THE PHYSICAL IMPACT OF MAFIC REPLENISHMENTS ON SILICIC MAGMA CHAMBERS IN THE VINALHAVEN INTRUSIVE COMPLEX, MAINE, USA

The Vinalhaven Intrusive Complex is a bimodal, shallow level pluton dominated by granite (72-74% SiO2). Within it, basaltic injections produced a gabbro-diorite unit more than 2 km thick with gabbroic layers between 10 and 100 meters thick. The bases of these layers are chilled against more felsic rocks and commonly grade upward through hybrid rocks to granite. After basaltic magma is injected, one would expect convective velocities on the order of 0.5 to 2 meters/day in overlying crystal-poor silicic magma (Snyder, 2000). Granite 10s of meters above mafic sheets contains mafic enclaves up to 20 cm in diameter; their settling velocity approximates the expected maximum due to mafic input. Their occurrence appears to indicate that 10s of meters of granitic crystal mush accumulated as convective velocity decayed and larger mafic enclaves settled out.

Heat from the basaltic injections went dominantly upward, via convection into an overlying silicic magma. The nature of preservation does not reveal the thickness of crystal-poor silicic magma at the time of mafic injection, but a rough, order-of-magnitude estimate can be made via thermal arguments. Application of the Ti in quartz thermometer (Wark and Watson, 2006) to Ti zoning in quartz crystals in the overlying granite indicates temperature increases of 20° to 120°C. These temperatures place an approximate upper bound on the temperature increase in the silicic magma and a lower bound on the thickness of silicic magma (melt and crystals) needed to absorb the heat provided by the mafic injections. Given the thickness of the mafic magma and the thermodynamic properties of the two magmas, the lower bound on the thickness of silicic magma was at least 100s of meters, and perhaps as much as a kilometer. If it were thinner, it would have responded by a greater temperature increase.