USING GEOCHEMISTRY AND GEOCHRONOLOGY TO STUDY CRYPTIC INTRUSIVE CONTACTS IN THE CRETACEOUS WESTERN MOJAVE BATHOLITH

During the Cretaceous the Farallon plate was subducted below the North American plate, creating a magmatic belt along its margin, but this magmatism was interrupted around the beginning of the Laramide orogeny ~80 million years ago. The Sierra Nevada and western Mojave batholiths have since been exposed by tectonic uplift and erosion. This study focused on the western Mojave batholith, using major-element geochemistry, geochronology, and geologic mapping of a ~100km2 area of the western Mojave batholith. The three main rock units are a conspicuously sphene-bearing biotite hornblende quartz monzonite, a columnar-biotite granite, and a hornblende biotite quartz monzonite. All samples had phaneritic, equigranular, medium- to coarse-grained textures, but biotite shapes varied from columnar to platy, and some of the more felsic samples contained a few 2 cm K feldspar crystals. Wavelength-dispersive XRF geochemical data show that from east to west there is a trend from felsic to intermediate. The linear spread of iron index with respect to SiO₂ indicates mixing of two magmas with different histories of magnetite crystallization. The broad gradational contact between two magmatic units suggest that they mixed while still warm as opposed to one pluton intruding another cooled pluton. Geochronology data were collected from two rock samples using laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) at CSU Northridge. The columnar-biotite granite was dated at 84 Ma ± 1.9 Ma and the hornblende biotite quartz monzonite was 85 Ma ± 5.4 Ma. These two ages are statistically indistinguishable from each other, and from a nearby sphene-bearing biotite hornblende quartz monzonite that was dated at 84.8 ± 0.8 Ma by a previous study, via SHRIMP. Further refinement of the geologic mapping will elucidate the crustal structure of the western Mojave and provide piercing points to estimate offset across the active Mirage Valley Fault.