cambrian and not as basement to the Precordillera terrane.
    Print ISSN: 0278-7407
    Electronic ISSN: 1944-9194
    Topics: Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Decoding polyphase migmatites using geochronology and phase equilibria modelling (2014)
    Wiley
    Details
    Publication Date: 2014-11-14
    Description: In this study, in situ U–Pb monazite ages and Lu–Hf garnet geochronology are used to distinguish mineral parageneses developed during Devonian–Carboniferous and Cretaceous events in migmatitic paragneiss and orthogneiss from the Fosdick migmatite–granite complex in West Antarctica. SHRIMP U–Pb monazite ages define two dominant populations at 365–300 Ma (from cores of polychronic grains, dominantly from deeper structural levels in the central and western sectors of the complex) and 120–96 Ma (from rims of polychronic grains, dominantly from the central and western sectors of the complex, and from monochronic grains, mostly from shallower structural levels in the eastern sector of the complex). For five paragneisses and two orthogneisses, Lu–Hf garnet ages range from 116 to 111 Ma, c. 12–17 Ma older than published Sm–Nd garnet ages of 102–99 Ma from three of the same samples. Garnet grains in the analysed samples generally have Lu-enriched rims relative to Lu-depleted cores. By contrast, for three of the same samples individual garnet grains have flat Sm concentrations consistent with high-temperature diffusive resetting. Lutetium enrichment of garnet rims is interpreted to record the breakdown of a Lu-rich accessory mineral during the final stage of garnet growth immediately prior to the metamorphic peak, and/or the preferential retention of Lu in garnet during breakdown to cordierite in the presence of melt concomitant with the initial stages of exhumation. Therefore, garnet is interpreted to be part of the Cretaceous mineral paragenesis and the Lu–Hf garnet ages are interpreted to record the timing of close-to-peak metamorphism for this event. For the Devonian–Carboniferous event, phase equilibria modelling of the metasedimentary protoliths to the paragneiss and a diatexite migmatite restrict the peak P–T conditions to 720–800°C at 0.45–1.0 GPa. For the Cretaceous event, using both forward and inverse phase equilibria modelling of residual paragneiss and orthogneiss compositions, the P–T conditions after decompression are estimated to have been 850–880°C at 0.65–0.80 GPa. These P–T conditions occurred between c. 106 and c. 96 Ma, determined from Y-enriched rims on monazite that record the timing of garnet and biotite breakdown to cordierite in the presence of melt. The effects of this younger metamorphic event are dominant throughout the Fosdick complex. This article is protected by copyright. All rights reserved.
    Print ISSN: 0263-4929
    Electronic ISSN: 1525-1314
    Topics: Geosciences
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  • 4
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    A positive test of East Antarctica-Laurentia juxtaposition within the Rodinia supercontinent (2008)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2008-07-16
    Description: The positions of Laurentia and other landmasses in the Precambrian supercontinent of Rodinia are controversial. Although geological and isotopic data support an East Antarctic fit with western Laurentia, alternative reconstructions favor the juxtaposition of Australia, Siberia, or South China. New geologic, age, and isotopic data provide a positive test of the juxtaposition with East Antarctica: Neodymium isotopes of Neoproterozoic rift-margin strata are similar; hafnium isotopes of approximately 1.4-billion-year-old Antarctic-margin detrital zircons match those in Laurentian granites of similar age; and a glacial clast of A-type granite has a uraniun-lead zircon age of approximately 1440 million years, an epsilon-hafnium initial value of +7, and an epsilon-neodymium initial value of +4. These tracers indicate the presence of granites in East Antarctica having the same age, geochemical properties, and isotopic signatures as the distinctive granites in Laurentia.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Goodge, J W -- Vervoort, J D -- Fanning, C M -- Brecke, D M -- Farmer, G L -- Williams, I S -- Myrow, P M -- DePaolo, D J -- New York, N.Y. -- Science. 2008 Jul 11;321(5886):235-40. doi: 10.1126/science.1159189.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Geological Sciences, University of Minnesota-Duluth, Duluth, MN 55812, USA. jgoodge@d.umn.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/18621666" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 5
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    Transfer of metasupracrustal rocks to mid to lower crustal depths in the North Cascades continental magmatic arc, Skagit Gneiss Complex, Washington (2017)
    Wiley
    In: Tectonics
    Details
    Publication Date: 2017-12-03
    Description: The metasupracrustal units within the north-central Chelan block of the North Cascades Range, Washington are investigated to determine mechanisms and timescales of supracrustal rock incorporation into the deep crust of continental magmatic arcs. Zircon U-Pb and Hf-isotope analyses were used to characterize the protoliths of metasedimentary and metaigneous rocks from the Skagit Gneiss Complex, metasupracrustal rocks from the Cascade River Schist, and metavolcanic rocks from the Napeequa Schist. Skagit Gneiss Complex metasedimentary rocks have: (1) a wide range of zircon U-Pb dates from Proterozoic to latest Cretaceous and (2) a more limited range of dates, from Late Triassic to latest Cretaceous, and a lack of Proterozoic dates. Two samples from the Cascade River Schist are characterized by Late Cretaceous protoliths. Amphibolites from the Napeequa Schist have Late Triassic protoliths. Similarities between the Skagit Gneiss metasediments and accretionary wedge and forearc sediments in northwestern Washington and southern California indicate that the protolith for these units was likely deposited in a forearc basin and/or accretionary wedge in the Early to Late Cretaceous (ca. 134–79 Ma). Sediment was likely underthrust into the active arc by ca. 74–65 Ma, as soon as 7 m.y. after deposition, and intruded by voluminous magmas. The incorporation of metasupracrustal units aligns with the timing of major arc magmatism in the North Cascades (ca. 79–60 Ma) and may indicate a link between the burial of sediments and pluton emplacement.
