CONTRASTING SOURCES AND CONSTRUCTION OF CRETACEOUS GRANITE PLUTONS IN THE MOJAVE, CA USING WHOLE ROCK GEOCHEMISTRY AND ZIRCON GEOCHRONOLOGY AND GEOCHEMISTRY

South of the Cretaceous Sierra Nevada batholith, the Cordilleran arc intruded a full thickness of continental crust. The southern Mojave Desert and eastern Transverse Ranges preserve a cross-section through this continental margin arc, and plutons emplaced in this part of the arc contain pre-magmatic (or inherited) Proterozoic zircons that can be considered as xenoliths from the magma source region. Thus, plutons emplaced greater than 20 km can be observed directly and the character and dynamics of the magma source region can be inferred from zircon geochronology and geochemistry. We compare granites from the Cadiz Valley Batholith (CVB), a large-volume, upper-crustal, normally zoned intrusive complex (similar to those generated in the Sierra Crest magmatic event), with the Palms pluton, a ~150 km² granitic intrusive complex emplaced in a deeper and more structurally complex mid-crustal environment. Low Y concentrations, high Sr/Y ratios, and low 87Sr/86Sr(t) suggest that the CVB formed by melting of a primitive, garnet-bearing source. Its pre-magmatic zircons are a mixed population of 1400 Ma and 1700 Ma grains with tightly grouped REE concentrations and ratios. None have HREE depletion indicative of crystallization in the presence of garnet. These whole rock and pre-magmatic zircon characteristics define a ‘CVB-like’ component that can be identified in the Palms pluton, along with several additional components including a. low Y components similar to other mid-crustal plutons, b. components with unique pre-magmatic zircon populations, such as copious garnet-effected 1700 Ma zircons, and c. apparent hybrids between components. Conclusions include that 1. A 1400 Ma zircon-bearing source is present beneath the western Mojave Desert, 2. The large-volume, upper-crustal Cadiz Valley Batholith preserves more homogeneous source material than the small-volume, mid-crustal Palms pluton, 3. Internal sheets with varying source characteristics are identified in the Palms pluton, 4. The Cadiz Valley Batholith may have been constructed from non-garnet bearing crustal material that experienced pro-grade garnet growth before melt extraction, and 5. The Palms and Cadiz Valley Batholith units, both considered ‘granite plutons’ have significantly different source characteristics and construction histories.