Reference Number: 1843
GeoREF Number: 2004-026593
Publication Year: 2003
Abstract: "Mesozoic ophiolites in the Vizcaino Peninsula and Cedros Island region of Baja California Sur are suprasubduction zone Cordilleran-type ophiolites structurally juxtaposed with underlying high pressure-temperature subduction complex assemblages. The region is divided into three separate tectonostratigraphic terranes, but here we recognize stratigraphic, intrusive, and petrologic links between these terranes and interpret the evolution of the entire region within the same Late Triassic to Early Cretaceous tectonic framework. Several phases of extension are recognized, including two major phases that resulted in development of distinct ophiolite assemblages. The Late Triassic Vizcaino Peninsula Ophiolite (221+ or -2 Ma) represents the earliest stage of this history and comprises a complete spreading center sequence with depleted upper mantle and mafic crustal rocks, including sheeted dike complex. Jurassic arc magmatic rocks with low-Ti arc tholeiite and boninite geochemical affinities were intruded through and constructed on the Triassic ophiolite basement. Ultra-depleted arc-ankaramites on Cedros Island may represent an initial phase of arc rifting that was followed by major Middle Jurassic extension and production of the Cedros Island Ophiolite (173+ or -2 Ma). The Late Jurassic-Early Cretaceous Coloradito and Eugenia Formations contain mudflows and olistostrome blocks intercalated with arc volcanogenic sediment and rift-related pillow lavas; these units record extension and/or transtension and provide the earliest definite evidence of arc-continent interaction in the region. Middle Jurassic to Early Cretaceous arc plutonic rocks (ca. 165-135 Ma) were shallowly intruded into low greenschist-facies ophiolite and arc volcanic basement. Plutonic rocks range in composition from gabbro to granodiorite, but tonalite dominates. These intrusions are typical I-type Cordilleran batholithic rocks with relatively primitive arc geochemical affinities (initial (super 87) Sr/ (super 86) Sr range from approximately 0.704 to 0.706),but they are distinctly calcic in nature, a feature common to the adjacent Cretaceous Peninsular Ranges batholith. The Vizcaino-Cedros region correlates to ophiolitic terranes of the western Sierra-Klamath belt and Coast Ranges of California and Oregon that were constructed in part across the North American margin. Age, stratigraphic, and petrochemical data from the Vizcaino-Cedros region support previously proposed forearc rifting models developed for the U.S. sector of the Cordilleran orogen that interpret the ophiolite assemblages as autochthonous or parautochthonous forearc lithosphere constructed outboard of the Mesozoic continental margin arc."
Descriptors: absolute-age; alkali-metals; alkaline-earth-metals; allochthons-; Ar-40-Ar-39; argon-; Baja-California; Baja-California-Sur-Mexico; basins-; California-; Cedros-Island; chemical-composition; Choyal-Formation; Coast-Ranges; Coloradito-Formation; Cordilleran-Geosyncline; correlation-; deformation-; Eugenia-Formation; fore-arc-basins; gabbros-; geochemistry-; granites-; granodiorites-; I-type-granites; igneous-rocks; intrusions-; island-arcs; isotope-ratios; isotopes-; Klamath-Mountains; lithosphere-; mafic-composition; major-elements; Mesozoic-; metals-; metamorphic-rocks; Mexico-; mineral-composition; models-; nesosilicates-; noble-gases; ophiolite-; ophiolite-complexes; Oregon-; orthosilicates-; paleoenvironment-; paleogeography-; petrology-; plate-tectonics; plutonic-rocks; radioactive-isotopes; Rb-87-Sr-86; rubidium-; San-Andres-Cedros-Complex; sedimentary-rocks; silicates-; stable-isotopes; stratigraphic-units; stratigraphy-; strontium-; subduction-zones; tectonics-; tectonostratigraphic-units; terranes-; trace-elements; U-Pb; United-States; Valle-Group; Vizcaino-Peninsula; volcanic-rocks; volcanism-; zircon-
Source: Special Paper - Geological Society of America. 374; Pages 43-71. 2003.
map_coordinates: LAT: N270000; N280000; LONG: W1140000; W1151500.
