MESOZOIC MAGMATISM AND ARC CESSATION IN THE CENTRAL MOJAVE DESERT, CALIFORNIA

The Mesozoic magmatic arc was once continuous throughout the Western North American Cordillera until Laramide-age disruption. Although the Sierra Nevada and Peninsular Ranges batholiths (PRB) are well studied, many granitic intrusions scattered throughout the Mojave block have been little studied and solid age and geochemical control is only locally available. Here we present reconnaissance bulk geochemistry, zircon U-Pb geochronology (LA-ICP-MS and SHRIMP-RG), and oxygen isotope geochemistry of Mesozoic granitoids predominantly sampled from areally significant plutons in the Central Mojave. Combined with published data, this allows us to propose models for the geochemical evolution of the Mesozoic magmatic arc and arc cessation in the Central Mojave Desert. The Jurassic magmatic arc within the Central Mojave has a wide range of ages, from ~175-145 Ma, and variable major elemental geochemistry (58.3-77.8 wt% SiO₂, calc-alkalic to alkalic). In contrast, early Cretaceous ferroan granites have more major elemental restricted geochemistry (75-77.5 wt% SiO₂, alkalic-calcic) and ages (~142–138 Ma). We propose Late Cretaceous magmatism occurred in four stages before arc cessation at 73 Ma. Stage 1 (100-92 Ma) consists of normal subduction and arc magmatism. Stage 2 (91 – 82 Ma) involves shallowing of the subducting slab due to the putative onset of subduction of an oceanic plateau. Subducted sediments/schist related to tectonic erosion generated the voluminous Western Mojave batholith in volcaniclastic basement (average δ¹⁸O_quartz = +11.3 - +12.8‰). Stage 3 (~81 Ma) is defined by a magmatic lull, whereas Stage 4 (80-73 Ma) is hypothetically an episode of slab tear magmatism. As the tear propagated to the east, it widened, and asthenospheric upwelling generated the Ludlow batholith (~78 Ma) in cratonic basement (average δ¹⁸O_quartz = +9.3 - +9.8‰). Similar isotopic trends are seen in the PRB, and can be considered evidence for the continuation of the PRB trend upward into the Mojave block.