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In Sawmill Canyon, located near the eastern margin of the Tuolumne batholith, central Sierra Nevada, California, a series of petrologically and structurally complex, magmatic sheeted zones intrude older granodioritic units (Kuna Crest and equigranular Half Dome) and in one case truncate these units along a sharp contact. These sheeted zones (a) consist of numerous batches of (now frozen) magma, (b) display clear outward growth directions, (c) were actively deforming during and after emplacement resulting in magmatic folds, faults and multiple magmatic mineral fabrics, and (d) are the location of numerous, but localized magma flow structures (schlieren-bounded tubes, troughs, megacryst-rich pipes) and instabilities (load casts, flame structures, slumps, diapirs, ridge and pillar structures). Geochemical data indicate that the sheeted zones largely consist of magmas derived from the Half Dome granodiorite with some late Cathedral Peak granodiorite pulses, and with fractionation and flow sorting forming widespread layering in the above structures. We interpret these sheeted zones to record the pulsing of magma during propagation and expansion of opening-mode (Mode I), submagmatic fractures at the margins of large blocks of older, fairly solidified magmatic pulses that were subsequently removed from the present crustal level. Elsewhere in the Tuolumne batholith we see similar features suggesting that a "recycling" process, i.e., the breaking off of older parts of the magma chamber and incorporation into younger intrusive units, occurred in this batholith. This recycling removed a significant portion of older units and resulted in the formation of sheeted zones and local instabilities in this batholith. Finally this recycling is one process responsible for transfer of zircon crystals between units and for obscuring whole-rock geochemical signatures. © 2008 Elsevier B.V. All rights reserved.
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Reference Number: 2069 GeoREF Number: 2008-900201 Publication Year: 2008
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Reference Number: 7950 GeoREF Number: 2007-051507 Publication Year: 2007 Abstract: "North-central and northeast Nevada contains numerous large plutons andsmaller stocks but also contains many small, shallowly emplacedintrusive bodies, including dikes, sills, and intrusive lava domecomplexes. Decades of geologic investigations in the study areademonstrate that many ore deposits, representing diverse ore deposittypes, are spatially, and probably temporally and genetically,associated with these igneous intrusions. However, despite the numberand importance of igneous intrusions in the study area, no synthesisof geochemical data available for these rocks has been completed. Thisreport presents a synthesis of geochemical data for these rocks. Theproduct represents the first phases of an effort to evaluate thetime-space-compositional evolution of Mesozoic and Cenozoic magmatismin the study area and identify genetic associations between magmatismand mineralizing processes in this region." Descriptors: cartography; chemical composition; data bases; data processing;geochemistry; histograms; igneous rocks; intrusions; magmatism;Nevada; north-central Nevada; northeastern Nevada; statisticalanalysis; United States; USGS Classification: "05A Igneous and metamorphic petrology; 02C Geochemistry of rocks,soils, and sediments" Source: "U. S. Geological Survey Data Series, Report: DS-0244, 62 pp., 2007" map_coordinates: "Latitude:N383000,N413000 Longitude:W1140000,W1183000"
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Reference Number: 2870 GeoREF Number: 2007-900087 Publication Year: 2007 Abstract: _NULL_ Classification: _NULL_ Source: "Journal of Petrology, V. 48, No. 5, P. 901-950"
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Reference Number: 8231 GeoREF Number: 2007-075911 Publication Year: 2007 Abstract: "The Tehachapi complex lies at the southern end of the Sierra Nevadabatholith adjacent to the Neogene-Quaternary Garlock fault. Thecomplex is composed principally of high-pressure (8-10 kbar)Cretaceous batholithic rocks, and it represents the deepest exposedlevels of a continuous oblique crustal section through the southernSierra Nevada batholith. Over the southern approximately 100 km ofthis section, structural/petrologic continuity and geochronologicaldata indicate that > or =35 km of felsic tointermediate-composition crust was generated by copious arc magmatismprimarily between 105 and 99 Ma. In the Tehachapi complex, thesebatholithic rocks intrude and are bounded to the west bysimilar-composition gneissic-textured high-pressure batholithic rocksemplaced at ca. 115-110 Ma. This lower crustal complex is boundedbelow by a regional thrust system, which in Late Cretaceous timetectonically eroded the underlying mantle lithosphere, and in seriesdisplaced and underplated the Rand Schist subduction assemblage bylow-angle slip from the outboard Franciscan trench. Geophysical andmantle xenolith studies indicate that the remnants of this shallowsubduction thrust descend northward through the crust and into themantle, leaving the mantle lithosphere intact beneath the greaterSierra Nevada