Reference Number: 1793
GeoREF Number: 2003-018070
Publication Year: 2003
Abstract: "Subduction-related and continental intraplate type magmatism coexisted in the San Pedro-Ceboruco SPC graben in western Mexico in a rather unusual close association. The magmatic systems belong to the Trans-Mexican Volcanic Belt TMVB, the active volcanic arc linked to subduction of the Rivera and Cocos plates beneath the North American plate. Three different magmatic series are recognized in the San Pedro-Ceboruco graben: calc-alkaline, Na-alkaline, and transitional. The transitional series can be further subdivided into: Low-Ti; High-Ti and Amado Nervo groups. The different magmatic series are characterized by variable LILE/HFSE and LREE/HFSE ratios. The Na-alkaline series has the most radiogenic Nd compositions and the lowest super 87 Sr/ super 86 Sr 0.70320-0.70344. The calc-alkaline series shows the lowest epsilon sub Nd and the highest super 87 Sr/ super 86 Sr 0.70395-0.70402 overlapping the values for the transitional High-Ti group 0.70390-0.70404. Sr isotopic compositions for transitional Low-Ti 0.70367-0.70394 and transitional Amado Nervo 0.70351-0.70389 groups are intermediate between those of the Na-alkaline and calc-alkaline rocks. All the studied rocks show similar super 207 Pb/ super 204 Pb 15.58-15.61 and super 208 Pb/ super 204 Pb 38.41-38.65, but super 206 Pb/ super 204 Pb discriminates well the Na-alkaline series 18.90-19.03 from all the others 18.68-18.75. Compositional and isotopic data suggest that the different series derive from distinct parental magmas, which were generated by partial melting of a heterogeneous mantle source characterized by two different components. A depleted mantle DM component with low super 206 Pb/ super 204 Pb and an enriched EM component characterized by high super 206 Pb/ super 204 Pb. These two components mix together in various degrees and are both overprinted, although in different proportions, by metasomatic processes. Metasomatism of the mantle wedge was induced by subduction-related agents resulting from the dehydration/melting of subducted sediments and characterized by variable fluid/melt proportions."
Descriptors: alkalic-composition; alkaline-earth-metals; calc-alkalic-composition; chemical-ratios; dehydration-; experimental-studies; faults-; geochemistry-; grabens-; heterogeneity-; high-field-strength-elements; ICP-mass-spectra; igneous-rocks; intraplate-processes; isotope-ratios; isotopes-; lead-; lithophile-elements; magmas-; magmatism-; major-elements; mantle-; mantle-wedges; mass-spectra; melting-; metals-; metasomatism-; Mexico-; Nayarit-Mexico; Nd-144-Nd-143; neodymium-; partial-melting; Pb-206-Pb-204; Pb-207-Pb-204; Pb-208-Pb-204; plate-tectonics; radioactive-isotopes; rare-earths; San-Pedro; San-Pedro-Ceboruco-Graben; spectra-; Sr-87-Sr-86; stable-isotopes; strontium-; subduction-; systems-; thermal-ionization-mass-spectra; trace-elements; volcanic-rocks; western-Mexico; X-ray-fluorescence-spectra
Source: "Chemical Geology. 193; 1-2, Pages 1-24. 2003."
map_coordinates: LAT| N210000; N212500; LONG: W1041000; W1050000.
Reference Number: 1814
GeoREF Number: 2003-050832
Publication Year: 2003
Abstract: "In order better to understand the origin and significance of across-strike geochemical variations in the Eocene Absaroka volcanic province AVP, this article compares compositional data for magmatic rocks from two eruptive centers that define type localities for major stratigraphic subdivisions of the Absaroka Volcanic Supergroup. Geochemical variations for evolved lavas indicate that the calc-alkaline differentiation trend at Washburn volcano results from mixing compositionally diverse mafic and silicic magmas, whereas the shoshonitic trend at the more easternly Sunlight volcano reflects assimilation-fractional crystallization. The mantle source for both volcanoes is, as shown by incompatible trace element contents and radiogenic isotopic ratios of mafic lavas, largely within ancient depleted lithospheric mantle later enriched by a metasomatic event characterized by low Sm/Nd and Rb/Sr ratios. The approximate timing of the enrichment event, as calculated from Nd isotopic data 2.2-2.0 Ga, is similar to estimates from other magmatic fields in the Wyoming Province. Large variations in trace element ratios of mafic lavas involving large ion lithophile elements LILE and light rare earth element that are not obviously reflected in isotopic ratios also indicate a recent <100 Ma metasomatic enrichment event. Superficially, the mantle source for Sunlight volcano appears to contain larger amounts of the young metasomatic component, as measured by high LILE/high field strength element HFSE ratios. Parental magmas at both volcanoes have similar HFSE contents and HFSE/heavy rare earth element ratios, however, suggesting comparable degrees of source region melting and metasomatism. Rather than variable degrees of metasomatism and partial melting, the geochemical differences between the mafic rocks are best explained by compositional heterogeneity in metasomatic fluids inherited from breakdown of different phases in hydrous peridotite. A consistent geodynamic model to account for generation of AVP magmas involves metasomatism of asthenosphere and subcontinental mantle lithosphere during shallow subduction of the Farallon plate during the Late Cretaceous through Paleocene, followed in the Eocene by foundering of the slab and primary basalt generation through major heating associated with upwelling and influx of hot asthenosphere."
