TIMESCALES OF MAGMATISM DURING LARGE-SCALE EXTENSION: NEW CA-TIMS U-PB ZIRCON GEOCHRONOLOGY OF LATE-MIOCENE INTRUSIONS, CENTRAL DEATH VALLEY, CALIFORNIA

Large-scale continental extension and magmatism are complementary processes that shape landscapes. However, the relative timing of the two processes and potential causal links are often less obvious. Intrusive rocks record a snapshot of crustal processes (e.g., during partial melting, thickening, and crustal reorganization), and thus zircon crystallization histories in plutons can illuminate the timescales of magma flux and generation during regional extension. Syn-extensional late-Miocene intrusions in central Death Valley, including the Willow Spring Diorite (WSD), Smith Mountain Granite (SMG), and several quartz monzonite intrusions (QM) were emplaced and unroofed during a major phase of Basin and Range extension. Using new higher-precision CA-TIMS U-Pb zircon dates and zircon trace element analyses, we will (1) identify the temporal and spatial extent of magmatism at the onset of regional extension, and (2) document their petrogenetic relationships.

WSD samples from different structural levels within the Black Mountains show broad agreement, spanning ~900 kyr, starting at ~11 Ma. SMG samples from Smith Mountain and a QM sample from Gold Valley overlap with the youngest WSD ages at ~10 Ma. Another QM sample from the central part of the Greenwater Range yielded ages of ~8 Ma. New U-Pb zircon ages and prior Sr-Nd whole-rock isotope data (e.g., Asmerom et al., 1990, 1994) suggest that starting at ~11 Ma, the mantle was a major source of melt and heat. These conditions persisted throughout the late-Miocene, likely related to lithosphere thinning and decompression melting. Roughly 0.5 Myr later, more evolved melts with higher crustal contamination (e.g., SMG and QM) were emplaced at the same structural level as the WSD. Field relations and overlapping WSD U-Pb ages with the SMG and QM suggest that the intrusions could be petrogenetically related to the WSD. CA-TIMS U-Pb zircon crystallization histories, coupled with zircon trace element analyses, have the potential to expose the complex interplays between regional extension and magmatism in Death Valley, where field relations are well-exposed, and can provide analogies to other extensional environments.