Contributors:Boroughs, Scott; Wolff, John; Bonnichsen, Bill; Godchaux, Martha;Larson, Peter
Reference Number: 1989
GeoREF Number: 2005-074680
Publication Year: 2005
Abstract: "The Miocene Bruneau-Jarbidge and adjacent volcanic fields of thecentral Snake River Plain, southwest Idaho, are dominated byhigh-temperature rhyolitic tuffs and lavas having an aggregate volumeestimated as 7000 km super 3 . Samples from units representing atleast 50% of this volume are strongly depleted in super 18 O, withmagmatic feldspar delta super 18 O sub VSMOW Vienna standard meanocean water values between -1.4ppm and 3.8 ppm. The magnitude of thesuper 18 O depletion and the complete lack of any rhyolites withnormal values 7 ppm-10 ppm combine to suggest that assimilation ormelting of a caldera block altered by near- contemporaneoushydrothermal activity is unlikely. Instead, we envisage generation ofthe high-temperature rhyolites by shallow melting of Idaho Batholithrocks, under the influence of the Yellowstone hotspot, affected byEocene meteoric-hydrothermal events. The seeming worldwide scarcity ofstrongly super 18 O-depleted rhyolites may simply reflect a similarscarcity of suitable crustal protoliths."
Descriptors: Bruneau-Jarbridge volcanic field; calderas; Cenozoic; crust;eruptions; feldspar group; framework silicates; geochemistry;hydrothermal alteration; Idaho; Idaho Batholith; igneous rocks;isotope ratios; isotopes; Jacks Creek volcanic field; lava; magmatism;major elements; melting; metasomatism; meteoric water; Miocene;Neogene; O-18/O-16; oxygen; pyroclastics; quartz; rhyolite tuff;rhyolites; silica minerals; silicates; Snake River plain; southwesternIdaho; stable isotopes; Tertiary; trace elements; Twin Falls volcanicfield; United States; volcanic features; volcanic fields; volcanicrocks; volcanism; volume; welded tuff; Yellowstone Hot Spot
Classification: 02D Isotope geochemistry; 05A Igneous and metamorphic petrology
Source: "Geology Boulder, vol.33, no.10, pp.821-824, Oct 2005"
Contributors:Pickett, Katharine Elizabeth
Reference Number: 1970
GeoREF Number: 2005-006856
Publication Year: 2004
Abstract: Quaternary basaltic rocks of the bimodal Blackfoot Volcanic FieldBVF in southeastern Idaho are normatively classified as olivine andquartz tholeiites. Eruptive vents and lava flows are located in theBasin and Range tectonic province, adjacent to the southern border ofthe eastern Snake River Plain ESRP volcanic province. This studycompares physical volcanology, petrography, and geochemistry of BVFbasalts with contemporaneous basalts in the Basin and Range and on theESRP in order to determine possible influences of ESRP magmatism onBasin and Range magmatism. Basalts in the study area represent bothstrombolian and hawaiian styles of volcanism, which are typicallyfound in the Basin and Range, and not on the ESRP. Geochemicalcomparisons between the above provinces and the study area indicatethat BVF basalts share many characteristics with ESRP basalts and notwith Basin and Range basalts. Geochemical and isotopic evidencesuggests a lithospheric source for BVF basalts, similar to the sourcebelieved to have produced the ESRP basalts. Typical Basin and Rangebasalts are not likely represented in the study area. The proposedsimilarity in ESRP and BVF source regions has significant implicationfor the extent of ESRP magmatic influence in southeastern Idaho; thisis consistent with recently proposed geophysical models that depictESRP-like source material beneath the BVF.
