MINERALOGY OF PEGMATITES AND HOST GRANODIORITES IN THE SPRUCE PINE PLUTONIC SUITE

The Spruce Pine plutonic suite consists of numerous Devonian (390-410 ma) peraluminous granitic plutons, dikes, sills, and associated pegmatites. The plutons and associated pegmatites intrude the biotite schists and amphibolites of the Ashe Metamorphic Suite. Contacts between the coarse-to very coarse-grained garnet-bearing muscovite granodiorite and pegmatites varies from sharp to gradational, reflecting late-stage crystallization of the pegmatites. Some pegmatites display compositional and/or textural zoning. The mineralogy of pegmatites and host granodiorites from seven different locations was studied to determine the extent of fractionation of the pegmatites relative to the host granitic rock. Localities sampled and studied include an exposure on Crabtree Creek Road, Field Mine, Hootowl Mine, Bon Ami Mine, and an exposure on Roan Road. Compositions of garnet, muscovite, plagioclase, and alkali feldspar were determined by electron microprobe. Garnets in the plutons range from 61.9 % almandine to 52.6 % spessartine. Some garnets display epitaxial overgrowths of up to 80.3 % grossular, in which the Ca preferentially replaces Mn or Fe. Mg is always very low, lending pyrope components of 0 to 3.3 %. Garnet compositions from the pegmatites are chemically similar to garnet compositions from the host granodiorite. No systematic variation is seen in garnet compositions from different bodies or different zones in a pegmatite body. Primary igneous muscovite from Spruce Pine plutons and pegmatites has 1.21 to 7.11 % total Fe. Pegmatitic muscovite is compositionally similar to muscovite from the host granodiorite. Plagioclase compositions in the granodiorite range from Ab99 to Ab74, and Or3 to Or0.3. Pegmatitic plagioclase compositions range from Ab98 to Ab69, and Or4 to Or0.3. Alkali feldspar composition in the host granodiorite ranges from Or99 to Or95, and Ab5 to Ab0.7. Pegmatitic alkali feldspar compositions range from Or98 to Or95, and Ab5 to Ab2. The lack of compositional variation between minerals in pegmatites and host granodiorites indicates the pegmatites were not the result of extreme fractional crystallization.