LATE JURASSIC PLUTONISM IN THE SOUTHWEST U.S. CORDILLERA

Although plate reconstructions suggest that subduction beneath the southwest U.S. Cordillera was an approximately steady-state process from the mid-Mesozoic through the early Tertiary, recent precise geochronologic studies suggest that emplacement of voluminous continental margin batholiths in the Cordillera was episodic. In central and southern California and western Arizona, geochronologic studies suggest that major episodes of batholithic magmatism occurred in Permo-Triassic, Middle Jurassic and late Early to Late Cretaceous time. However, recently published studies of Great Valley forearc basin sediments and ash beds in Colorado Plateau continental interior sediments suggest that Late Jurassic time was probably also a period of significant magmatism, though few dated plutons of this age have been recognized. Our recent focused geochronologic studies have identified a belt of Late Jurassic plutonic and hypabyssal rocks at least 200 km in length, extending from the northwestern Mojave Desert and San Bernardino Mountains on the northwest, through the Pinto Mountains and Eagle Mountains, to the San Gabriel Mountains on the southeast (prior to dextral slip on the San Andreas fault system). The plutons range in age from 156 to 149 Ma. The plutons include two intrusive suites: a calc-alkaline suite compositionally unlike Permo-Triassic and Middle Jurassic monzonitic suites but similar to Late Cretaceous arc plutons emplaced across this region ca. 85-72 Ma, and a contemporaneous but not comagmatic alkaline suite. The alkaline suite cannot have been derived from the calc-alkaline suite by low pressure fractionation, suggesting that two discrete sources were melted to form these plutonic suites. The belt lies outboard of both the voluminous Middle Jurassic arc and the ca. 148 Ma Independence dike swarm at these latitudes. The Late Jurassic was thus a time of both tectonic and magmatic transitions in the southern Cordillera.