Descriptors: basalts; Basin and Range Province; Blackfoot volcanic field;geochemistry; Idaho; igneous rocks; isotopes; lithosphere; magmatism;North America; petrography; Snake River plain; southeastern Idaho;tholeiitic basalt; United States; volcanic fields; volcanic rocks;volcanism
Classification: 05A Igneous and metamorphic petrology; 02D Isotope geochemistry
Contributors:Shervais, John W; Shroff, Gaurav; Vetter, Scott K; Matthews, Scott;Hanan, Barry B; McGee, James J
Contributors:Coleman, Drew S; Gray, Walt; Glazner, Allen F
Reference Number: 901
GeoREF Number: 2004-047843
Publication Year: 2004
Abstract: "New U-Pb geochronologic data indicate that the Tuolumne Intrusive Suite, California, was assembled over a period of at least 10 m.y. between 95 and 85 Ma, and that the Half Dome Granodiorite intruded over a period approaching 4 m.y. Simple thermal considerations preclude the possibility that a magma chamber the size of the Half Dome pluton could have existed as a liquid at shallow crustal depths for that long. Rather, field evidence for sheeting along the margins of the suite, the range of ages, and the regular decrease of ages toward the center of the suite and within individual plutons suggest incremental assembly. Geochronologic evidence for incremental assembly is consistent with the failure of geophysical methods to detect large magma chambers with more than approximately 20% melt, even in active volcanic areas. Because it is unlikely that the individual plutons composing the Tuolumne ever coexisted as liquid-rich magmas, the chemical evolution of the suite cannot be the result of simple fractionation and/or mixing between exposed units, but instead must reflect processes occurring during magma generation."
Descriptors: absolute age; California; Cretaceous; dates; emplacement; igneous rocks; intrusions; magma chambers; magmas; Mesozoic; nesosilicates; orthosilicates; plutonic rocks; plutons; Sierra Nevada; silicates; Tuolumne County California; Tuolumne Suite; U/Pb; United States; Upper Cretaceous; zircon; zircon group; zoning
Classification: 05A Igneous and metamorphic petrology; 03 Geochronology
Source: Geology (Boulder) 32.5 (May 2004): 433-436
map_coordinates: "Latitude:N373500,N381500 Longitude:W1190499,W1194500"
Contributors:Sherrod, David R; Taylor, Edward M; Ferns, Mark L; Scott, William E;Conrey, Richard M; Smith, Gary A
Reference Number: 661
GeoREF Number: 2004-076790
Publication Year: 2004
Abstract: "This map presents the stratigraphic and structural setting of volcanicand sedimentary strata deposited during the past 35 million yearsacross 4,430 km (super 2) in central Oregon. Snowfall in the CascadeRange (west part of map area) recharges important aquifers in theDeschutes basin (central part of map). The area includes the majesticpeaks of the Three Sisters volcanoes, where continued eruptions ofbasalt and rhyolite in the past 3,000 years indicate an ongoingvolcanic hazard. The Sisters fault zone, with several potentiallyactive faults, crosses the map from southeast to northwest."
Descriptors: areal geology; Bend Quadrangle; central Oregon; geochemistry; geologicmaps; intrusions; lithostratigraphy; maps; Oregon; petrology;stratigraphic units; surficial geology; tectonic elements; UnitedStates; USGS; volcanism
Classification: "13, Areal geology"
Source: "Geologic Investigations Series - U. S. Geological Survey, Report:I-2683, 48 pp., 2 sheets, 2004"
map_coordinates: "Latitude:N440000,N443000 Longitude:W1210000,W1220000"
Reference Number: 1971
GeoREF Number: 2005-022484
Publication Year: 2004
Abstract: "This map is located just east of the Columbia River about 30 km north of Portland, Oregon. Bedrock, exposed only in the north part of the quadrangle, consists of eroded and tilted Paleogene volcanic rocks unconformably overlain by lava flows of the Miocene Columbia River Basalt Group and by sedimentary deposits that accumulated in the late Neogene Portland Basin. In most areas, the older deposits are covered by either slack-water silt deposited by latest Pleistocene Missoula floods or by Holocene alluvium."
Descriptors: areal-geology; bedrock-; Clark-County-Washington; Columbia-River-basin; Cowlitz-County-Washington; geologic-maps; landforms-; lithostratigraphy-; maps-; petrology-; Ridgefield-Quadrangle; stratigraphic-units; surficial-geology; tectonic-elements; United-States; USGS-; volcanism-; Washington-
Source: Scientific Investigations Map. 2004.
map_coordinates: LAT: N454500; N455230; LONG: W1223730; W1224500.
