METAMORPHISM, PARTIAL MELTING, AND DEFORMATION IN THE FUNERAL MOUNTAINS METAMORPHIC CORE COMPLEX, DEATH VALLEY, CALIFORNIA

Amphibolite facies Proterozoic metasediments below the low-angle Cenozoic Boundary Canyon fault (BCF) record disparate deformational and metamorphic histories above and below the structurally deeper, subparallel Monarch Spring fault (MSF). The MSF truncates a steeply dipping foliation in its footwall; kinematic indicators in quartz mylonites, breccias, and gouge along and above the fault indicate top-to-the NE slip. Rocks above and below the MSF record similar peak conditions (600-700°C, 7-9 kbar), but contrasting retrograde histories. 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 complex history of partial melting and intrusion absent above the MSF. Discordant SHRIMP U-Pb zircon ages (144-199 Ma lower intercepts) from leucosomal segregations in paragneisses suggest Jurassic melting of Proterozoic sedimentary protoliths. Monazites from a Bt-Grt-Sil schist at the same locality yield a U-Pb age of 93 ± 3 Ma, similar to the 98 ± 27 Ma lower intercept of discordant, low Th/U zircon rims from the same sample; two zircon cores are 1.8 Ga and 2.6 Ga. Zircons from a weakly deformed two-mica leucogranite dike which cuts the dominant foliation yield concordant 62.7 ± 1.5 Ma ages with 1.5-1.6 Ga cores. Above the MSF, the older fabric is overprinted by a pervasive low greenschist-facies retrogression, high strain subhorizontal mylonitic foliation, and a prominent WNW-ESE stretching lineation parallel to slickensides on the BCF. 70-72 Ma pegmatites (Applegate et al. 1992, Geology 20: 519-522) are deformed, rotated into parallelism and boudinaged by the mylonitic foliation. A 26 ± 1 Ma mafic dike (Hbl Ar/Ar total fusion age) cuts the ductile fabrics and is brittlely offset <5 m by the MSF, bracketing the mylonite formation and high extensional strain between 70 Ma and 26 Ma.