FARALLON SLAB REMOVAL AS A DRIVING FORCE OF CORDILLERAN METAMORPHIC CORE COMPLEX FORMATION: DETAILS FROM THE ANACONDA RANGE OF WESTERN MONTANA

Metamorphic core complexes (MCCs) are a product of crustal extension, but their dynamics are still debated. Early research suggests that the formation of MCCs in the western USA was due to gravitational collapse of crust that had been thickened by thrust faulting and addition of arc magmas during Cordilleran orogenesis. However, the instability of overthickened crust alone cannot explain the diachronous formation of core complexes with a strong spatial dependency, as there was relatively uniform thickening in magnitude and time during development of the Cordillera. For this reason, there is growing interest in what role other lithospheric processes (such as subducted slab rollback or slab detachment) play in the initiation and evolution of MCCs. We investigate the role of such processes by determining the temporal relationship between magmatism and extension in the Anaconda metamorphic core complex of western Montana. Geologic mapping, zircon U-Pb geochronology, zircon (U-Th)/He thermochronology, and Lu-Hf isotopic analysis reveal that extension in the AMCC initiated in the Eocene (ca. 53 Ma), at least 7 Myr after the cessation of main-phase plutonism. Therefore, we interpret that the AMCC is an example of a core complex that was primed for large magnitude extension through crustal thickening and voluminous magmatism. We suggest that buckling and rollback of the Farallon flat slab and the onset of the ignimbrite flareup—a transgressive migration of calc-alkaline volcanism—in western Montana was responsible for the initiation of AMCC extension via asthenospheric mantle heating of the hydrated continental lithosphere and elevation of geotherms. Furthermore, we compile MCC cooling ages and ages of ignimbrite flareup related rocks across the western USA to suggest that removal of the Farallon Plate was a primary driver of Cordilleran core complex formation.