CONSTRAINTS FROM MONAZITE AND XENOTIME PETROCHRONOLOGY ON BURIAL AND EXHUMATION FROM THE FUNERAL MOUNTAINS, CALIFORNIA WITH IMPLICATIONS ON U.S. CORDILLERAN JURASSIC PLATE BOUNDARY TECTONICS

The Funeral Mountains in eastern California expose Barrovian metamorphic rocks in the lower plate of a Miocene detachment fault. Garnets from greenschist facies rocks at Indian Pass (IP) and amphibolite facies rocks, 9 km NW at Chloride Cliff (CC) yielded pressure-temperature (PT) paths and Lu-Hf garnet ages that indicate crystallization was concurrent with tectonic burial from 158-167 Ma and was followed by exhumation. Ar-Ar thermochronology from IP indicate cooling through ~425 °C by 146-153 Ma, and CC cooled through ~500°C by 113 Ma and ~425 °C by 77 Ma.

Further constraints on the timing of Jurassic burial at CC were obtained using the laser ablation split stream (LASS) method to determine REE+Y compositions and U-Pb ages simultaneously on monazite and xenotime. Rim to rim garnet REE+Y element data were also collected. Xenotime yielded ages of 170-157 Ma (n=6). Monazite yielded 3 age populations, 176-149 Ma (n=174), 145-115 Ma (n=7), and 76-74 Ma (n=5). Monazite, xenotime and garnet trace element data help to better constrain the PT path: (1) Xenotime breakdown occurred during early burial before or during garnet crystallization. (2) Monazite domains 176-166 Ma grew from prograde recrystallization of detrital grains before garnet growth. (3) We infer that monazite and xenotime domains yielding ages of 166-149 Ma grew via allanite breakdown during garnet growth and prograde garnet resorption (~165 Ma in the Johnnie Fm. and ~158 Ma in the Kingston Peak Fm.). (4) Monazite domains heavily depleted in HREE (145-130 Ma) and a monazite inclusion (143 Ma) constrain the timing of retrograde garnet regrowth along the exhumation portion of the PT path. (5) The 76-75 Ma monazite grains grew after garnet crystallization during continued exhumation, coinciding with cooling ages.

The nature and timing of tectonic reorganization along the U.S. western margin, which led to Franciscan subduction, remains controversial. However, these data suggest that westward underthrusting in the back arc was ongoing by 176 Ma, supporting the development of E-dipping subduction by this time. Also ongoing by this time was retroarc crustal shortening represented by several thrust belts that collectively extend the length of the continental margin and ~75 km east of the magmatic arc in the SW U.S. (accounting for Neogene extension).