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 be tween the infrastructure and suprastructure of the Ruby Mountains core comp lex. Emplacement was at similar to3 kbar pressure and was in two stages: ea rly stage tonalitic to monzogranitic magmas, followed by late-stage monzogr anites and mafic dikes. The early stage began with emplacement of biotite /- hornblende granodiorite of Toyn Creek, followed by the biotite monzogran ite of Corral Creek. Quenched equivalents of these units are preserved as p orphyritic dike sin the roof. Leucocratic monzogranite that forms cupolas i n the roof zone represents the product of fractional crystallization of th early magmas. Al-in-hornblende barometry and the presence of magmatic epido te suggest that early stage magmas resided at a pressure of similar to5-6 k bar before emplacement in the upper crust. Compositional variation in the T oyn Creek and Corral Creek units is essentially linear, and can be explaine d 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 m ember was itself a hybrid that formed by interaction of mafic magma with cr ustal melts. The monzogranitic end member is a crustal melt that escaped hy bridization. Nd isotopic compositions of the early stage are heterogeneous and do not correlate with degree of differentiation, which is consistent wi th a compositionally heterogeneous felsic end member. Elemental variation i n the early stage of the pluton is unusual because of its essentially linea r trends, which suggest a single mixing event before emplacement in the upp er crust. Late-stage activity consisted of three pulses of monzogranitic ma gma plus sparse mafic dikes. The largest and youngest of these pulses, the two-mica monzogranite of Green Mountain Creek, is distinct from all other u nits in the presence of restitic enclaves, low epsilon (Nd), and high initi al Sr-87/Sr-86. Pod-like bodies of leucocratic biotite +/- amphibole monzog ranite and sheets and dikes of leucocratic two-mice monzogranite make up th e other late-stage granites. These rocks display deep negative Eu anomalies and show wide, nonsystematic concentrations of high field strength element s; 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 m iddle crust. Their elemental compositions are thought to result from the ef fects of residual plagioclase and accessory minerals. The variable and non- systematic isotopic compositions of all granitic units in the pluton sugges t a heterogeneous source region, such as the Proterozoic Mojave province.