THERMAL REJUVENATION OF THE VINALHAVEN GRANITE, COASTAL MAINE

A porphyry unit in the Vinalhaven intrusive complex (VIC) appears to record thermal rejuvenation of a silicic mush in response to a basaltic influx, comparable to rejuvenation events inferred from volcanics (Couch et al. 2001). The VIC, about 80 sq km in area, is located on Vinalhaven Island, Maine. A part of the Silurian coastal Maine magmatic province, it consists mainly of cg and fg granites with abundant inward-dipping gabbroic and hybrid sheets.

The porphyry unit sits above a unit of locally chilled gabbroic and hybrid rocks in the cg granitic core of the VIC. The porphyry and the mafic rocks form an E-W trending complex roughly 0.5 by 2.5 km. The southern (lower) part of the complex is dominated by basaltic and hybrid rocks and the northern (upper) part is dominated by porphyry with highly corroded and reacted phenocrysts in an aphanitic to fine-grained felsic matrix. Immediately above mafic hybrid rocks, the porphyry is phenocryst-rich (>40%), and the percent of phenocrysts decreases upward toward to the top of the complex (<20%). Contacts of the porphyry unit with cg granite are irregular and sharp but do not cut across crystals. Porphyry dikes project from the main body and locally disintegrate into globular masses within cg granite. The aphanitic matrix of some porphyry dikes may reflect pressure quenching related to volcanic eruption.

Corroded and reacted phenocrysts (plagioclase, quartz, K-feldspar and biotite) in the porphyry are equivalent in composition and comparable in size to crystals in the cg granite. Major and trace element compositions of the cg granite and the porphyry are nearly identical. These observations indicate that the porphyry was derived by melting the enclosing cg granite. Phenocrysts in a single thin-section show greatly varying degrees of corrosion and reaction, suggesting the porphyry was well stirred. The presence of biotite phenocrysts in the porphyry indicates that dehydration melting was not important and suggests that hydrous interstitial melt was present in the cg granite when the basaltic magma was emplaced. Assuming the areas of basalt and porphyry reflect their volumes, thermal calculations suggest that the amount of basalt is adequate to generate the porphyry from a granite crystal mush containing about 10-20% melt.