CONSTRAINTS FROM MONAZITE AND XENOTIME PETROCHRONOLOGY ON JURASSIC PT PATHS FROM THE FUNERAL MOUNTAINS METAMORPHIC CORE COMPLEX, DEATH VALLEY NATIONAL PARK

The Funeral Mountains in eastern California exposes Barrovian metamorphism in the lower plate of a Tertiary detachment fault. Garnets from greenschist facies rocks at Indian Pass (IP) and amphibolite facies rocks 9 km to the northwest at Chloride Cliff (CC) yielded pressure temperature (PT) paths and Lu-Hf garnet ages that indicate crystallization occurred as a result of thrusting at 158.2 ± 2.6 and 165 ± 9.2 Ma, respectively. Ar-Ar geochronology indicate that IP moved through Ar closure ~150 Ma and that CC moved through hornblende closure by 113 Ma and through muscovite closure by ~77 Ma. Further constraints on the timing of garnet growth at CC were obtained from the same samples analyzed for PT paths. Monazite and xenotime were analyzed using the LASS method to determine trace element compositions and U-Pb ages simultaneously. Garnet trace element data were collected along the same traverse used to model the PT paths utilizing a quadrupole mass spectrometer. Xenotime, from the matrix, yielded ages of 170-157 Ma (n=6). Garnet Y patterns suggest that xenotime breakdown occurred during early P increase (thrust burial). Monazite, found as inclusions in garnet and in the matrix, yielded 3 age populations, 176-149 (n=174), 144-115 (n=7), and 76-74 (n=5) Ma. Monazite and garnet trace element patterns suggest that (1) the oldest monazite grew via xenotime breakdown concurrent with garnet growth during initial P increase (thrust burial), (2) between ~144-130 Ma garnet grew via the reversal of the prograde staurolite-producing reaction along the P decrease portion (exhumation) of the PT path but had creased regrowth by 115 Ma, and (3) the youngest monazite grew after garnet crystallization, but from an event that elevated the available HREEs in the system. Although the timing of tectonic reorganization leading to the development of Franciscan subduction remains controversial, the timing and sequence of Middle Jurassic westward underthrusting followed by early Cretaceous exhumation is consistent with a record of high-flux magmatism followed by delamination. The 76-75 Ma monazite ages coincide with Laramide flat-slab subduction, suggesting that they crystallized as a result of slab derived fluids that mobilized P and REEs.