193 Mineralogy/Crystallography; Petrology, Experimental; Igneous; Metamorphic (Posters)

Monday, 6 October 2008
8:00 AM - 6:00 PM

Mineralogy and Petrography of the Metamorphosed and Altered Country Rock Surrounding the Beryl and Tourmaline-Bearing Crabtree Pegmatite, Spruce Pine District, Mitchell County, North Carolina

Paper number: 193-41
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

  • James A. Dockal Presenting Author
    University of North Carolina Wilmington, Wilmington, NC
  • Michael Smith
    University of North Carolina Wilmington, Wilmington, NC

Abstract

The Crabtree pegmatite is a beryl (emerald, green beryl, aquamarine, yellow beryl) and tourmaline-bearing granitic pegmatite emplaced within amphibolite facies, kyanite grade gneiss and schist of the Ashe Metamorphic Suite. Texturally unzoned, it has weakly developed differentiation and variously assimilated xenoliths. Fluids accompanying the pegmatite caused extensive hydrothermal alteration of the country rock resulting in the formation of retrograde greenschist facies granofels and three-mica schist. The granofels consists of calcite, chlorite (high-Mg), cordierite, plagioclase (An ~30%), quartz, and tremolite with (trace) allanite, biotite, garnet, titanite, and detrital zircon. The three-mica schist has biotite (high-Fe), chlorite, white mica, quartz, and plagioclase with (trace) magnetite, titanite, vesuvianite, and detrital zircon. The granofels does not retain any fabric elements of events prior to the hydrothermal event beyond detrital zircon. The schist retains a well-formed mica preferred orientation, a parallel but weak compositional banding foliation, quartz ribbons, and detrital zircon. Protoliths may have been a dolomitic mudstone (granofels) and compositionally immature sandstone (schist).

The Crabtree Pegmatite and country rock were variably affected by a brittle-ductile deformation event. Weakly formed pegmatite contains plagioclase that has developed strongly curved and kinked deformation twins, cordierite with sweeping extinction, and some quartz exhibiting well formed deformation lamellae and prismatic subgrains. Weakly deformed granofels displays Type I calcite twins and sparse deformation twinning in plagioclase. Strongly sheared pegmatite contains fragmented feldspar, cordierite, quartz and tourmaline. Strongly sheared granofels displays fragmented and fractured feldspar and cordierite with fractures healed with small equant quartz grains. In both rock types a matrix consisting of variable amounts of epidote, sericite, and quartz occurs between the fragments. This matrix is well foliated and crenulated. Calcite, chlorite, and tremolite are absent in the strongly sheared granofels. Surprisingly, the schist appears not effected by this deformation even though proximal to the nearby pegmatite and granofels.