EMPLACEMENT MECHANISMS AND MAGMA DRIVING PRESSURE OF THE PROTEROZOIC CURECANTI PLUTON; THE BLACK CANYON OF THE GUNNISON, COLORADO

There is significant obliquity between the margins of the Curecanti pluton, an internal foliation, a coeval swarm of ~400 pegmatite dikes just west of the pluton, and the host rock foliation. This pluton is a 5 km-long, 3 km-wide, and 0.4 km-thick sheet of monzogranite exposed in the Black Canyon of the Gunnison, CO. The pluton is discordant along most of its length, but has a >100-m-thick root at its western margin subparallel to the foliation in the host rock gneisses. A cordierite + anthophyllite + staurolite + garnet schist from the Vernal Mesa pluton aureole was previously dated (1.4 Ga) and indicates emplacement occurred at 600*C [degrees C] ± [plus or minus] 50*C and 300 ± 100 MPa. We present evidence that indicates emplacement of the Curecanti pluton, 25 kilometers-southeast of the Vernal Mesa pluton, may have occurred during similar conditions. The Vernal Mesa pluton was intruded subparallel to, and contemporaneous with movement on the NE-striking, subvertical Black Canyon shear zone. In contrast, Curecanti monzogranite was emplaced as a tongue-shaped sheet that tapers out in the hinge zone of the kilometer-scale F2 Curecanti antiform. The discordance between the pluton margins, an internal foliation, and the host rock foliation is contrary to observations of many other tabular granitoids worldwide that are emplaced parallel to host rock foliation and display a margin-parallel foliation. Three transects through the Curecanti pluton display evidence for solid-state foliation development localized in the pluton floor and in correlative dikes just beneath the pluton. Evidence for submagmatic flow is preserved near the roof of the pluton. Strain accumulated in the pluton at least 75 m above the floor, but did not result in the development of a foliation. The decoupling of wall-rock fabric and Curecanti pluton foliations, along with the presence of high-temperature quartz deformation mechanisms in the pluton, indicate high-temperature subsolidus deformation. In addition, Curecanti pluton geochemistry and magma driving pressure are evaluated to show that a combination of neutral buoyancy, depth to the magma source region, and a rheological impediment are necessary conditions to form this partly discordant peraluminous pluton.