Geology and petrology of Cretaceous and Tertiary granitic rocks, Lamoille Canyon, Ruby Mountains, Nevada

Abstract

Lamoille Canyon cuts through the northern Ruby Mountain metamorphic core complex and exposes the deep interior, which is characterized by highly metamorphosed miogeoclinal sedimentary rocks and various late Cretaceous and Tertiary granitic intrusions.

Late Cretaceous granitic rocks in Lamoille Canyon can be broadly divided into two groups based on field relations: equigranular two-mica granitic gneiss and pegmatitic sillimanite-bearing two-mica granitic gneiss. The former rock unit is distinct in its higher concentrations of Fe, Mg, Na, Ca, Sr, V, Zr, Zn, Hf, but lower K, Rb, Th. In spite of these elemental differences, both of the late Cretaceous granitic units in this region are strongly peraluminous, have similar S^18 values, and are closely associated in the field. The origin of the pegmatitic two-mica granitic gneiss is best modeled by muscovite dominated dehydration melting of a metapelitic source, whereas the equigranular twomica granitic gneiss formed by plagioclase-imited biotite dehydration melting of a metapelitic source.

Oligocene biotite monzogranite and related tonalitic dikes were emplaced in middle crustal levels (5-6 kbar). The biotite monzogranite suite consists of three geochemically distinct sub-groups. Group I shows characteristic geochemical features of A-type granite and is likely to have been generated by reaction of mantle-derived basaltic magma with either Archean orthogneiss or Proterozoic metapelite, leaving abundant plagioclase, orthopyroxene, and clinopyroxene as cumulative phases. Compared to group I, group II lacks characteristics of A-type granite. Partial melting of a metapelitic source explains most of the observed geochemical and stable isotopic data of group II. Geochemical variations within group I and group II can be explained by fractional crystallization but the possibility of minor crustal assimilation cannot be excluded. The compositions of tonalitic dikes are best explained by magma mixing between mantle derived (?) basaltic magma and granitic magma of the biotite monzogranite suite.

Systematic geochemical comparison between the late Cretaceous and Oligocene granitic intrusions suggests temporal transition from early deep-seated crustal anatexis of a garnet-bearing source in a thickened crust, to later interaction between mantle-derived magma and crust, or high-temperature biotite-dehydration melting in a shallower crustal environment.