Reference Number: 2691
GeoREF Number: 2004-014651
Publication Year: 2004
Abstract: "A detailed geochemical characterization of 19 representative Proterozoic basement rocks in the Quitovac region in northwestern Sonora, Mexico, has identified two distinct Paleoproterozoic basement blocks that coincide spatially with the previously proposed Caborca and ""North America"" blocks. New U-Pb zircon geochronology revises their age ranges, the Caborca (1.78-1.69 Ga) and ""North America"" (1.71-1.66 Ga) blocks at Quitovac, and precludes a simple age differentiation between them. In addition, Grenvillian-age granitoids (ca. 1.1 Ga), spatially associated with the Caborca block have been identified at Quitovac. Nd isotopes and major- and trace- element geochemistry support the distinction of these Paleoproterozoic blocks. Granitoids of the ""North America"" block are characterized by depleted epsilon Nd values (3.4-3.9) and younger Nd model ages (1800-1740 Ma) and have lower K (sub 2) O, Y, Rb, Ba, Th, REE, and Fe/Mg values than coeval rocks of the Caborca block. The Caborca block granitoids are likewise characterized by slightly less depleted epsilon Nd (0.6-2.6) and older Nd model ages (2070-1880 Ma). Despite the subtle differences, granitoids from both the Caborca and ""North America"" blocks exhibit island arc-like affinities. We propose that the Proterozoic basement rocks from the Quitovac region are an extension of the Proterozoic crustal provinces in the southwestern United States. Specifically, rocks of the Caborca block exhibit an affinity to rocks of either the Yavapai province or the Mojave-Yavapai transition zone, whereas rocks of the ""North America"" block have signatures similar to those of the Mazatzal province or possibly the Yavapai province of Arizona. The new isotopic ages and geochemical data do not support the existence of the Late Jurassic Mojave-Sonora megashear at Quitovac, as originally proposed. However, the Quitovac region accounts only for a small fraction of the Proterozoic basement in Sonora, so these findings do not eliminate the possibility of a megashear elsewhere in northern Sonora. Our new data create the possibility of alternative hypotheses for the distribution of Paleoproterozoic crustal provinces in southwestern North America that affect reconstructions of the original southwestern margin of Laurentia, and reduce uncertainties in the configuration, timing, and existence of the Proterozoic supercontinent, Rodinia."
Descriptors: absolute-age; basement-; block-structures; crust-; dates-; faults-; geochemistry-; granitic-composition; igneous-rocks; isotope-ratios; isotopes-; Laurentia-; lead-; major-elements; metals-; metamorphic-rocks; metasedimentary-rocks; Mexico-; Nd-144-Nd-143; neodymium-; nesosilicates-; orthosilicates-; paleogeography-; Pb-206-Pb-204; Pb-208-Pb-204; Precambrian-; Proterozoic-; Quitovac-Mexico; radioactive-isotopes; rare-earths; shear-zones; silicates-; Sm-Nd; Sonora-Mexico; stable-isotopes; systems-; tectonics-; trace-elements; U-Pb; upper-Precambrian; zircon-
Source: "Geological Society of America Bulletin. 116; 1-2, Pages 154-170. 2004."
map_coordinates: LAT: N312000; N313000; LONG: W1124000; W1125500.
Contributors:Henry, Christopher D; Faulds, James E; dePolo, Craig M; Davis, DavidA
Reference Number: 452
GeoREF Number: 2005-036660
Publication Year: 2004
Abstract: "A 1:24,000-scale, full-color geologic map of the Dogskin Mountain7.5-minute Quadrangle in Washoe County, Nevada, with three crosssections and descriptions of 76 rock units. Accompanying textdescribes the geology of the quadrangle, which includes two majorstrike-slip faults of the Walker Lane."
Descriptors: areal geology; Dogskin Mountain Quadrangle; geologic maps; maps;Nevada; United States; Warm Springs Valley; Washoe County Nevada
Classification: "14, Geologic maps"
Source: "Map - Nevada Bureau of Mines and Geology, Report: 148, 20 pp., 1sheet, 2004"
map_coordinates: "Latitude:N395230,N400000 Longitude:W1194500,W1195230"