Late-Stage Mafic Injection and Thermal Rejuvenation of the Vinalhaven Granite, Coastal Maine

The Vinalhaven intrusive complex consists mainly of coarse-grained granite, inward-dipping gabbro–diorite sheets, and a fine-grained granite core. Small bodies of porphyry occur throughout the coarse-grained granite. The largest porphyry body (roughly 0·5 km by 2·5 km) occurs with coeval gabbro, hybrid rocks, and minor fine-grained granite in the Vinal Cove complex, which formed during the waning stages of solidification of the coarse-grained Vinalhaven granite. Porphyry contacts with surrounding coarse-grained granite are irregular and gradational. Compositions of whole rocks and minerals in the porphyry and the coarse-grained granite are nearly identical. Neighboring phenocrysts in the porphyry vary greatly in degree of corrosion and reaction, indicating that the porphyry was well stirred. Thermal rejuvenation of a silicic crystal mush by a basaltic influx can explain the composition and texture of the porphyry. Comparable rejuvenation events have been recognized in recent studies of erupted rocks. Weakly corroded biotite phenocrysts in the porphyry require that hydrous interstitial melt existed in the granite during remelting. Field relations, along with thermal calculations, suggest that cooling and crystallization of coeval mafic magma could have generated the porphyry by thermal rejuvenation of granite crystal-mush containing about 20% melt. Field relations also suggest that some of the porphyry matrix may represent new felsic magma that was emplaced during remelting.