GEOLOGIC MAPPING AND METAMORPHIC PETROLOGY OF PART OF THE EASTERN PIEDMONT GOOCHLAND TERRANE, VIRGINIA: EVIDENCE FOR EXTENDING THE GOOCHLAND GRANULITE TERRANE

The Goochland terrane of southern-central Virginia lies in the Piedmont between the Hylas mylonite zone on the east and the Spotsylvania lineament on the west. This terrane is further defined by its areal extent of relict granulite facies metamorphic assemblages. Recent research has established that rocks comprising the Goochland terrane are Grenville in age, and that the granulite event that produced these mineral assemblages is also Grenvillian.

This study involved geologic mapping and extensive petrography that may indicate an extension of granulite facies mineral assemblages northward into the Po River terrane. The study area lies approximately 40 mi NW of Richmond, VA in parts of the Beaverdam, Lake Anna East and Hewlett quadrangles.

Discontinuous layers or pods of intermediate to mafic gneisses, some containing relict granulite facies mineral assemblages, are distributed throughout the study area. Granulite assemblages found in these rocks include opx + cpx + plagioclase and cpx + garnet + plagioclase. A pelitic unit, possibly containing relict, high pressure granulite assemblages was mapped as a continuous unit through the study area. The dominant mineral assemblage in the pelitic gneisses include kyanite + K-feldspar + garnet. The abundance of kyanite and K-feldspar (no muscovite) in the pelitic unit may be a stable granulite (Grenville) assemblage at high pressure.

Other rock units mapped here display strikingly similar characteristics to the granulites and to the granulites that have been mapped in the Goochland terrane to the south. The units mapped here do reveal retrogressive textures like those in the southern Goochland terrane, but it is reasonable to suggest that the granulite event produced cpx + garnet + plagioclase in intermediate to mafic gneisses and kyanite + K-feldspar in pelitic rocks as stable granulite assemblages. This and other petrographic, textural, mineral chemical, and geographic proximity evidence support a northern continuation of the Goochland terrane.