Document Type

Thesis

Abstract

The State Farm Gneiss, which may be the oldest member of Virginia’s Goochland Terrane, was originally mapped as a single unit. Later studies have determined that the State Farm Gneiss was intruded by a number of small granite bodies in the Neoproterozoic (~ 600 Ma). Because the gneiss and granites were both originally granitic, and both were affected by the same metamorphic events during the Acadian and Alleghenian orogenies, it is not surprising that they are quite similar, both in field appearance and mineralogy. Two plutons had been previously mapped in the southern exposed area of the State Farm Gneiss, but were only recently recognized to be Neoproterozoic (Owens and Tucker, 2000). This project was intended to map the areal extent of a previously unknown body of Neoprterozoic granite in the central portion of the main State Farm Gneiss dome. This body had been previously recognized on the basis of only a few, closely spaced outcrops, and an age of ~ 630 Ma was reported from one of these outcrops by Owens and Tucker (submitted). Based strictly on field appearance and mineralogy, this body does not appear to extend much beyond the originally recognized outcrops. However, one locality in the northern part of the map area contains somewhat leucocratic rocks that superficially resemble the Neoproterozoic rocks. These also contain the distinctive mineral fluorite, which is known from some other Neoproterozoic plutons. In one outcrop in this same area, the State Farm gneiss is intruded by a fluorite bearing dike, clearly indicating a younger age. Thus, Neoproterozoic magmatism in this map area may be somewhat more widespread than is indicated by the original outcrops.

Date Awarded

2002

Department

Geology

Advisor 1

Brent E. Owens

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