Cenozoic Volcanic Rocks in the Southern Shoshone Mountains and Paradise Range, Nevada

Abstract

The Cenozoic volcanic rocks in the southern part of the Shoshone Mountains and most of the Paradise Range consist predominantly of lava flows and pyroclastic rocks ranging in composition from rhyolitic to basaltic, and minor volcaniclastic sedimentary rocks. In the Shoshone Mountains, the stratigraphic column is more than 3,000 m thick, of which all but 150 m consist of lava flows and pyroclastic rocks, the remainder being volcaniclastic sediments. New formation names are given to two units in the lower part of the section that are well exposed near Mission Spring and Third Canyon, respectively. In the Paradise Range, lava flows and pyroclastic rocks of rhyolitic to andesitic composition 625 m thick constitute the entire Cenozoic section. In the lower part of the section, only the Third Canyon Formation of the Shoshone Mountains section is recognized in the Paradise Range; it is identified on the basis of stratigraphic position and characteristic propylitic alteration. A prominent ignimbrite unit higher in the section in both ranges is obviously the correlative of the Toiyabe Quartz Latite originally described in the Round Mountain quadrangle. Published K/Ar dates give an absolute age range of 19.3 ± 0.6 to 21.5 ± 0.6 for this formation in the area of the lone and Paradise Peak quadrangles. Dikes and pluglike intrusive rocks range in composition from rhyolite to trachyandesite. The relative position assigned to the various intrusive rocks, purely on the basis of superposition, is in good chemical and mineralogical agreement with the chemistry and mineralogy of the intruded and overlying rocks in each case, suggesting that most dikes and plugs are feeders. Chemical analyses and C.I.P.W. norms for 14 lava flows and pyroclastic and associated intrusive rocks show that all are oversaturated with respect to quartz, all are quartz-hypersthene normative, and all belong to the calc-alkaline suite. Some of the rocks have co-menditic and potassic rhyolite tendencies.