    Print ISSN: 0278-7407
    Electronic ISSN: 1944-9194
    Topics: Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    New constraints on Eocene extension within the Canadian Cordillera and identification of Phanerozoic protoliths for footwall gneisses of the Okanagan Valley shear zone (2012)
    Geological Society of America (GSA)
    Details
    Publication Date: 2012-08-01
    Description: The Okanagan Valley shear zone delineates the SW margin of the Shuswap metamorphic complex, the largest core complex within the North American Cordillera. The Okanagan Valley shear zone is a major Eocene extensional fault zone that facilitated exhumation of the southern Shuswap metamorphic complex during the orogenic collapse of the SE Canadian Cordillera when convergence at the western margin of North America switched from transpression to transtension. This study documents the petrology, structure, and age of the Okanagan gneiss, the main lithology within the footwall of the Okanagan Valley shear zone, and constrains its history from protolith to exhumed shear zone.The Okanagan gneiss is an ~1.5-km-thick, west-dipping panel composed of intercalated orthogneiss and paragneiss in which intense ductile deformation of the Okanagan Valley shear zone is recorded. New U-Pb zircon ages from the gneiss and crosscutting intrusions constrain the development of the Okanagan gneiss to the Eocene, contemporaneous with widespread extension, intense deformation, high-grade metamorphism, and anatexis in the southern Canadian Cordillera. Thermobarometric data from the paragneiss domain indicate Eocene exhumation from between 17 and 23 km depth, which implies 64–89 km of WNW-directed horizontal extension based on an original shear zone angle of ~15°. Neither the Okanagan gneiss nor its protolith represents exhumed Proterozoic North American cratonic basement as previously postulated. New U-Pb data demonstrate that the protolith for the gneiss is Phanerozoic, consisting of Mesozoic intrusions emplaced within a late Paleozoic–Mesozoic layered sequence of sedimentary rocks.
    Print ISSN: 1941-8264
    Electronic ISSN: 1947-4253
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 7
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    Timing and deformation conditions of the western Idaho shear zone, West Mountain, west-central Idaho (2017)
    Geological Society of America (GSA)
    Details
    Publication Date: 2017-03-18
    Description: The western Idaho shear zone is a major, lithospheric-scale structure separating accreted terranes of the Blue Mountains from continental North America. We document the occurrence of the western Idaho shear zone in West Mountain, west-central Idaho. Rocks deformed by the western Idaho shear zone at West Mountain are dominantly orthogneisses, although exposures on West Mountain containing screens of metamorphosed sedimentary rocks are also present. Steeply E-dipping, N-NNE–oriented foliations and downdip lineations characterize the fabric in the orthogneisses, consistent with dextral transpressional kinematics. The foliation orientation changes from 005° to 024° from the northern to the southern part of the field area, and this is interpreted to reflect a primary along-strike variation in the orientation of the western Idaho shear zone. The westernmost unit in West Mountain (Four Bit Creek tonalite) has a U-Pb zircon age of 101 ± 3.0 Ma, yet it is only weakly deformed. We interpret this unit to have been emplaced pretectonically, thus constraining the initiation of the western Idaho shear zone. The youngest unit at West Mountain is the undeformed Rat Creek granite (88.2 ± 3.3 Ma). U-Pb analyses of zircons from orthogneisses at West Mountain span ages of 111–91 Ma, indicating both precursory and continuous magmatism coeval with western Idaho shear zone deformation. Two Lu-Hf garnet isochron ages, 97.3 ± 0.7 Ma and 99.5 ± 1.4 Ma, are interpreted to indicate peak metamorphism during western Idaho shear zone deformation. Geochemical analyses suggest that the westernmost exposed orthogneiss units are dominantly derived from continental material in West Mountain, and yet there is also evidence for a component of accreted terrane rocks at depth east of the western Idaho shear zone.
