FIELD AND PETROGRAPHIC INVESTIGATIONS OF ENCLAVES FROM THE OAK POINT GRANITE, DEER ISLE, MAINE

Several plutons of coastal Maine show evidence for mingling/mixing of mafic and felsic magmas (i.e. Pleasant Bay, Isle au Haut). Such evidence includes cuspate/lobate contacts between felsic and mafic units, diapirs, pillow structures, composite dikes, and concentrations of mafic magmatic enclaves. These enclaves are fine-grained relative to the host granite and have been suggested to represent magma blobs separated from an injected body.

The Deer Isle pluton (~300 km²) lies in Penobscot Bay, coastal Maine. The pluton consists of two distinct facies, the Stonington and Oak Point Granites. Composite dikes and abundant magmatic enclaves within the Oak Point Granite suggest interaction of mafic magma with the granitic melt.

The Oak Point facies (371±2 Ma) is coarse-grained, seriate, biotite granite, including orange 1-2 cm alkali feldspars that often show rapakivi textures. The granite is well exposed along the coastlines of eastern Deer Isle, Brooklyn and Flye peninsulas and several small islands. Feldspar foliation is prominent in some outcrops. Composite dikes and two distinct mafic units occur on both the Brooklyn and Flye peninsulas. Other features in the Oak Point Granite include aplite dikes and schlieren layers.

Fine-grained, equigranular mafic magmatic enclaves are common with in the Oak Point granite. Shapes are planar/tabular with ellipsoid to disk forms. Thickness of enclaves range from 1 to 10 cm. Length of enclaves typically range from 30 to 60 cm, however some are 1 to 3 m long. Orientations of enclaves west of Eggemoggin Reach are generally NW to SE (115-145°), and those to the east are generally NE-SW (40-70°). Dip directions are towards the west (30-50°). Feldspar foliation shows similar attitudes. Enclaves often have feldspar megacrysts similar to those in the host granite. Ocellar textures exists in some enclaves. Current petrographic and geochemical investigations are intended to determine source and petrogenesis of the enclaves.