PRESSURE AND TEMPERATURE CONSTRAINTS ON CRETACEOUS PLUTONS OF THE WESTERN MOJAVE DESERT

Subduction of the Shatsky Rise conjugate beneath the North American Plate after 80 Ma is thought to have disrupted the Cordilleran batholith in the area of the Mojave Desert. The resulting flat-slab subduction would have caused a significant amount of tectonic shuffling in this area. In order to help resolve the locations of these cryptic Laramide-age structures in the western Mojave, sample mineral compositions were analyzed via electron microprobe wavelength-dispersive x-ray spectrometry to determine emplacement pressures using aluminum in hornblende barometry, in conjunction with hornblende-plagioclase thermometry. Calculated temperatures fell in the expected magmatic range of ~660-740°C. Pressures were calculated to vary across a wide range from ~2 to 6 kbar. Corresponding depths range from approximately 7 km in the Victorville area up to 26 km in the northwestern Mojave Desert, south of the Rand Mountains. Samples from the central Mojave west of Hwy 395 indicate mid- to upper crustal depths of ~7 to 26 km. The sample with the highest pressure is from outside the currently mapped bounds of the Central Mojave Metamorphic Core Complex and therefore may represent the effects of unrecognized pre-Tertiary exhumation. In general, the broad mix of emplacement pressures encountered over the western Mojave suggests tectonic shuffling of the area is more complicated than previously documented. Ongoing efforts to refine pressure constraints from the western Mojave using thermobarometry of aplites and metamorphic rocks can further constrain the relationship between high flux magmatism and tectonic deformation processes in the Late Cretaceous.