LOW-PRESSURE, HIGH-TEMPERATURE CRUSTAL XENOLITHS OF THE VINALHAVEN PLUTON, VINALHAVEN ISLAND, MAINE

The Silurian Vinalhaven pluton is part of the Coastal Maine Magmatic Province. The pluton consists of granite to the north and becomes gabbroic to the south. The host rocks to the north are andesite and metamorphosed quartzite; pluton boundaries elsewhere are below water. As part of a KECK undergraduate research project, R.A. Wiebe and others are developing a model of magma intrusion at low pressure for the Vinalhaven Pluton.

Contact metamorphosed, detached roof pendants of host rocks occur at 17 locations in two types—a migmatite, and a gneissic plagioclase biotite quartzite. The melanosome of the migmatite contains andalusite, corundum, spinel, plagioclase feldspar and biotite in a cordierite matrix. The leucosome typically consists of quartz and alkali feldspar with late muscovite and is separated from the melanosome by biotite selvage. Garnet is present in three migmatite localities in both leucosome and melanosome assemblages.

Mineral compositions for the migmatite samples are determined with SEM/EDS. The resulting data are compared to petrogenetic grids and used to calculate pressure and temperature of equilibration. Assemblages of grt+plag+qtz+bt as well as grt+cord+and+qtz provide information for pressure calculations. Assemblages of grt+cord in addition to grt+bt are used to calculate temperatures. Preliminary findings suggest that the migmatite unit experienced temperatures near 750° C based on the grt-bt thermometer of Ferry and Spear with Berman (1990). These results are similar to those obtained from nearby xenoliths by Porter (1999).

An important objective is to determine the source of the migmatite—was the leucosome produced by injection of granite from the pluton? Or, do these migmatites result from contact anatexis? If so, to what extent is the melting caused by dehydration vs. injection of water upon crystallization of the pluton.