Contributors:Glazner, Allen F; Coleman, Drew S; Bartley, John M
Reference Number: 9041
GeoREF Number: 2008-062604
Publication Year: 2008
Abstract: "The trace element compositions of aplite dikes in the Sierra Nevada batholith of California differ profoundly from high-silica rhyolites (HSRs) and contradict a genetic connection to them. The aplites are strongly depleted in all middle rare earth elements (REEs), whereas HSRs are strongly depleted only in Eu and enriched in other REEs; the aplites are strongly depleted in Y and variably enriched in Sr, whereas HSRs are enriched in Y and strongly depleted in Sr. Volcanic rocks with the trace element characteristics of these aplites are rare to absent in the geologic record. Aplite REE patterns are likely controlled by titanite, which has large distribution coefficients for REEs, whereas HSRs cannot have equilibrated with titanite. Titanite may crystallize late in dacitic magma and thus HSR may be extracted before titanite saturation is reached; aplites would form after titanite appears, but when the melt percentage is too low and the water content of the melt too high (at fluid saturation) for the magma to ascend without rapid crystallization, thus preventing eruption. HSRs could also form by low-degree partial melting of granodiorite plutons in which titanite melts out early. Alternatively, HSRs may be extracted from silica-rich plutons that lack titanite; leucogranite plutons with REE contents that could be complementary to HSRs are present but uncommon in the Sierra Nevada."
Descriptors: "aplite, California, dikes, feldspar group, framework silicates, geochemistry, granites, granodiorites, igneous rocks, intrusions, magmas, major elements, metals, nesosilicates, orthosilicates, plutonic rocks, plutons, rare earths, rhyolites, Sierra Nevada, Sierra Nevada Batholith, silica, silicates, titanite, titanite group, trace elements, United States, volcanic rocks"
Classification: "02C: Geochemistry of rocks, soils, and sediments, 05A: Igneous and metamorphic petrology"
Source: Geology (Boulder)36.2 (February 2008): 183-186
Contributors:Feeley, T C; Clynne, M A; Winer, G S; Grice, W C
Reference Number: 7582
GeoREF Number: 2008-130075
Publication Year: 2008
Abstract: "This study reports oxygen isotope ratios determined by laserfluorination of mineral separates (mainly plagioclase) from basalticandesitic to rhyolitic composition volcanic rocks erupted from theLassen Volcanic Center (LVC), northern California. Plagioclaseseparates from nearly all rocks have delta (super 18) O values(6.1-8.4ppm) higher than expected for production of the magmas bypartial melting of little evolved basaltic lavas erupted in the arcfront and back-arc regions of the southernmost Cascades during thelate Cenozoic. Most LVC magmas must therefore contain high (super 18)O crustal material. In this regard, the delta (super 18) O values ofthe volcanic rocks show strong spatial patterns, particularly foryoung rhyodacitic rocks that best represent unmodified partial meltsof the continental crust. Rhyodacitic magmas erupted from ventslocated within 3.5 km of the inferred center of the LVC haveconsistently lower delta (super 18) O values (average 6.3ppm+ or-0.1ppm) at given SiO (sub 2) contents relative to rocks erupted fromdistal vents (>7.0 km; average 7.1ppm+ or -0.1ppm). Further, magmaserupted from vents situated at transitional distances haveintermediate values and span a larger range (average 6.8ppm+ or-0.2ppm). Basaltic andesitic to andesitic composition rocks showsimilar spatial variations, although as a group the delta (super 18) Ovalues of these rocks are more variable and extend to higher valuesthan the rhyodacitic rocks. These features are interpreted to reflectassimilation of heterogeneous lower continental crust by mafic magmas,followed by mixing or mingling with silicic magmas formed by partialmelting of initially high (super 18) O continental crust (approximately 9.0ppm) increasingly hybridized by lower delta (super18) O ( approximately 6.0ppm) mantle-derived basaltic magmas towardthe center of the system. Mixing calculations using estimatedendmember source delta (super 18) O values imply that LVC magmascontain on a molar oxygen basis approximately 42 to 4% isotopicallyheavy continental crust, with proportions declining in a broadlyregular fashion toward the center of the LVC. Conversely, the delta(super 18) O values of the rhyodacitic rocks suggest that thecontinental crust in the melt generation zones beneath the LVC hasbeen substantially modified by intrusion of mantle-derived basalticmagmas, with the degree of hybridization ranging on a molar oxygenbasis from approximately 60% at distances up to 12 km from the centerof the system to 97% directly beneath the focus region. These resultsdemonstrate on a relatively small scale the strong influence thatintrusion of mantle-derived mafic magmas can have on modifying thecomposition of pre-existing continental crust in regions of meltproduction. Given this result, similar, but larger-scale, regionaltrends in magma compositions may reflect an analogous but moreextensive process wherein the continental crust becomes progressivelyhybridized beneath frontal arc localities as a result of protractedintrusion of subduction-related basaltic magmas."
