Petrology and geochemistry of the late Eocene Harrison Pass pluton Ruby Mountains core complex, Northeastern Nevada

The late Eocene Harrison Pass pluton was emplaced in the transition zone between the infrastructure and suprastructure of the Ruby Mountains core complex. Emplacement was at ∼ 3 kbar pressure and was in two stages: early stage tonalitic to monzogranitic magmas, followed by late-stage monzogranites and mafic dikes. The early stage began with emplacement of biotile ± hornblende granodiorite of Toyn Creek, followed by the biotile monzogranite of Corral Creek. Quenched equivalents of these units are preserved as porphyritic dikes in the roof. Leucocratic monzogranite that forms cupolas in the roof zone represents the product of fractional crystallization of the early magmas. Al-in-hornblende barometry and the presence of magmatic epidote suggest that early stage magmas resided at a pressure of ∼ 5-6 kbar before emplacement in the upper crust. Compositional variation in the Toyn Creek and Corral Creek units is essentially linear, and can be explained by mixing of a tonalitic end member with a monzogranitic end member such as evolved samples of the monzogranite of Corral Creek. The tonalitic end member was itself a hybrid that formed by interaction of mafic magma with crustal melts. The monzogranitic end member is a crustal melt that escaped hybridization. Nd isotopic compositions of the early stage are heterogeneous and do not correlate with degree of differentiation, which is consistent with a compositionally heterogeneous felsic end member. Elemental variation in the early stage of the pluton is unusual because of its essentially linear trends, which suggest a single mixing event before emplacement in the upper crust. Late-stage activity consisted of three pulses of monzogranitic magma plus sparse mafic dikes. The largest and youngest of these pulses, the two-mica monzogranite of Green Mountain Creek, is distinct from all other units in the presence of restitic enclaves, low ε_Nd and high initial ⁸⁷Sr/⁸⁶Sr. Pod-like bodies of leucocratic biotite ± amphibole monzogranite and sheets and dikes of leucocratic two-mica monzogranite make up the other late-stage granites. These rocks display deep negative Eu anomalies and show wide, non-systematic concentrations of high field strength elements; however, their isotopic compositions are identical to those of the early stage rocks. They are thought to be small melt fractions of the lower to middle crust. Their elemental compositions are thought to result from the effects of residual plagioclase and accessory minerals. The variable and non-systematic isotopic compositions of the granitic units in the pluton suggest a heterogeneous source region, such as the Proterozoic Mojave province.