FEEDER SYSTEMS OF MAFIC REPLENISHMENTS INTO THE VINALHAVEN INTRUSIVE COMPLEX, COASTAL MAINE

The Vinalhaven intrusive complex, located in coastal Maine, is about 12 km in diameter and consists mainly of coarse-grained granite above an inward-dipping gabbro-diorite unit comprised of interlayered gabbro, diorite and granite sheets; both units are cut by a fine-grained granite core about 3 km in diameter. The gabbro-diorite unit was formed by multiple mafic replenishments that broke into and spread across the floor of a silicic magma chamber. Excellent coastal outcrops on Lane Island expose the feeder systems of two major mafic replenishments that break through well-layered gabbro, diorite and granite. The older feeder is exposed along 200 meters of the coast and dips moderately to the east. Its western base consists of tightly packed basaltic pillows, tubes and sheets commingled with and chilled against a granitic matrix. These pillows grade laterally and upward into massive and then layered gabbro with variable grain sizes. At higher levels, the gabbro grades upward into moderately dipping, thin sheets and pillows with increasingly prominent seams of felsic material between them. These and higher-level gabbros were contaminated by mixing with felsic magmas and have distinctive hybrid textures. Rocks within 30 meters of the eastern wall of the feeder consist of scattered and broken chilled pillows and meter-scale blocks of older gabbro in a hybrid quartz-bearing diorite. This body is cut by a younger intrusion of tightly packed chilled basaltic pillows in a felsic porphyry matrix. These relations indicate that, as gabbro magma broke its way upward through the layered rocks, silicic magma was also emplaced contemporaneously and commingled with it.