Descriptors: andesitic composition; basaltic composition; California; continentalcrust; crust; geochemistry; igneous rocks; isotope ratios; isotopes;Lassen Volcanic Center; Lassen Volcanic National Park;lithogeochemistry; magmatism; mantle; O-18/O-16; oxygen; rhyoliticcomposition; stable isotopes; United States; volcanic rocks
Classification: 02D Isotope geochemistry; 05A Igneous and metamorphic petrology
Source: "Journal of Petrology, vol. 49, no. 5, pp.971-997, May 2008"
map_coordinates: "Latitude:N402500,N403500 Longitude:W1211700,W1213700"
Contributors:Saleeby, Jason B; Ducea, M N; Busby, Cathy J; Nadin, Elisabeth S;Wetmore, P H
Reference Number: 7924
GeoREF Number: 2008-110001
Publication Year: 2008
Abstract: "Cretaceous plutonic rocks of the southern Sierra Nevada batholithbetween latitudes 35.5 degrees N and 36 degrees N lie in a strategicposition that physically links shallow, subvolcanic levels of thebatholith to lower-crustal (-35 km deep) batholitnic rocks. Thisregion preserves an oblique crustal section through the southernSierra Nevada batholith. Prior studies have produced large U/Pb zircondata sets for an aerially extensive region of the batholith to thenorth of this area and for the lower-crustal rocks of the Tehachapicomplex to the south. We present a large set of new U/Pb zircon agedata that ties together the temporal relations of pluton emplacementand intra-arc ductile deformation for the region. We define fiveinformal intrusive suites in the area based on petrography, structuralsetting, U/Pb zircon ages, and patterns in initial (super 87) Sr/(super 86) Sr (Sr (sub 1) ). Two regionally extensive intrusivesuites, the 105-98 Ma Bear Valley suite and 95-84 Ma Domelands suite,underlie the entire southwestern and eastern regions of the studyarea, respectively, and extend beyond the limits of the study area. Athird regionally extensive suite (101-95 Ma Needles suite) cuts outthe northern end of the Bear Valley suite and extends for an unknowndistance to the north of the study area. The Bear Valley and Needlessuites are tectonically separated from the Domelands suite by theproto-Kern Canyon fault, which is a regional Late Cretaceous ductileshear zone that runs along the axis of the southern Sierra Nevadabatholith. The 105-102 Ma Kern River suite also lies west of theproto-Kern Canyon fault and constitutes the subvolcanic plutoniccomplex for the 105-102 Ma Erskine Canyon sequence, an approximately2-km-thick silicic ignimbrite-hypabyssal complex. The 100-94 Ma SouthFork suite lies east of the proto-Kern Canyon fault. It recordstemporal and structural relations of high-magnitude ductile strain andmigmatization in its host metamorphic pendant rocks commensurate withmagmatic emplacement. Integration of t!
