NEW CONSTRAINTS ON THE DEFORMATIONAL AND METAMORPHIC HISTORY OF THE FUNERAL MOUNTAINS METAMORPHIC CORE COMPLEX, DEATH VALLEY, CALIFORNIA

Proterozoic metasedimentary rocks in the lower plate of the Cenozoic Boundary Canyon detachment fault (BCDF) in the northern Funeral Mountains, Death Valley, record a complex metamorphic, deformational, and intrusive history. They are separated by the BCDF from relatively unmetamorphosed Upper Proterozoic, Paleozoic, and Tertiary sedimentary and volcanic rocks in the upper plate. Despite numerous studies, the absolute timing of events in the lower plate is not well understood.

The Monarch Spring fault (MSF) is a previously unmapped feature structurally deeper than and subparallel to the BCDF. The fault truncates steeply dipping units in the footwall and separates rocks with disparate deformational and metamorphic histories. Quartz mylonites, breccias, and gouge occur along and above the fault with kinematic indicators showing a top-to-the NE sense of motion. Hornblende from a mafic dike cutting the ductile fabrics and only slightly offset (<5m) by the MSF yields an Ar/Ar isochron age of 25 ± 1 Ma placing an Oligocene age limit on the formation of high-strain fabrics associated with the MSF.

Amphibolite facies metasedimentary rocks above the MSF were subjected to low greenschist facies retrogression and pervasive deformation resulting in a high strain subhorizontal foliation and NW-SE stretching and mineral elongation lineations. 70 to 72 Ma coarse grained pegmatites (Applegate et al., 1992) are variably deformed, rotated into parallelism and boudinaged with the dominant foliation, thus bracketing the foliation as post 70 Ma and pre 25 Ma.

Rocks below the MSF preserve their high temperature metamorphic history with little retrogression. Leucocratic granitoid dikes and sills mostly cut the dominant foliation and are themselves relatively undeformed. SHRIMP U-Pb zircon analyses from leucosomal segregations in paragneisses yield ages that suggest Jurassic melting of Proterozoic sedimentary protoliths. Monazites from a biotite-garnet-sillimanite schist at the same locality yield a U-Pb age of 93 ± 3 Ma. Zircons from a weakly-deformed fine grained two-mica leucogranite from the same general locality yield concordant 62.7 ± 1.5 Ma ages. Thus rocks were metamorphosed in the Jurassic and maintained at high T's or reheated to high T's in the Late Cretaceous, with a later magmatic event in the Paleocene.