    Print ISSN: 1941-8264
    Electronic ISSN: 1947-4253
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 8
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    Intrusive and depositional constraints on the Cretaceous tectonic history of the southern Blue Mountains, eastern Oregon (2017)
    Geological Society of America (GSA)
    Details
    Publication Date: 2017-03-18
    Description: We present an integrated study of the postcollisional (post–Late Jurassic) history of the Blue Mountains province (Oregon and Idaho, USA) using constraints from Cretaceous igneous and sedimentary rocks. The Blue Mountains province consists of the Wallowa and Olds Ferry arcs, separated by forearc accretionary material of the Baker terrane. Four plutons (Lookout Mountain, Pedro Mountain, Amelia, Tureman Ranch) intrude along or near the Connor Creek fault, which separates the Izee and Baker terranes. High-precision U-Pb zircon ages indicate 129.4–123.8 Ma crystallization ages and exhibit a north-northeast–younging trend of the magmatism. The 40 Ar/ 39 Ar analyses on biotite and hornblende indicate very rapid (〈1 m.y.) cooling below biotite closure temperature (~350 °C) for the plutons. The (U-Th)/He zircon analyses were done on a series of regional plutons, including the Lookout Mountain and Tureman Ranch plutons, and indicate a middle Cretaceous age of cooling through ~200 °C. Sr, Nd, and Pb isotope geochemistry on the four studied plutons confirms that the Izee terrane is on Olds Ferry terrane basement. We also present data from detrital zircons from Late Cretaceous sedimentary rocks at Dixie Butte, Oregon. These detrital zircons record only Paleozoic–Mesozoic ages with only juvenile Hf isotopic compositions, indicating derivation from juvenile accreted terrane lithosphere. Although the Blue Mountains province is juxtaposed against cratonic North America along the western Idaho shear zone, it shows trends in magmatism, cooling, and sediment deposition that differ from the adjacent part of North America and are consistent with a more southern position for terranes of this province at the time of their accretion. We therefore propose a tectonic history involving moderate northward translation of the Blue Mountains province along the western Idaho shear zone in the middle Cretaceous.
    Print ISSN: 1941-8264
    Electronic ISSN: 1947-4253
    Topics: Geosciences
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  • 9
    Electronic Resource
    Electronic Resource
    Constraints on early Earth differentiation from hafnium and neodymium isotopes (1996)
    [s.l.] : Nature Publishing Group
    Nature 379 (1996), S. 624-627 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The neodymium isotope record of juvenile, mantle-derived rocks is quite well documented12'13. In general terms, the record of the highest £Nd values4'13'14 can be described in three main segments (Fig. 1): (1) a nearly linear increase from about 2.5 Gyr ago to the present; (2) an essentially ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/379624a0
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  • 10
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    Redefining the metamorphic history of the oldest rocks in the southern Rocky Mountains (2016)
    Geological Society of America (GSA)
    Details
    Publication Date: 2016-06-24
    Description: Proterozoic rocks exposed in the Rocky Mountains of New Mexico record the history of the assembly of North America, providing insight into crustal growth, accretion, and recycling. Here, we present six well-constrained Lu-Hf garnet ages showing that regional amphibolite-facies metamorphism and deformation occurred between 1.46 and 1.40 Ga. The samples come from the kyanite, sillimanite, and andalusite zones of the "Al 2 SiO 5 triple-point terrane" of northern New Mexico. Porphyroblast-matrix textures in dated samples provide definitive evidence for garnet growth during three phases of deformation associated with crustal thickening during the development of a regional-scale north-vergent contractional orogenic belt. The development of first-generation deformation fabrics is constrained to a period between 1.46 and ca. 1.45 Ga; second-generation deformation fabrics are constrained to the period 1.42–1.40 Ga; and third-generation fabrics were formed after ca. 1.40 Ga. Our results demonstrate that crustal-scale structures and regional metamorphism in northern New Mexico are both the result of a Mesoproterozoic orogenic event, the Picuris orogeny. We found no evidence for amphibolite-facies metamorphism during the Paleoproterozoic Mazatzal or Yavapai orogenies in northern New Mexico.
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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