Descriptors: absolute age; alkaline earth metals; Bear Valley; California;Cretaceous; dates; deformation; emplacement; Erskine Canyon Sequence;faults; geochemistry; intrusions; isotope ratios; isotopes; KernCanyon Fault; magmatism; Mesozoic; metals; nesosilicates;orthosilicates; plutons; Sierra Nevada; Sierra Nevada Batholith;silicates; Sr-87/Sr-86; stable isotopes; strontium; tectonics; U/Pb;United States; zircon; zircon group
Classification: 03 Geochronology; 16 Structural geology
Source: "Special Paper - Geological Society of America, vol. 438, pp.397-427,2008"
map_coordinates: "Latitude:N353000,N354500 Longitude:W1180000,W1183000"
Contributors:MacDonald, James H, Jr; Harper, Gregory D; Miller, Robert B; Miller,Jonathan S; Mlinarevic, Ante N; Schultz, Cynthia E
Reference Number: 7654
GeoREF Number: 2008-109991
Publication Year: 2008
Abstract: "The polygenetic Ingalls ophiolite complex in the central Cascades,Washington, is one of several Middle to Late Jurassic ophiolites ofthe North American Cordillera. It consists primarily of mantletectonites. High-temperature mylonitic peridotite, overprinted byserpentinite melange (Navaho Divide fault zone), separates harzburgiteand dunite in the south from lherzolite in the north. Crustal units ofthe ophiolite occur as steeply dipping, kilometer-scale fault blockswithin the Navaho Divide fault zone. These units are the IronMountain, Esmeralda Peaks, and Ingalls sedimentary rocks. Volcanicrocks of the Iron Mountain unit have transitionalwithin-plate-enriched mid-ocean-ridge basalt affinities, and arhyolite yields a U-Pb zircon age of ca. 192 Ma. Minor sedimentaryrocks include local oolitic limestones and cherts that contain LowerJurassic (Pliensbachian) Radiolaria. This unit probably formed as aseamount within close proximity to a spreading ridge. The EsmeraldaPeaks unit forms the crustal section of the ophiolite, and it consistsof gabbro, diabase, basalt, lesser felsic volcanics, and minorsedimentary rocks. U-Pb zircon indicates that the age of this unit isca. 161 Ma. The Esmeralda Peaks unit has transitionalisland-arc-mid-ocean ridge basalt and minor boninitic affinities. Apreferred interpretation for this unit is that it formed initially byforearc rifting that evolved into back-are spreading, and it wassubsequently deformed by a fracture zone. The Iron Mountain unit isthe rifted basement of the Esmeralda Peaks unit, indicating that theIngalls ophiolite complex is polygenetic. Ingalls sedimentary rocksconsist primarily of argillite with minor graywacke, conglomerate,chert, and ophiolite-derived breccias and olistoliths. Radiolaria fromchert give lower, Oxfordian ages. The Ingalls ophiolite complex issimilar in age and geochemistry to the Josephine ophiolite and itsrelated rift-edge facies and to the Coast Range ophiolite ofCalifornia and Oregon. The Ingalls and Josephine ophiolites arepolygenetic, while the Coast Range ophiolite is not, and sedimentaryrocks (Galice Formation) that sit on the Josephine and its rift-edgefacies have the same Radiolaria fauna as Ingalls sedimentary rocks.Therefore, we correlate the Ingalls ophiolite complex with theJosephine ophiolite of the Klamath Mountains. Taking known Cretaceousand younger strike-slip faulting into account, this correlationimplies that the Josephine ophiolite either continued northwardapproximately 440 km-thus increasing the known length of the Josephinebasin-or that the Ingalls ophiolite was translated northwardapproximately 440 km along the continental margin."
