15 Petrology and Tectonics of the Southern Appalachians

Thursday, 29 March 2007
1:20 PM - 5:20 PM

PETROGRAPHY OF THE COUNTRY ROCK SURROUNDING THE CRABTREE PEGMATITE, SPRUCE PINE DISTRICT, MITCHELL COUNTY, NORTH CAROLINA

Paper number: 15-3
Program: THEME SESSIONS
Time: 2:00 PM – 2:20 PM

Authors

Abstract

The beryl-cordierite-tourmaline-bearing Crabtree pegmatite was emplaced within amphibolite facies, kyanite grade gneiss and schist of the Ashe Metamorphic Suite. 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 is characterized by a suite of minerals consisting of calcite, chlorite (high-Mg), cordierite, plagioclase (An ~30%), quartz, and tremolite with trace amounts of allanite, biotite, garnet, sapphirine (?), titanite, and detrital zircon. The three-mica schist is composed of biotite (high-Fe), chlorite, white mica (muscovite), quartz, and plagioclase with trace amounts of magnetite, titanite, vesuvianite, and detrital zircon. The granofels does not retain any fabric elements of events prior to the hydrothermal event beyond the presence of detrital zircon. The schist retains a well-formed mica preferred orientation, a parallel but weak compositional banding foliation, quartz ribbons, and detrital zircon. Protolith of the granofels may have been a dolomitic mudstone whereas the protolith of the schist could have been compositionally immature sandstone (greywacke).

The Crabtree pegmatite and country rock were affected by a brittle-ductile deformation event. Where weakly deformed, the pegmatite contains plagioclase that has developed strongly curved and kinked deformation twins, cordierite with a sweeping extinction, and some quartz with well-formed deformation lamellae and prismatic subgrains. Weakly deformed granofels displays Type I calcite twins and sparse deformation twinning in plagioclase. However, where it is strongly sheared, the 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. Calcite, chlorite, and tremolite are absent in the strongly sheared granofels. 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. The three-mica schist appears not effected by this deformation even though nearby pegmatite and granofels display considerable deformation features.