Descriptors: Absaroka Supergroup; alkaline earth metals; calc-alkalic composition; Cenozoic; Eocene; geochemistry; heterogeneity; high-field-strength elements; igneous rocks; isotope ratios; isotopes; lava; lithophile elements; mafic composition; magmas; magmatic differentiation; major elements; metals; metasomatism; Nd-144/Nd-143; neodymium; Paleogene; Park County Wyoming; rare earths; Sr-87/Sr-86; stable isotopes; strontium; Sunlight; tectonics; Tertiary; trace elements; United States; vents; volcanic rocks; volcanoes; Washburn; water-rock interaction; Wyoming; Yellowstone National Park
Classification: "02C Geochemistry of rocks, soils, and sediments; 05A Igneous and metamorphic petrology"
Source: "Journal of Geology, vol.111, no.3, pp.329-346, May 2003"
Contributors:Coleman, Drew S; Briggs, Stephanie; Glazner, All F; Northrup, C J
Reference Number: 1184
GeoREF Number: 2003-014288
Publication Year: 2003
Abstract: "New mapping and U-Pb zircon geochronology help establish the timing of contractional deformation and magmatism in the White Mountains of California. In the Redding Canyon area of the west-central White Mountains, Mesozoic deformation characterized by east-directed movement along reverse faults as well as recumbent folding was followed by development of upright folds with axes that plunge moderately to the north. This later folding event produced penetrative, vertical, north-striking axial-planar cleavage that is present along much of the western flank of the range. Deformed units include folded and/or boudinaged diorite dikes (ca. 165 Ma; U-Pb zircon), that contain the later penetrative cleavage. The cleavage is clearly cut by the Redding Canyon pluton (ca. 164 Ma; U-Pb zircon), demostrating that at least some of the intense deformation preserved in the area is Middle Jurassic and correlative with the East Sierran thrust system identified elsewhere in California. Dates for the Beer Creek pluton (ca. 179 Ma; U-Pb zircon) and the Sage Hen Flat pluton (ca. 175 Ma; U-Pb zircon), which cut deformation in their wall rocks, suggest that East Sierran thrust deformation did not propagate as far eastward as these plutons at the present level of exposure. The new dates also cast doubt on the presence of any Late Jurassic-Early Cretaceous plutonism in the White Mountains. Throughout the east-central Sierra Nevada and White Mountains, high-precision U-Pb zircon geochronolgy is resolving significant plutonism into two short-lived events that occured at ca. 180-165 Ma and 102-86 Ma."
Descriptors: absolute age; Beer Creek Pluton; boudinage; California; cleavage; dates; deformation; dikes; emplacement; faults; folds; foliation; intrusions; Inyo County California; Inyo Mountains; magmatism; Mesozoic; Mono County California; nesosilicates; orthosilicates; plutons; Poleta Canyon; recumbent folds; Redding Canyon Pluton; reverse faults; Sage Hen Flat Pluton; silicates; structural analysis; U/Pb; United States; White Mountains; zircon; zircon group
Source: "Geological Society of America Bulletin, vol. 115, no. 1, pp. 48-57, Jan 2003"
Reference Number: 1845
GeoREF Number: 2004-026598
Publication Year: 2003
Abstract: The Peninsular Ranges batholith of southern California and Baja California, Mexico, is well recognized as a prime example of an I-type Cretaceous batholith. Often overlooked, however, is a volumetrically significant amount of pre-Cretaceous gneissic granite in the axial zone of the batholith. New U-Pb zircon age data confirm that the metaluminous and peraluminous plutonic bodies were emplaced during the middle Jurassic. Also reported in this paper is a Jurassic U-Pb age for a metaluminous (I-type) tonalite-quartz diorite pluton that is spatially related to the peraluminous suites. This result suggests that other unrecognized Jurassic I-type plutons may also be present in the batholith. Within San Diego County, Todd and Shaw (1985) recognized and mapped two suites of strongly deformed gneissic granites and migmatites. One is peraluminous (Harper Creek suite) while the other is transitional between metaluminous and peraluminous (Cuyamaca Reservoir suite). These rocks bear a striking resemblance to deformed and, in places, migmatitic, peraluminous (S-type) examples from the Lachlan fold belt and New England batholith of eastern Australia. The gneissic granite suites are known to extend north along the axial zone of the Peninsular Ranges batholith and cover an area at least 45 km wide by 150 km long. To the south, rocks of similar type are known to extend into Baja California, Mexico, for at least 300 km. Chemical and isotopic studies of these Jurassic suites confirm that they meet the criteria necessary to define them as S-type and transitional I- to S-type, respectively. However, unlike the majority of Lachlan fold belt S-type granites that are high level and often associated with their volcanic equivalents, the Peninsular Ranges batholith suites were emplaced at much deeper levels, possibly as much as 11-16 km. The Harper Creek suite, of S-type gneissic granodiorite-tonalite plutons and associated Stephenson Peak migmatitic schist and gneiss facies, is strongly peraluminous and contains biotite, cordierite, sillimanite, abundant graphite, and ilmenite. It has elevated delta (super 18) O up to +20 per mil, initial (super 87) Sr/ (super 86) Sr ratios (Sr (sub i) ) to 0.713, a high aluminum saturation index, and Na (sub 2) O/K (sub 2) O ratios that overlap those of the Lachlan fold belt S-type granites. The Cuyamaca Reservoir suite contains gneissic granodiorite-tonalite plutons, transitional between metaluminous and moderately peraluminous (I-to S-type), containing reduced biotite, subaluminous amphibole, orthopyroxene, titanite and ilmenite. It has values of delta (super 18) O and Sr (sub i) greater than the Cretaceous I-type granites but less than the Harper Creek suite. Leucosome melt phase accumulation from the Julian Schist diatexites to produce a restite-rich magma is considered the most likely origin for the Harper Creek suite. For the Cuyamaca Reservoir suite, possible source components include young mantle-derived magma, metaigneous and metasedimentary rocks formed in an arc environment, and evolved basinal fill metasedimentary rocks. The evolved metasedimentary component may be of Julian type. The Harper Creek and Cuyamaca Reservoir suites comprise deformed, steepwalled, north-northwest-trending bodies up to 20 km long and with length-to-width ratios of 4:1. Textures range from strongly foliated to gneissic or mylonitic. Internal foliation that strikes parallel to the long dimension of the bodies and dips steeply to the east is defined by alignment of relict magmatic feldspar and quartz grains and recrystallized aggregates of quartz and biotite. The concordance of magmatic and subsolidus foliations in the Jurassic plutons and the continuity of these structures with regionally developed metamorphic fabrics in their wallrocks indicate that magmatic foliation was overprinted by high-temperature, post-magmatic solid-state foliation. Foliation in the Harper Creek and Cuyamaca Reservoir suites is concordant with the axial pla
Descriptors: absolute-age; alkali-metals; alkaline-earth-metals; Australasia-; Australia-; Baja-California; batholiths-; California-; chemical-composition; chemical-ratios; Cuyamaca-Reservoir-Suite; dates-; deformation-; diorites-; folds-; fractures-; geochemistry-; gneisses-; granites-; Harper-Creek-Suite; I-type-granites; ICP-mass-spectra; igneous-rocks; intrusions-; island-arcs; isotope-ratios; isotopes-; Jurassic-; Lachlan-fold-belt; magmas-; major-elements; mass-spectra; Mesozoic-; metals-; metaluminous-composition; metamorphic-rocks; Mexico-; mineral-assemblages; mineral-composition; nesosilicates-; North-America; O-18-O-16; orthosilicates-; oxygen-; Peninsular-Ranges; Peninsular-Ranges-Batholith; peraluminous-composition; petrography-; petrology-; plate-tectonics; plutonic-rocks; radioactive-isotopes; Rb-87-Sr-86; restites-; rubidium-; S-type-granites; San-Diego-County-California; sedimentary-rocks; silicates-; spectra-; stable-isotopes; stratigraphic-units; strontium-; tectonics-; trace-elements; U-Pb; United-States; volcanic-rocks; whole-rock; zircon-
Source: Special Paper - Geological Society of America. 374; Pages 157-183. 2003.
map_coordinates: LAT: N323000; N333000; LONG: W1161000; W1174000.LAT: N321500; N330000;LONG: W1161500; W1163000.