Descriptors: absolute age; basalts; Cascade Range; Cenozoic; Chiwaukum Schist;Chumstick Formation; correlation; crust; dates; De Roux Unit; Eocene;Esmeralda Peaks Unit; fault zones; faults; geochemistry; harzburgite;igneous rocks; Ingalls Complex; Invertebrata; Iron Mountain Unit;Jurassic; lithostratigraphy; Lower Jurassic; major elements; melange;Mesozoic; metaigneous rocks; metamorphic rocks; metasomatic rocks;microfossils; mid-ocean ridge basalts; Mount Stuart Batholith; NavahoDivide fault zone; oceanic crust; ophiolite; ophiolite complexes;Paleogene; peridotites; plate tectonics; plutonic rocks; Protista;Radiolaria; sea-floor spreading; serpentinite; spreading centers;Swauk Formation; tectonics; Tertiary; trace elements; U/Pb;ultramafics; United States; volcanic rocks; Washington
Classification: "02C Geochemistry of rocks, soils, and sediments; 16 Structuralgeology"
Source: "Special Paper - Geological Society of America, vol. 438, pp.133-159,2008"
map_coordinates: "Latitude:N472500,N474000 Longitude:W1193500,W1200500"
Reference Number: 2894
GeoREF Number: 2008-900303
Publication Year: 2008
Abstract: "This open-file report presents WDXRF major-element chemical data for late Pliocene to Holocene volcanic rocks collected from Lassen Volcanic National Park and vicinity, California. Data for Rb, Sr, Ba, Y, Zr, Nb, Ni, Cr, Zn and Cu obtained by EDXRF are included for many samples. Data are presented in an EXCEL spreadsheet and are keyed to rock units as displayed on the Geologic Map of Lassen Volcanic National Park and vicinity (Clynne and Muffler, in press). Location of the samples is given in latitude and longitude in degrees and decimal minutes and in decimal degrees."
Source: Open-File Report - U. S. Geological Survey. 2008. Report # OF. 2008-1091
Contributors:Barth, Andrew P; Wooden, Joseph L; Howard, Keith A; Richards, JoshuaL
Reference Number: 7655
GeoREF Number: 2008-110000
Publication Year: 2008
Abstract: "Although plate reconstructions suggest that subduction was anapproximately steady-state process from the mid-Mesozoic through theearly Tertiary, recent precise geochronologic studies suggest highlyepisodic emplacement of voluminous continental-margin batholiths inthe U.S. Cordillera. In central and southern California and westernArizona, major episodes of batholithic magmatism are known to haveoccurred in Permian-Triassic, Middle Jurassic, and late Early to LateCretaceous time. However, recent studies of forearc-basin andcontinental-interior sediments suggest that Late Jurassic time wasprobably also a period of significant magmatism, although few datedplutons of this age have been recognized. We describe a belt of LateJurassic plutonic and hypabyssal rocks at least 200 km in length thatextends from the northwestern Mojave Desert through the TransverseRanges. The belt lies outboard of both the voluminous Middle Jurassicarc and the ca. 148 Ma Independence dike swarm at these latitudes. Theplutons include two intrusive suites emplaced between 157 and 149 Ma:a calc-alkaline suite compositionally unlike Permian-Triassic andMiddle Jurassic monzonitic suites but similar to Late Cretaceous arcplutons emplaced across this region, and a contemporaneous but notcomagmatic alkaline suite. The Late Jurassic was thus a time of bothtectonic and magmatic transitions in the southern Cordillera."
Descriptors: absolute age; basins; batholiths; California; Cenozoic; chemicalcomposition; dates; fore-arc basins; geochemistry; intrusions; ionprobe data; Jurassic; magmatism; major elements; mass spectra;Mesozoic; Mojave Desert; North America; North American Cordillera;paleoenvironment; Paleogene; Paleozoic; Permian; plate tectonics;reconstruction; SHRIMP data; Southern California; Southwestern U.S.;spectra; subduction; Tertiary; thermal ionization mass spectra; traceelements; Transverse Ranges; U/Pb; United States; Upper Jurassic
Classification: "05A, Igneous and metamorphic petrology"
Source: "Special Paper - Geological Society of America, vol. 438, pp.379-396,2008"
map_coordinates: "Latitude:N333000,N343000 Longitude:W1153000,W1171500"
Contributors:Cousens, Brian; Prytulak, Julie; Henry, Christopher; Alcazar, Al;Brownrigg, Tim