Petrogenesis of the Spruce Pine pegmatites, North Carolina

Pegmatites in the Spruce Pine district, North Carolina, are an example of muscovite class pegmatites that were derived from differentiation of a magma. There are eight granodiorite plutons in the Spruce Pine area ranging in size from 1 to 20 square kilometers that contain and are surrounded by hundreds of pegmatites. The granodiorite and the pegmatites are mineralogically and chemically identical, indicating that they formed from a common magma. The pegmatites appear to have formed in the granodiorite, becoming larger and more abundant near the margins of the plutons before intruding into the surrounding country rock.

Major and trace element data from muscovite plotted on a map of the area reveal a systematic regional trend where either the highest or lowest concentrations occur along a central northeast-trending belt in the Spruce Pine district, with the concentration either decreasing or increasing, respectively, to the northwest and southeast. Similarly, more calcic plagioclase is located in the same central portion of the district, whereas more sodic plagioclase is found at the margins of the district. These regular variations in mineral chemistry are the result of fractionation of the pegmatites away from the core of a common magmatic source. K-feldspar, garnet, beryl and tourmaline also have similar regional trends. Although the chemical core of the area revealed by the mineral chemistry does not correspond to an exposure of granodiorite, it may correspond to the core of a larger plutonic body that has not been completely exposed by erosion. Variations in granodiorite texture may reflect zoning within this larger magmatic body. The systematic regional variations in the major and trace element chemistry of minerals as well as the relationship between the granodiorite and the pegmatites support the conclusion that the Spruce Pine pegmatites were derived from differentiation of the granodiorite magma.