THE ROLE OF MAIDENS GNEISS IN UNDERSTANDING THE RELATIONSHIP BETWEEN MOREFIELD PEGMATITE AND OTHER GOOCHLAND TERRANE PEGMATITES

In central Virginia at the northern end of the southern Appalachians, exposed terranes include, from west to east, the Blue Ridge, the Western Piedmont, the Chopawamsic, the Goochland, and the Southeastern Piedmont. The Goochland terrane, located between the Spotsylvania fault zone in the west and Hylas fault zone in the east, is composed of multiply deformed and metamorphosed gneiss, amphibolite, granite, and anorthosite. The Maidens gneiss is a garnet-biotite-quartz-plagioclase gneiss containing lenses of pelitic gneiss and leucocratic gneiss. Although well documented, the pegmatite belt and Elk Hill Complex have been excluded from geologic maps since the second half of the twentieth century. No published geochronology exists for any of the rocks in these two fault blocks. Some later workers included a similar area of pegmatite on their geologic maps. However, the 1963 geologic map of Virginia depicts this area as an extension of the Columbia granite. Other workers recognized pegmatite in this area and interpreted it to be associated with the intrusion of the Columbia pluton. However, on the 1993 geologic map of Virginia, this area was mapped as biotite gneiss with small intrusions of biotite granite, all within the Chopawamsic terrane. These rocks are lithologically distinct and separated by faults from the Chopawamsic Formation and the Elk Hill Complex. The recently described Little Fork Church fault forms the western boundary of the pegmatite belt. The eastern boundary is defined by the Lakeside fault, which separates the pegmatite belt from the Elk Hill Complex. The pegmatite belt in Goochland terrane has not been studied in detail. The Morefield pegmatite, which is in the same unit as the other pegmatites, has been studied by several researchers. Our focus involves understanding the relationships and crystallization history of minerals in the rocks and descriptions of the internal zonation of pegmatites in Goochland terrane. The following specific research questions will also be tried to be answered in this study. (1) What is the crystallization history of pegmatites in Goochland terrane? (2) Are the pegmatites zoned and if so, how did the zonation develop? (3) Is there any relationship between these pegmatites and Morefield pegmatite?