The Curecanti pluton – evaluating emplacement mechanisms and magma driving pressure of a mid-crustal Proterozoic intrusion
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There is a significant obliquity between the Curecanti pluton’s (1420 ± 15 Ma) margins, its internal foliation, and its host rock’s foliation. This pluton is a 5 km-long, 3 km-wide, and 0.5 km-thick discordant sheet of quartz monzonite exposed in the Black Canyon of the Gunnison, CO. Previous pressure-temperature estimates from the Vernal Mesa pluton’s aureole (1434 ± 2 Ma), 25 km northwest of the Curecanti pluton, suggests emplacement occurred at 600˚ ± 50˚C and 300 ± 100 MPa. The Vernal Mesa pluton was intruded as a wedge-shaped stock subparallel to and contemporaneous with movement on the NE-striking subvertical Black Canyon shear zone. In contrast, the Curecanti quartz monzonite apparently travelled some distance along its host rock’s foliation and then was emplaced as a subhorizontal tongue-shaped sheet that tapers out in the hinge zone of the km-scale F2 Curecanti antiform. The discordancy between the pluton’s margins, its internal foliation, and its host rock’s foliation is contrary to observations of many other tabular granitoids worldwide that are emplaced parallel to their host rock’s foliation and display a margin-parallel magmatic and/or solid-state foliation. Three transects across and through the Curecanti pluton are examined and display evidence for solid-state foliation development localized at the floor of the pluton, up to 10 m-up from the floor contact, and in correlative dikes just beneath the pluton. Evidence for submagmatic flow is preserved near the roof contact of the pluton. Strain accumulated in the pluton at least 75 m-up from its floor contact, but did not result in the development of a foliation. The discontinuity of wallrock and Curecanti pluton foliations along with the presence of high-temperature quartz deformation mechanisms in the pluton are consistent with the interpretation of high-temperature subsolidus deformation. In addition, the Curecanti pluton’s magma driving pressure is evaluated to show that a combination of neutral buoyancy, depth to the magma’s source region, and a rheological impediment are necessary to have formed this discordant pluton. Furthermore, geochemical data of the Curecanti pluton adds to a growing set of data on the worldwide suite of 1.4 Ga “A-type” igneous rocks emplaced during what may have been a regional orogenic event.