SYSTEMATIC MIDDDLE TO LATE JURASSIC CHANGES IN MAGMA SOURCE IN THE MOJAVE SEGMENT OF CALIFORNIA’S CONTINENTAL ARC

The southern Mojave Desert contains plutonic and volcanic level exposures of California’s Jurassic continental arc. Because these exposures are fragmented across several ranges, and are profoundly hydrothermally altered (Solomon and Taylor, GC Sp. Pub. 3, 1991), the value of this arc section for understanding regional magmatism and volcanic-to-plutonic transitions has been greatly compromised. To obtain a clearer sense of the magmatic history of this arc section, we have employed a regional campaign of U-Pb dating, O, and Hf isotope analysis of zircon to: 1) draw temporal and spatial links among disparate plutonic and volcanic fragments; 2) obtain pristine records of original magma chemistry. U-Pb dating shows that Middle Jurassic pluton ages cluster at: 175–174 Ma (n = 2 samples); 169–167 Ma (n = 2); and 165–162 Ma (n=14). Late Jurassic ages continuously span 154 to 147 Ma (n = 11). Assuming a conservative ±2% (2SD) external error, these ages match well with magmatic pulses recognized elsewhere in the Jurassic arc. Younger plutons (77–78 Ma; n = 5) are associated with the waning stages of magmatism in the Cretaceous arc. Values of δ¹⁸O from Middle Jurassic plutons are higher and more variable (5.77±0.78‰, 2SD, n=22), whereas most Late Jurassic plutons lie in a narrow range of δ¹⁸O (5.16±0.30‰,2SD, n=10). Late-Jurassic tuffs from caldera complexes are lower still (4.81±0.34‰,2SD, n=3). Average εHf_(T) of zircon varies by ± 1.5 epsilon units (2SD) from sample to sample, but values overall are highly bimodal: εHf_(T) is –12.3 to –11.7 in four samples ranging from 148 to 162 Ma; three plutons >165 Ma are –16.6 to –15.4. Values of εHf_(T) for Mojave basement in the Late-Jurassic are –29 to –33. Overall, Hf and O isotopes indicate a shift toward greater proportions of mantle, and less Proterozoic crust between Middle and Late Jurassic magmatic pulses associated with lithospheric extension that likely facilitated melting and influx of mantle melts into lower Mojave crust. In the Late Jurassic, lower average δ¹⁸O values in tuffs compared to co-existing plutons suggest that caldera complexes saw melting and recycling of hydrothermally altered rock. Compared to other parts of the Jurassic arc in the Owens Valley and Sierra Nevada, the Mojave segment has distinctly lower δ¹⁸O suggesting fundamentally different source composition.