METAMORPHIC EVOLUTION OF THE HOUSE MOUNTAIN GNEISS COMPLEX, SOUTH WEST IDAHO

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 lower Neoproterozoic and upper Jurassic orthogneiss units that both intrude into a central Neoproterozoic paragneiss unit composed of pelites, mafic amphibolites, quartzofeldspathic gneisses, and calc-silicates. All of the gneisses are crosscut by deformed (P1; 82 Ma), and undeformed (P2; 66 Ma) pegmatites; intrusion of the regional Idaho Batholith peraluminous biotite granodiorite is bracketed between these pegmatite ages. U-Pb geochronology of zircon, titanite, monazite, and rutile, collected from each of the different lithologies in the paragneiss sequence documents the timing of mineral growth during metamorphism. The oldest metamorphic accessory minerals are interpreted to record a portion of the prograde limb of the P-T-t path. Single grains of titanite collected from diopside marbles yield an age range from 90-103 Ma, while a nearby thick, structurally coherent mafic amphibolite unit yields overlapping titanite ages ranging from 95-102 Ma, along with metamorphic zircon ages of 124 and 121 Ma. In other portions of the paragneiss sequence, discontinuous amphibolite boudins are often observed mantled by a pelitic unit. Strain shadows of these structures contain leucosomal melts accumulated during deformation, indicating that deformation was coincident with magma injection. Titanite from one amphibolite boudin dates to 81 ± 2 Ma and monazite from associated pelites dates between 90 and 72 Ma, with half of the analyses clustering between 82 and 84 Ma; these ages are synchronous with emplacement of the P1 pegmatites and may represent peak metamorphic conditions. U-Pb ages from rutile indicate cooling at ~66 Ma, shortly after emplacement of the P2 pegmatite. These geochronologic data will be augmented with trace element thermobarometry (Ti-in-Zircon, Zr-in-Titanite, Zr-in-Rutile) and metamorphic phase equalibria analyses to develop a detailed P-T-t path for the gneiss complex.