Contributions to mineralogy and petrology

A geochemically and mineralogically diverse group neous aspect can form as products of different proof granitoids is present within an area of 900 km2 in the cesses . . . " Within an area of 900 km2 in the southern Snake southern Snake Range of eastern Nevada. The granitoids Range, White Pine County, Nevada, six igneous masses exposed range in age from Jurassic through Cretaceous to are well exposed in discrete outcrops. These granitoids Oligocene and include two calcic intrusions, two different range in age from Jurassic to Oligocene, in SiO, content types of two-mica granites, and aplites. The younger intrufrom 63 to 76 wt.%, in 6 '80 from -2.6 to + 13.2 permil, sions appear to have been emplaced at progressively more and in initial 87Sr/86Sr from 0.7071 to 0.7157. The petroshallow depths. All of these granitoid types are represented logic types exposed include two calcic (Peacock 1931) intruelsewhere in the eastern Great Basin, but the southern sions, two different kinds of two-mica granites, and aplites. Snake Range is distinguished by the grouping of all these The purpose of this report is to summarize and integrate types within a relatively small area. the information in some 30 papers on the petrology, age The Jurassic calcic pluton of the Snake Creek-Williams relations, and systematic mineralogy of these rocks, and Canyon area displays large and systematic chemical and to speculate on the processes that may have been involved mineralogical zonation over a horizontal distance of five in their formation. km. Although major element variations in the pluton comWe first describe a well-studied pluton from the southpare closely with Daly's average andesite-dacite-rhyolite ern Snake Range using models of its trace element and over an SiO, range of 63 to 76 percent, trace element (Rb, isotopic composition to demonstrate the role of fractional Sr, Ba) variations show that the zonation is the result of crystallization in developing its strong compositional zonain situ fractional crystallization, with the formation of relation. We then compare it with other plutons from the area tively mafic cumulates on at least one wall of the magma and attempt to explain their origins from Sr and 0 isotope chamber. Models of trace element and isotopic data indirelationships. cate that relatively little assimilation took place at the level In this report we refer to a classification of granites of crystallization. Nonetheless, an initial 87Sr/86Sr value according to source materials (Chappell and White 1974). of 0.7071 and d S 8 0 values of 10.2 to 12.2 permil suggest In the scheme, granites inferred to be derived from predomia lower crustal magma that was contaminated by upper nantly sedimentary protoliths are S-types, whereas those crustal clastic sedimentary rocks before crystallization. The derived from igneous source materials are I-types. The involvement of mantle-derived magmas in its genesis is diffichemical, mineralogical, and isotopic criteria used to distincult to rule out. Two other Jurassic plutons show isotopic guish between Sand I-type granites have been summarized and chemical similarities to the Snake Creek-Williams Canby Chappell and White (1974), O'Neil and Chappell (1977), yon pluton. Beckinsale (1979), and Furgeson et al. (1980). As is apparCretaceous granites from eastern Nevada that contain ent in the discussion to follow, application of these criteria phenocrystic muscovite are strongly peraluminous, and to a particular pluton sometimes leads to ambiguous results. have high initial Sr-isotope ratios and other features characThe central part of the area studied is about 60 km teristic of S-type granitoids. They were probably derived southeast of Ely, Nevada, in the southern part of the Snake from Proterozoic metasediments and granite gneisses that Range of eastern Nevada (Fig. 1). The granitoid rocks of comprise the middle crust of this region. this area intruded part of a lower Paleozoic miogeosynclinal Another group of granitoids (including the Tertiary apsequence of predominantly quartzite and carbonate rocks; lites) show chemical, mineralogic, and isotopic characteristhey are exposed east of the Mississippian Antler orogenic tics intermediate between the first two groups and may have belt (Gilluly 1963, pp 139, 140) and just west of the Cretabeen derived by contamination of magmas from the lower ceous Sevier orogenic belt (Armstrong 1968, p 436). None crust by the midcrustal metasediments. of these plutons has been deeply unroofed. With the possible exception of the Osceola intrusion, probably none of these granitoid masses has been eroded to a depth of more Introduction than 300 m. All of the plutons to be described (Fig. 2) are exposed beneath the Snake Range decollement. Parts or Faure and Powell (1 972, p 43) stated: "the crux of the granite problem is that it is possible that granitic rocks of igall of the area studied have been mapped by Drewes (1958), Misch and Hazzard (1962), Whitebread (1969), and Hose Offprint requests to: D.E. Lee and Blake (1976).