POLYMETAMORPHISM IN THE FUNERAL MOUNTAINS METAMORPHIC CORE COMPLEX RECODED IN MONAZITE, TITANITE, AND ZIRCON

In the Funeral Mountains metamorphic core complex, a polymetamorphic geologic history is preserved in highly strained rocks in the footwall of the Boundary Canyon detachment fault. Here, a NW-deepening Jurassic metamorphic field gradient is preserved but is overprinted in the deepest structural levels of this core complex in Monarch Canyon. Prograde Jurassic and mid Cretaceous metamorphism, and retrograde Late Cretaceous and Miocene metamorphism, is recorded by U-Th-Pb titanite, monazite, and zircon petrochronology. Cretaceous monazite grains have heavy rare earth element (HREE) trends that decrease from 100-90 Ma, suggesting that following Jurassic metamorphism, prograde metamorphism and garnet growth at ca. 100-90 Ma records hinterland crustal thickening concurrent with an increase in foreland thrusting. At 90-85 Ma, the HREE trends transition from a decrease in HREE concentrations to an abrupt increase, with continued monazite growth during increasing HREEs until ca. 65 Ma, suggesting that monazite geochemistry records the initiation and duration of garnet breakdown during decompression. In the deepest level of the core complex, exhumation caused the replacement of kyanite by sillimanite synchronous with, and perhaps causing, partial melting. Depth profiles in metamorphic overgrowths of detrital zircons suggest that melt crystallized from 90-80 Ma. No zircon overgrowths were developed during prograde Jurassic and mid Cretaceous metamorphism; all zircon growth was synchronous with retrograde decompression and melting. In the overlying non-migmatitic shearzone, extension is recorded in a top-NW extensional shear fabrics that has associated 77 and 80 Ma sheared titanites. This polyphase Jurassic and Cretaceous deformation was exposed during Miocene unroofing that is associated with greenschist facies shearing that is recorded in titanite.