Grand Tour - Part 4: Geology and geochemistry of the Harrison Pass pluton, central Ruby Mountains, Nevada

Harrison Pass pluton is a late Eocene, composite granitoid body with related mafic and hypabyssal rocks. The pluton was emplaced at upper-crustal levels probably during the initiation of a phase of large-magnitude crustal extension that resulted in the eventual exhumation of the Ruby Mountains core complex. Post-emplacement tilting facilitated the exposure of a partial cross section of the pluton from a roof zone exposed on the east side of the range to granitic rocks on the west side of the range that were originally about 5 km deeper than the roof zone. The pluton is composed of two distinct suites of granitic rocks. The oldest suite consists of granodiorite and minor, related rock types that occupy the structurally higher eastern half of the pluton. Compositional variation within the weakly peraluminous to metaluminous granodioritic suite can be explained by crystal fractionation and magma-mixing processes. This plutonic suite was the magmatic source of hypabyssal dikes and sills that cut country rocks in the roof zone. Emplacement of the granodioritic suite was accommodated by stoping, wall-rock shortening, and uplift. Slightly younger granitic rocks comprise the monzogranitic suite which underlies much of the western half of the pluton. Three distinct rock types are recognized: two-mica monzogranite which forms a body informally called the "granite of Green Mountain Creek," other two-mica monzogranitic rocks, and biotite monzogranite. These granitoids are peraluminous and chemically, but not isotopically, similar to Jurassic monzogranite exposed in the central Ruby Mountains. Except for the granite of Green Mountain Creek, much of the monzogranitic suite consists of layered, tabular units that were emplaced as composite sheets intruded into rocks of the granodioritic suite.