MAGMATICALLY INDUCED DIAPIRIC RISE OF THE ALBION-RAFT RIVER-GROUSE CREEK METAMORPHIC CORE COMPLEX

Metamorphic core complexes are structural culminations of the middle and lower crust that form during continental extension, exposing fabrics associated with ductile crustal flow. Thus, they offer unique opportunities to investigate the processes that led to their formation. Our study of the Albion-Raft River-Grouse Creek (ARG) metamorphic core complex in the Basin and Range focuses on the interplay between magmatism and the development of extensional high-T ductile fabrics that formed episodically between 32-25 Ma and are bracketed by cross-cutting relations. We conclude that regional mantle-derived magmatic events are important in transferring heat to the lower and middle crust, and play the dominant role in melting and weakening the lower crust, thus allowing large scale crustal flow.

Zircon U-Pb geochronologic and geochemical data (major element, Sr-Nd isotopes, zircon O-isotopes) were collected from Cenozoic igneous rocks exposed at three structural levels of the ARG. Our results demonstrate that the 41-31 Ma Emigrant Pass plutonic complex, emplaced in the upper plate of the core complex, and the 32-25 Ma Cassia plutonic complex, emplaced in the lower plate of the complex, share a common deep crustal “hot-zone”. We interpret these magmas to be part of the southward propagation of magmatism in the western U.S. between ~55-21 Ma, which has been previously inferred to reflect asthenospheric upwelling after the progressive delamination of the Farallon flat slab during Middle Cenozoic.

We have developed a model for the coupled evolution of magmatism and the formation of the high-T extensional fabric in the ARG, that involves a prolonged (41-25 Ma) significant influx of mantle-derived magmas. Between 41-31 Ma, crustal melting and progressively greater hybridization of basalt with lower crustal melts gave rise to calc-alkaline magmas. Between 32-25, large portions of the partially molten, weak and mobile lower crust and the magmatic “hot-zone” flowed in diapiric ascent, forming granite-cored gneiss domes with strain localization along the bounding high-T shear zones. Our model emphasizes the importance of mantle-derived magmatism as the driving mechanism in the formation of the ARG, and our data preclude the collapse of partially molten thickened crust as the sole driver for the ARG granite-cored gneiss domes.