MONAZITE PETROCHRONOLOGY REVEALS THE LIFECYCLE OF AN OROGEN: EXAMPLE FROM THE UPPER GRANITE GORGE, GRAND CANYON

The timescales, durations, and 4-D evolution of metamorphism and ductile deformation throughout continental collision control orogenic expression and influence dynamics. Yet the evolution of these processes often remain relatively unconstrained due to incomplete exposures or lack of detailed in-situ petrochronology to quantify P-T-d-t. The Grand Canyon’s Upper Granite Gorge (Arizona, USA) preserves a 68 km (42-mi) long 100% exposure of Paleoproterozoic crystalline basement that formed within a long-lived convergent boundary along southern Laurentia. We investigate timescales of orogenic evolution using in-situ monazite Electron Microprobe chemical age data from Vishnu Schist metapelites in two adjacent tectonometamorphic blocks. Both blocks experienced prograde metamorphism and deformation (D1) but were variably overprinted and transposed during a second phase of deformation (D2). We contrast new monazite petrochronology from the Clear Creek block (CCb), which preserves D1 structures and reached peak conditions of 500°C and 6 kbar during D2, with new and previously published dates from the Mineral Canyon block (MCb), which is pervasively deformed during D2 and reached peak conditions of 700°C and 6 kbar during pegmatite emplacement. Across both blocks, metamorphic and magmatic monazite and xenotime yield U-Pb ages spanning deposition (ca. 1745 Ma), prograde (ca. 1727-1704 Ma), peak and retrograde thermotectonism (ca. 1702-1690 Ma), and rapid decompression to mid-crustal residence (ca. 1690-1674 Ma), reflecting the entire duration of crustal accretion during the Yavapai orogeny. Monazite rims yield U-Pb ages that are interpreted to reflect reactivation of Yavapai structures during Mazatzal orogenesis (ca.1660-1647 Ma), mesoproterozoic tectonism (ca. 1560-1522 Ma), and Picarus orogenesis (ca. 1497-1413 Ma), although the latter are exclusive to MCb. Monazite rims yielding U-Pb These temporal patterns are consistent with previous geochronological studies in the region suggesting the Yavapai juvenile island arc accreted to the proto-North American continent over a 60 My interval. Additionally, results raise new questions regarding how subsequent deformation is accommodated in pre-existing zones of weakness in the middle crust throughout the lifespan of a collisional boundary.