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