ONE-FELDSPAR GRANITES IN THE SODA MOUNTAINS: MESOZOIC A-TYPE GRANITES IN THE MOJAVE?

Widespread intermediate to felsic intrusive rocks in the Mojave Desert are generally considered to be a product of arc magmatism, mostly produced in higher-flux periods during the mid to late Jurassic and the mid to late Cretaceous. New field and petrographic study suggest the ~ 200 km² dark-weathering granitoid of the Soda Mountains, in the central Mojave, does not fit this pattern. Investigation of the study area was accomplished by reconnaissance sampling and more detailed mapping of the area north of I-15 before the Zzyzx Road exit. Samples were processed for petrographic, geochemical (XRF + ICP-MS), and zircon U-Pb analysis.

In hand samples, these granitoids are fine to medium grained, with ca. 1 cm purplish feldspar phenocrysts, as well as miarolitic cavities in some areas. Petrographic analysis shows that these rocks are perthitic one-feldspar syenogranites to quartz alkali feldspar syenites with micrographic textures. Mafic and accessory minerals include clinopyroxene, rutile, magnetite, and pyrite. The presence of accessory pyrite may account for the dark patina acquired by these rocks upon weathering. Geochemical analysis shows that these granitoids are alkali-rich and ferroan but iron poor. Zircon dating is still in progress, but petrographically similar rocks in surrounding areas give ages around Late Jurassic to Early Cretaceous. The perthitic feldspars and accessory pyrite suggest that these rocks were formed in an anhydrous, reduced melting environment not associated with arc magmatism. Rather, these A-type granites could be a symptom of extensional tectonics or mantle delamination at this time.