A WINDOW INTO THE MIDDLE CRUST OF THE NORTHERN U.S. CORDILLERA AT HOUSE MOUNTAIN, SOUTHERN IDAHO BATHOLITH

The House Mountain gneiss complex comprises a suite of penetratively deformed, amphibolite-facies ortho- and paragneisses located near the southern edge of the Atlanta lobe of the Idaho batholith. This study aims to understand the timing and drivers of metamorphism at House Mountain, within the context of the local magmatic and broader Cordilleran tectonic evolution. The House Mountain gneiss complex is characterized by Cryogenian and late Jurassic plutonic orthogneisses intruding Archean (?) banded gneisses and a Cryogenian paragneiss cover sequence composed of quartzofeldspathic schists, calc-silicates, quartzites, mafic amphibolites and rare metapelites. All of the gneisses are crosscut by deformed (P1; 82 Ma), and nondeformed (P2; 66 Ma) pegmatites and aplites; intrusion of the regional Idaho Batholith peraluminous biotite granodiorite is bracketed between these pegmatite ages. U-Pb and Rb-Sr geochronology of zircon, titanite, monazite, rutile and phlogopite from various lithologies in the paragneiss sequence document over 70 million years of metamorphic mineral growth and subsequent cooling during exhumation. The oldest metamorphic accessory minerals, including zircon (amphibolites), monazite (metapelites) and titanite (amphibolites and calc-silicates) are interpreted to record a portion of the prograde limb of the P-T path from 124 to 90 Ma. Climactic temperatures and penetrative deformation are recorded by many chronometers coeval with the injection of P1 intrusives at 82 Ma, but deformation abruptly ceased prior to the intrusion of voluminous peraluminous granites. Renewed mineral growth is associated with the pervasive injection of 66 Ma aplites and pegmatites into the gneiss complex. Grain-size dependence in the ages of phlogopite and rutile records cooling of the complex below 400°C from 59 to 50 Ma, prior to emplacement of the Challis intrusive suite. These data will be presented within an interpretational framework of Early Cretaceous retro-arc crustal thickening, Late Cretaceous syn-contractional extension immediately prior to peraluminous granitoid flooding, and Paleocene unroofing of the Idaho Batholith.