Petrology of the Potassic Silver Peak Volcanic Center, Western Nevada

Abstract

The Silver Peak volcanic center is part of a late Tertiary volcanic province in western Nevada and eastern California characterized by slightly alkaline, potassic lavas. The rocks of the Pliocene Silver Peak center have an alkali-lime index of 55.5, a K2O/K2O + Na2O ratio of 0.5 at 60 percent SiO2, and range in composition from trachybasalt to rhyolite, with latite the most voluminous. Chemical and mineralogical variations in the intermediate and silicic lavas are compatible with differentiation by crystal fractionation. The differentiation trend is similar to calc-alkaline suites with progressive enrichment in silica and alkalis but no relative enrichment in iron. Relatively high water pressures during crystallization are indicated by the ubiquitious presence of biotite or hornblende. The parent magma of the intermediate and silicic part of the series is believed to have a composition of trachyandesite with approximately 56 percent silica. The potassic nature of trachyandesite in this province is tentatively related to depths of a Tertiary subduction zone beneath western North America. Associated trachybasalts of the Silver Peak center are nepheline normative and characterized by high strontium contents, low Rb/K and Rb/Sr ratios, and relatively high Sr87/Sr86 ratios (.7061). These are believed to represent a separate parent magma derived from material previously depleted in rubidium.