batholith. This north-dipping regional structure recordsan inflection in the Farallon plate, which was segmented into ashallow subduction trajectory to the south and a normal steepertrajectory to the north. We combine new and published data from abroad spectrum of thermochronometers that together form a coherentdata array constraining the thermal evolution of the complex.Integration of these data with published thermobarometric andpetrogenetic data also constrains the tectonically drivendecompression and exhumation history of the complex. The timing of arcmagmatic construction of the complex, as denoted above, is resolved bya large body of U/Pb zircon ages. High-confidence thermochronometricdata track a single retrogressing path commencing from widelyestablished solidus conditions at ca. 100 Ma, and traversing throughtime-temperature space as follows: (1) Sm/Nd garnet approximately770-680 degrees C at ca. 102-95 Ma, (2) U/Pb titanite approximately750-600 degrees C at ca. 102-95 Ma, (3) Ar/Ar hornblende approximately570-490 degrees C at ca. 94-91 Ma, (4) Rb/Sr biotite approximately390-260 degrees C at ca. 90-86 Ma, (5) Ar/Ar biotite approximately320-240 degrees C at ca. 88-85 Ma, and (6) (U-Th)/He zirconapproximately 230-170 degrees C at ca. 88-83 Ma. Additionalstratigraphic constraints place the complex at surface conditions inPaleocene-early Eocene time (ca. 66-55 Ma). Integration of theseresults with thermobarometric and structural data, including publisheddata on the underlying Rand Schist, reveals a profound tectonic eventwhereby rapid cooling and exhumation at rates potentially as high as100s degrees C/m.y. and >5 mm/yr initiated at ca. 98 Ma and peakedbetween 96 and 94 Ma. Between 93 and 85 Ma, cooling rates remainedhigh, but decelerated with or without significant exhumation.Subsequent cooling and exhumation rates are poorly constrained butwere much slower and ultimately resulted in Paleocene-Eocene surfaceexposure. Initial rapid exhumation and cooling are hypothesized tohave been driven by abrupt flattening in the corresponding segment ofthe Farallon plate and the resulting tectonic erosion of theunderlying mantle lithosphere. Protolith as well as metamorphicpressure-temperature and age constraints on the Rand Schist indicateits rapid low-angle subduction between 93 and 88 Ma. Comparison of theRand Schist and Tehachapi complex pressure-temperature-time paths inconjunction with structural relations strongly suggest that the schistascended the equivalent of approximately 4 kbar relative to theTehachapi complex by low-angle normal displacement along the Randfault between 88 and 80 Ma to attain its current underplatedstructural position. Such exten Descriptors: (U-Th)/He; absolute age; batholiths; California; cooling; Cretaceous;crust; dates; exhumation; geologic barometry; geologic thermometry;intrusions; Kern County California; magmatism; Mesozoic; metamorphicrocks; metamorphism; nesosilicates; orthosilicates; Paleozoic;protoliths; Rand Schist; rates; retrograde metamorphism; schists;Sierra Nevada; Sierra Nevada Batholith; silicates; tectonics;Tehachapi Mountains; thermal history; thermochronology; U/Pb; UnitedStates; zircon; zircon group Classification: 16 Structural geology; 03 Geochronology Source: "Special Paper - Geological Society of America, vol.419, pp.39-66,2007" map_coordinates: "Latitude:N344500,N351500 Longitude:W1181500,W1185000"
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Reference Number: 2063 GeoREF Number: 2007-900167 Publication Year: 2007 Source: Idaho Geological Survey DAD-4
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Reference Number: 173 GeoREF Number: 2007-900101 Publication Year: 2007 Abstract: _NULL_ Classification: _NULL_ Source: "U.S. Geological Survey, Reston, Virginia"
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Reference Number: 2044 GeoREF Number: 2007-014825 Publication Year: 2007 Abstract: "The Tepic-Zacoalco rift, a NW-trending corridor approximately 50Xapproximately 250 km, is one arm of a triple-rift system in westernMexico. Together with the Colima rift and the Middle America Trench,it bounds the Jalisco block, a portion of western Mexico that may bemoving independently of North America. The predominant basement rocktypes in the Tepic-Zacoalco rift are rhyolitic ash-flow tuffs andlavas, which were previously assumed to be Oligocene-Miocene in age,related to the Sierra Madre Occidental volcanic province, or older.New (super 40) Ar/ (super 39) Ar dates on 41 volcanic samples reveal apreviously unrecognized, voluminous flare-up of rhyolitic ignimbritesbetween 5 and 3 Ma throughout the entire corridor of theTepic-Zacoalco rift; they are often associated with Pliocene high-Tibasalts. The eruption rate during this Pliocene time period was anorder of magnitude higher (hundreds of m/m.y.) than that documented inthe Tepic-Zacoalco rift over the last 1 m.y. The P!" Descriptors: absolute age; Ar/Ar; ash-flow tuff; basalts; Cenozoic; dates; digitalterrain models; East Pacific; eruptions; experimental studies;extension; geochemistry; igneous rocks; ignimbrite; Jalisco Block;lithosphere; major elements; Mexico; microplates; Miocene; Neogene;North America; North Pacific; Northeast Pacific; outcrops; PacificOcean; plate tectonics; Pliocene; pyroclastics; rates; rhyolite tuff;rhyolites; rift zones; rifting; Rivera Plate; Sierra Madre Occidental;Tepic-Zacoalco Rift; Tertiary; trace elements; triple junctions;volcanic rocks; volcanism Classification: 18 Solid-earth geophysics; 05A Igneous and metamorphic petrology Source: "Geological Society of America Bulletin, vol.119, no.1-2, pp.49-64, Jan2007" map_coordinates: "Latitude:N180000,N220000 Longitude:W1030000,W1080000"
