RESOLVING STRATIGRAPHIC COMPLEXITIES OF THE PAHRUMP GROUP AND MIGMATITE PROTOLITH IN THE FUNERAL MOUNTAINS METAMORPHIC CORE COMPLEX USING U-PB DZ GEOCHRONOLOGY

The northern Funeral Mountains contains an attenuated sequence of amphibolite facies Neoproterozoic Johnnie Formation to Mesoproterozoic and Neoproterozoic Pahrump Group in the footwall of the Boundary Canyon Detachment. At the deepest structural levels are migmatitic paragneisses and gneissic amphibolites interpreted to represent alternatively Paleoproterozoic crystalline basement unconformably underlying lower Crystal Spring Formation (CSF), migmatitic CSF separated from non-migmatitic CSF by the Monarch Spring fault, or an insitu anatectic front within the CSF. Recent documentation of an unconformity between a Neoproterozoic part of the CSF (now Horse Thief Spring Fm) and Mesoproterozoic CSF in unmetamorphosed sections in the region, and their differences in detrital zircon (DZ) age spectra, allows a test of these alternatives. We performed LA-ICPMS DZ U-Pb geochronology on metasedimentary rocks in Monarch Canyon, from the deepest levels of the migmatite complex upwards to the base of the Beck Springs Dolomite. Our DZ data allows distinction between the Neoproterozoic and Mesoproterozoic parts of the Pahrump Group and permits correlation to unmetamorphosed sections. The new DZ U-Pb ages confirm the stratigraphic correlation of this metamorphic section to the CSF and Horse Thief Springs Fm of the Pahrump Group and places the unconformity that separates them significantly deeper structurally than previously recognized. Two different DZ age spectra patterns are noted: patterns that exhibit variable modes at 1.0-1.3, 1.3-1.5, 1.6-1.8 and 2.4-2.6 Ga that we correlate to the Horse Thief Springs Fm; and patterns that exhibit a single mode at 1.6-1.8 Ga that we correlate to the Lower CSF. The DZ age spectra suggest what was previously mapped as a thick section of upper, middle, and lower CSF in Monarch Canyon is actually the Horse Thief Springs Fm, and that the unconformity lies deep within the migmatite complex in Monarch Canyon, below the Monarch Spring Fault. Additionally, a repetition of part of the Horse Thief Springs Fm is present within the CSF deep within the migmatite complex, suggesting a cryptic pre-metamorphic juxtaposition. Our results are consistent with the boundary between migmatitic and non-migmatitic rocks representing an insitu anatectic front within the Horse Thief Springs Fm.