IMPLICATIONS OF NEW SIMS ZIRCON AGES FOR MAGMATIC EVOLUTION OF THE WHIPPLE MOUNTAINS METAMORPHIC CORE COMPLEX

Exhumation of lower plate mylonitic gneisses in Cordilleran core complexes provides a window into lower and mid-crustal magmatism during Mesozoic and Cenozoic orogeny in southeastern California. We present new SIMS Pb/U ages for zircons from Mesozoic and Paleogene plutonic and metaplutonic rocks from the Whipple Mountains, providing constraints on multiple phases of magmatism associated with crustal thickening and detachment faulting, and new insights into the interplay between magmatism, mylonitization and exhumation. The oldest recognized plutons in the lower plate in the Whipple Mountains comprise the marginally metaluminous to peraluminous Whipple Wash suite. Complex zircon systematics in four samples from this suite record peraluminous magmatism ~104 to 100 Ma and disturbance of the U-Pb system about 80 to 70 Ma. This younger disturbance was coeval with emplacement of the 75 Ma Axtel quartz diorite and granodiorite, and with regional Late Cretaceous (~79 to 71 Ma) magmatism in southeastern California. The Whipple Mountains lower plate is pervasively intruded by tonalitic to trondhjemitic sills inferred to be synkinematic to mylonitization and exhumation; one such sill yielded an age of 25.5 Ma. Mylonitization predates emplacement of the War Eagle plutonic complex, which yielded an age of 19.5 Ma, overlapping in age with the Chambers Well dike swarm. The majority of premagmatic zircons in samples of the Whipple Wash suite, Axtel quartz diorite and tonalitic sill form a coherent population with an age of ~1410 Ma. These new results suggest that: (1) early peraluminous magmatism in the lower plate occurred in mid-Cretaceous time and records mid- to deep-crustal magmatism associated with regional crustal thickening and re-melting of Mesoproterozoic crust, (2) the lower plate was affected by a pervasive thermal event associated with metaluminous magmatism in early Laramide time, and (3) Miocene footwall mylonitization and exhumation was completed by 19.5 Ma.