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The ∼ 5000 km3Fish Canyon Tuff (FCT) is an important unit for the geochronological community because its sanidine, zircon and apatite are widely used as standards for the40Ar/39Ar and fission track dating techniques. The recognition, more than 10 years ago [Oberli, F., Fischer, H. and Meier, M., 1990. High-resolution238U-206Pb zircon dating of Tertiary bentonites and Fish Canyon Tuff; a test for age "concordance" by single-crystal analysis. Seventh International Conference on Geochronology, Cosmochronology and Isotope Geology. Geological Society of Australia Special Publication Canberra, 27:74], of a ≥ 0.4 Ma age difference between the U-Pb zircon ages and40Ar/39Ar sanidine ages has, therefore, motivated efforts to resolve the origin of this discrepancy. To address this controversial issue, we initially performed 37 U-Pb analyses on mainly air-abraded zircons at ETH Zurich and nearly 20040Ar/39Ar measurements on hornblende, biotite, plagioclase and sanidine obtained at the University of Geneva, using samples keyed to a refined eruptive stratigraphy of the FCT magmatic system. Disequilibrium-corrected206Pb/238U ages obtained for 29 single-crystal and three multi-grain analyses span an interval of ∼ 28.67-28.03 Ma and yield a weighted mean age of 28.37 ± 0.05 Ma (95% confidence level), with MSWD = 8.4. The individual dates resolve a range of ages in excess of analytical precision, covering ∼ 600 ka. In order to independently confirm the observed spread in zircon ages, 12 additional analyses were carried out at the Berkeley Geochronology Center (BGC) on individual zircons from a single lithological unit, part of them pre-treated by the "chemical abrasion" (CA) technique [Mattinson, J.M., 2005. Zircon U-Pb chemical abrasion ("CA-TIMS") method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220(1-2): 47-66]. Whereas the bulk of the BGC results displays a spread overlapping that obtained at ETH, the group of CA treated zircons yield a considerably narrower range with a mean age of 28.61 ± 0.08 Ma (MSWD = 1.0). Both mean zircon ages determined at ETH and BGC are older than the ∼ 28.0 Ma40Ar/39Ar eruption age of FCT - even when considering the possibility that the latter may be low by as much as ∼ 1% due to a miscalibration of the40K decay constants - and is thus indicative of a substantial time gap between magma crystallization and extrusion. The CA technique further reveals that younger FCT zircon ages are likely to be associated with chemically unstable U-enriched domains, which may be linked to crystallization during extended magma residence or may have been affected by pre-eruptive and/or post-eruptive secondary loss of radiogenic lead. Due to their complex crystallization history and/or age bias due to Pb loss, the FCT zircon ages are deemed unsuitable for an accurate age calibration of FCT sandine as a fluence monitor for the40Ar/39Ar method. Even though data statistics preclude unambiguous conclusions,40Ar/39Ar dating of sanidine, plagioclase, biotite, and hornblende from the same sample of vitrophyric Fish Canyon Tuff supports the idea of a protracted crystallization history. Sanidine, thought to be the mineral with the lowest closure temperature, yielded the youngest age (28.04 ± 0.18 Ma at 95% c.l., using Taylor Creek Rhyolite [Renne, P.R. et al., 1998. Intercalibration of standards, absolute ages and uncertainties in40Ar/39Ar dating. Chemical Geology, 145: 117-152.] as the fluence monitor), whereas more retentive biotite, hornblende and plagioclase gave slightly older nominal ages (by 0.2-0.3 Ma). In addition, a laser step-heating experiment on a 2-cm diameter feldspar megacryst produced a "staircase" argon release spectrum (older ages at higher laser power), suggestive of traces of inherited argon in the system. Thermal and water budgets for the Fish Canyon magma indicate that the body remained above its solidus (∼ 700 °C) for an extended period of time (> 105 years). At these temperatures, argon volume diffusion is thought to be fast enough to prevent accumulation of radiogenic Ar. If this statement were true, an existing isotopic record should have been completely reset within a few hundred years, regardless of the phase and initial age of the phenocryst. As these minerals are unlikely to be xenocrysts that were incorporated within such a short time span prior to eruption, we suggest that a fraction of radiogenic Ar can be retained > 105 years, even at T ∼ 700 °C. © 2006 Elsevier B.V. All rights reserved.
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Reference Number: 8064 GeoREF Number: 2007-114028 Publication Year: 2007 Classification: 16 Structural geology; 12 Stratigraphy Source: "Special Paper - Geological Society of America, vol. 422, pp.41-70, 2007"
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