Late Jurassic plutonism in the southwest U. S. Cordillera

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.

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