Characterization of Mesozoic granitoids in the northern Coso Range Inyo County, California

Petrographic and geochemical characterization of a senes of granitic intrusions exposed within a northeast-trending swath through a portion of the northern Coso Range in Inyo County, California, provides a missing data-link between studies of granitoids within the Sierran batholith farther north and Sierran granitoids farther south. Additionally, granitoids of the study area have been intruded across proposed northwest-trending zones of continental truncation or interbatholithic breaks. Prebatholithic metasedimentary rocks, the oldest exposures in the study area, occur m a limited number of exposures, generally as screens and isolated pendants. Metasedimentary rocks exposed in the eastern portion of the field area have characteristics similar to Paleozoic Owens Valley Group Metasedimentary rocks described in the lnyo Range to the north and in the Argus Range immediately to the east. Metasedimentary rocks of undetermined Mesozoic or Paleozoic age consist of mafic schistose rocks, marbles, and quartzites. These rocks, occurring mostly in the western field area, are exposed in more limited outcrops than are Paleozoic metasedimentary rocks in the east. Batholithic rocks represent the largest percentage of rock exposures within the field area and modally range from melanocratic hornblende gabbro to leucogranite. Radiometric age determinations for Coso intrusions show that plutonism occurred in two main pulses; a Late Jurassic pulse from about 175 to 173 Ma, and a late Early to early Late Cretaceous pulse from about 102 to 89 Ma. Combining these criteria with field and cross-cutting relationships; major-element geochemistry; and geographical location, intrusions in the study area are segregated into Middle Jurassic intrusive rocks of Coso Junction (IRCJ) and Silver Peak (IRSP), and middle Cretaceous intrusive rocks of Cactus Flat (IRCF) and Coso Village (IRCV). Nearly all of the intrusive rocks in the study area contain textures and fabrics suggesting that they have experienced varying combinations of strain and strain recovery, low-grade metamorphism, and hydrothermal alteration. Major-oxide analyses show that Jurassic intrusive rocks have an A/CNK ranging from 0.73 to 1.06 (metaluminous to weakly peraluminous) and have a Peacock Index of -53 (alkalic-calcic). Cretaceous rocks range in A/CNK from 0.44 to 0.99 and have a Peacock Index of -58 (calc-alkalic). The slightly higher A/CNK values for Jurassic rocks may be the result of syn- to postbatholithic alkali enrichment related to tectonic deformation and hydrothermal alterations during later stages of regional plutonism. The overall similarity in major-oxide trends in both Jurassic and Cretaceous intrusive rocks suggests that both groups have followed similar courses of chemical evolution. In contrast to the rather homogenous major-oxide trends in both age groups of rocks, trace-element data shows considerable variation. In general, with increasing Si02, Jurassic granitoids show an increase in Ba, Nb, and Rb, and a sharp decrease in Sr. With increasing Si02, Cretaceous rocks show increases in Nb, Rb, and Th, whereas Sr decreases. Ba, Zr, Hf, and La show no definitive changes. Comparisons between rare-earth element values for Jurassic and Cretaceous intrusive rocks show only minor variations with respect to increasing Si02� Additionally, Eu anomalies are poorly defined. Spider diagram patterns show an overall decrease in the heavier incompatible elements with respect to the lighter ones. The majority of the samples have troughs for Nb-Ta and negative spikes for Ba, Th, Sr, and Ti. The troughs at Sr probably result from the fractional crystallization of plagioclase. The Th-Rb trough combined with a trough at Nb-Ta suggest contamination from lower continental crust. Subduction-related continental rocks thought to have a significant crustal component typically have a distinctive Nb-Ta trough and modest negative spikes for Ba, Th, Sr, P, and Ti, as do the Coso intrusions. The initial 87Sr/86Sr values (Sri) for both Jurassic suites and the Cretaceous suites have identical ranges, 0.7061 to 0.7071. These ratios are consistent with regional trends exhibited by Sri; igneous rocks in the western Sierra generally have ratios 0.706, whereas those in the eastern Sierra generally have ratios 0.706. The Sri = 0. 706 isopleth generally divides a region underlain by Paleozoic eugeoclinal rocks to the west from a region underlain by Paleozoic miogeoclinal rocks to the east. No direct field or laboratory evidence supports or refutes the presence in the Coso Range of either a cryptic or exposed major strike-slip fault zone that various workers have hypothesized as extending southward through or near the study area. Intrusive rocks across_ the full width of the study area have overlapping major-oxide and trace-element compositions that give no indication that they may have interacted with different kinds of fault-juxtaposed lithosphere. In addition, the initial 87Sr/86Sr values for both Jurassic suites and the Cretaceous suites have identical ranges, 0.7061 to 0.7071. These ratios are consistent with regional trends exhibited by Sri. The only possible indication of differences in geologic framework rocks of the study area are seen in contrasting types of prebatholithic rocks exposed at the east and west ends of the study area. Screens in the east consist of Paleozoic(?) metasedimentary rocks that include strata similar to those of Pennsylvanian and Permian age in the southern Inyo Mountains plus undifferentiated calcsilicate and metasedimentary rocks. Screens west of Coso Peak include metasedimentary rocks of unaffiliated Mesozoic/Paleozoic age and consist largely of mafic schistose rocks. Additional data, such as Pb and 0 isotopes, will be needed to further evaluate the presence of a crustal interbatholithic break.