GEOCHEMICAL ANALYSES OF PLUTONS IN THE EASTERN MOJAVE DESERT, CALIFORNIA

Mesozoic subduction generated the Cordilleran magmatic arc, once continuous from the Sierra Nevada to the Peninsular Ranges, before Laramide disruption of the Mojave block. Eastern Mojave granitoids likely differ from other Cordilleran arc segments, as they were emplaced into more continental-affinity lithosphere. Although the Sierra Nevada and Peninsular Ranges Batholiths are well-characterized geochemically, many granitic intrusions scattered throughout the Mojave block have been studied little and solid age control is only available locally. Major-element geochemistry of a reconnaissance sampling of 36 areally significant eastern Mojave plutonic rocks was collected via X-ray fluorescence spectrometry. Geochemical correlations among these rocks can be used to discriminate between intrusions of different ages and tectonic origins, allowing tentative assignment of inferred ages where geochronological data is unavailable. Five distinct groups have been identified: (1) Proterozoic ferroan quartz syenite and monzonite, low in SiO₂ (63 – 67%), but with a high Fe-Index of 0.85-1 and high Na₂O+K₂O (11-12%). (2), Jurassic ferroan granite with a similar Fe-Index to group 1 (0.78-1) but higher SiO₂ (75-77%), and lower Al₂O₃. (3) Cretaceous two-mica (± garnet) granites, high in SiO₂ (~77%), Fe-index (~0.9), and MnO. (4) 100-80 Ma magnesian to slightly ferroan monzodiorites, quartz monzonites, and granites, that vary along a roughly linear trend from about 61% SiO₂ and a Fe-index of 0.6, to 73% SiO₂ and a Fe-index of 0.8. This set of intrusions includes the Teutonia Batholith, and is similar to plutons in the western Mojave. A general eastward increase of about 0.02 %SiO₂ per kilometer is similar to trends found in the adjacent Sierra Nevada and Peninsular Ranges Batholith that may be related to eastward changes in intruded lithosphere. (5) 73-77 Ma magnesian granite and granodiorite, about 5% richer in SiO₂ for a given Fe-index than group 4. Future studies will better constrain the age ranges of these groups, their defining geochemical characteristics, and their different tectonic origins. Geochemical isopleths also have potential to act as piercing lines for reconstruction of post-batholith faults.