GEOLOGIC CONSTRAINTS ON MID-CRUSTAL MAGMATIC PROCESSES IN THE MOJAVE SEGMENT OF THE CRETACEOUS CORDILLERAN ARC

In continental arc settings, ponded magma bodies, created by rheological boundaries, are home to abundant magmatic processes, such as AFC. Though the geochemical signature of magmatic processes is seen in surface magmas, dispute surrounds the manner and depth at which these processes occur. The Little San Bernardino Mountains of the Eastern Transverse Ranges of Southern California, a tilted crustal section, preserves the ponded granitic plutons of the Cretaceous Cordilleran Arc, intruding Proterozoic metamorphic and metasedimentary host rock, allowing for the study of in-situ magmatic processing. Wide Canyon is an approximately 16 km long north/south trending canyon that crosscuts the tilted crustal section at approximately 18.65 m projected paleodepth (Needy et al., 2009), extending from the bottom of the sheeted complex into the underlying zone of AFC. The top of Wide Canyon is composed of fine-coarse grained granitic sheeted complex intrusions of Cretaceous to Late Jurassic age (Needy et al., 2009), disrupting the 1406.9 ± 22.9 Ma K-spar rich gneissic framework. Amphibolite blocks and gneiss begins to appear in greater abundances towards the bottom of the sheeted complex, gradually transitioning into an extensive zone of migmitization. An approximate 1400 Ma to 1700 Ma, km scale, garnetiferous gneissic unit marks the middle of the canyon and remains unaltered until being disrupted by an increasing abundance of granitic to granodioritic Cretaceous magmatism (74.6 ± 1.6 Ma), yielding a significant zone of melt injection with increased abundances of stoping and assimilation. Zircon trace element analysis reveals metamorphic ²³²Th/²³⁸U values (.02 - .1) and depleted HREE concentrations in zircons from garnet bearing samples, constraining metamorphic garnet growth at approximately 1.4 Ga. Similar analyses of synchronous non-garnet bearing samples reveals magmatic rims with an evolved ²³²Th/²³⁸U signature and no HREE depletion. Magmatic abundances continue to increase with depth in the section resulting in a massive unit of deformed blocks supported by a granitic matrix (80-88 Ma). U-Pb in Zircon geochronology reveals a bimodal population of ages surrounding 73 and 88 Ma, with synchronous but compositionally distinct magmas as a result of magmatic processing beneath or at the level of emplacement.