Research Training Program

Ellen Hissong
College of William and Mary
Williamsburg, Virginia

Michael Wise, Ph.D.
Project Advisor
Department of Mineral Sciences

"The RTP is a wonderful chance to make contacts in the fields you're interested in."

Petrogenesis of a Metamorphosed Gabbroic Pegmatite from the Virginia Piedmont

Mafic pegmatites have been rather neglected in the geological community. Most references are merely a description of the rock; few articles delve into the origin or fractionation of these mafic pegmatites.

A gabbroic pegmatite from the Piedmont of Virginia was studied in detail, and is found to consist of an altered amphibole mineral (hornblende to actinolite) and plagioclase. Zoisite, quartz, muscovite and sericite are observed in the host rock and the pegmatite, and from these trends, the two phases are found to be cogenetic. Electron microprobe analysis, thin section study and point counts reveal that secondary alteration, possibly retrograde metamorphism is taking place. This secondary alteration has changed the original composition of the rock, most notably altering calcium plagioclase to a sodium plagioclase. These secondary fluids could be coming from a variety of places, but most likely they are either late stage magmatic fluids, outside hydrothermal fluids, or a combination of the two.

This research was supported by a grant from the National Science Foundation Research Experiences for Undergraduates (Award: BIR-9300225).