NO LATE CRETACEOUS MAGMATIC GAP IN THE MOJAVE DESERT REGION (Invited Presentation)

The Mojave Desert region preserves structures and magmatic products of major tectonic events associated with peak and waning stages of the Cretaceous California arc. This region is critical for testing models of Laramide orogenesis, which is hypothesized to have developed through the subduction and shallow passage of one or more seamounts deep into the continental interior. Mapping and geochronology of igneous products in the Mojave region offer insights into the thermal and tectono-magmatic evolution of this part of the western US.

The earliest Mojave Desert Cretaceous plutons (ca. 105 - 100 Ma, e.g. Howard et al., 2023) are most commonly small, suggesting relatively localized thermal events. New geochronology from the Teutonia Batholith reveals extensive zircon inheritance of this age range, indicating more voluminous, zircon bearing magmatism of this age in the source region. The Teutonia Batholith region contains several plutons of 97 – 95 Ma age, including the Rock Springs (quartz) monzodiorite and the Black Canyon gabbro. The largest Teutonia Batholith plutons of ca. 92 – 87 Ma (Miller et al., 1996; Wells et al., 2005) are predominantly quartz monzonites. Younger, small volume monzonites with K-spar megacrysts at ca. 82 Ma (Barth et al., 2004; this study) overlap in age with the oldest Cretaceous units dated in Joshua Tree National Park by Needy et al., 2009. Younger (ca. 82 – 74 Ma) magmatism includes the voluminous Cadiz Valley Batholith and plutons in western Joshua Tree National Park (Economos et al., 2021). Finally, the last Cretaceous magmatism in this region are peraluminous dikes cross-cutting plutons near the San Andreas fault with provisional ages of ca. 69 Ma.

Thus, the Mojave region continuously produced calc alkaline magmatism from 92 – 74 Ma and lacks any demonstrable magmatic gap during this period, which would be predicted for the passage of a shallow oceanic slab during this time. Extensive evidence of magmatic shutdown and tectonic disruption is demonstrated for the post-65 Ma period (e.g. Chapman et al., 2020) occurred after the last Cretaceous igneous rocks we have dated in the Mojave. In summary, plutonism underwent geographic transitions during the late Cretaceous, from inboard to outboard at ~82 Ma and from more focused to more diffuse at ~76 Ma, but was continuous and of calc alkaline character throughout.