LATE CRETACEOUS TO PALEOCENE METAMORPHISM AND MAGMATISM IN THE FUNERAL MOUNTAINS METAMORPHIC CORE COMPLEX, DEATH VALLEY, CALIFORNIA

Amphibolite facies Proterozoic metasedimentary rocks below the low-angle Cenozoic Boundary Canyon detachment (BCD) are most deeply exposed in Monarch Canyon, northern Funeral Mountains. A 91.5 ± 1.4 Ma Th/Pb SHRIMP-RG monazite age from garnet-kyanite-staurolite schist provides the best constraint on the age of prograde metamorphism in the lower plate. Between the BCD and the structurally deeper, subparallel Monarch Spring fault (MSF), prograde metamorphic fabrics are overprinted by a pervasive greenschist-facies retrogression, high strain subhorizontal mylonitic foliation, and a prominent WNW-ESE stretching lineation parallel to corrugations on the BCD. Granitic pegmatite dikes are deformed, rotated into parallelism and boudinaged by the mylonitic foliation. High-U zircons from one muscovite granite dike yield an 85.8 ± 1.4 Ma age. Below the MSF, retrogression is minor, and amphibolite-facies mineral elongation lineations plunge gently N-NE. Multiple generations of variably deformed dikes, sills, and leucosomal segregations indicate a more complex history of partial melting and intrusion compared to that above the MSF, but thermobarometry on garnet amphibolites above and below the MSF record similar peak conditions of 620 - 680°C, 7 - 9 kbar, indicating minor (<3 - 5 km) structural omission across the MSF. Discordant SHRIMP-RG U/Pb zircon ages and 75 - 88 Ma Th/Pb monazite ages from leucosomal segregations in paragneisses suggest partial melting of Proterozoic sedimentary protoliths as a source for the 86 Ma pegmatites. Two weakly deformed two-mica leucogranite dikes which cut the high-grade metamorphic fabrics below the MSF yield 62.3 ± 2.6 and 61.7 ± 4.7 Ma U/Pb zircon ages, and contain 1.5 - 1.7 Ga cores. The similarity of metamorphic, leucosome, and pegmatite ages to the period of Sevier belt thrusting and the period of most voluminous magmatism in the Sierran arc suggests that both thrust burial and regional magmatic heating may have contributed to metamorphism and subsequent partial melting.