NATURE AND PROGRESSION OF MAGMATISM IN AND AROUND THE ALBION-RAFT RIVER-GROUSE CREEK METAMORPHIC CORE COMPLEX

The Albion-Raft River-Grouse Creek metamorphic core complex (ARG-MCC) is a classic example of a Cordilleran metamorphic core complex where deep crustal rocks were deformed, intruded and exhumed to the surface during Cenozoic extension. Cenozoic igneous rocks are exposed both within and around the ARG and represent three periods of magmatism, each linked to a different stage of its Tertiary evolution.

Earliest magmatism is recorded by ~ 42-34 Ma shallow-level plutonic and volcanic rocks; part of the early Cenozoic N to S sweep of magmatism in the Western U.S.. Eocene magmas are syn-extensional but extension is minor in magnitude and pre-dates Basin and Range faulting. Magmas are calc-alkaline, weakly peraluminous and range from intermediate to felsic. The next episode of magmatism is represented by 32-25 Ma plutons that intruded at depths of 12-15 km during extreme ductile attenuation of roof rocks. They are strongly peraluminous granodiorites to granites and contain zircons with inherited Archean cores and Oligocene rims, indicating significant involvement of older crust. The youngest magmas are represented by 10-7 Ma volcanic rocks, exposed along the east flank of the Albion Mts., that are similar in age and composition to Snake River Plain-type magmas. In the ARG, they erupted during slip along the Albion fault and Raft River detachment, which were responsible for the final exhumation of the core complex between ~14-7 Ma (Wells et al., 2000) and the formation of a syn-extensional system of basins. The Miocene volcanic rocks are bimodal (basalt and rhyolite), tholeiitic and weakly peraluminous.

This progression in the nature of magmatism suggests initial heating and melting of the crust at ~ 42 Ma continued into the Oligocene when sufficient partial melting remobilized deeper parts of the crust leading to the rise of the 32-25 Ma plutonic-metamorphic complex to depths of ~12 km. Miocene magmatism resulted in additional heating of the crust synchronous with Basin and Range faulting which exhumed the complex to the surface.