MAGMA CHAMBER PROCESSES IN THE VINALHAVEN INTRUSIVE COMPLEX, MAINE

Many recent field studies of granitic plutons have recognized evidence for (1) multiple replenishments of compositionally variable magmas, (2) silicic magma chambers that underwent convection and crystal accumulation, and (3) rejuvenation of previously solidified granite. These processes indicate that, during the growth of the pluton, crystal-poor magma chambers existed and were appropriate sources for associated volcanic rocks.

Silurian bimodal intrusions on the Maine coast provide exceptional evidence for these processes. The Vinalhaven intrusive complex is about 12 km in diameter and consists mainly of granite that includes a ~2 km thick section of inward-dipping, originally subhorizontal, gabbro-diorite sheets that extend > 10 km. Granite intrudes a roof of cogenetic rhyolite that overlaps in age with the oldest portions of the granite. U-Pb zircon ages show the magmatic system was active for about 2 My. Gabbroic rocks form layers from 10 to 100 meters thick that can be traced for several km; they have chilled basaltic bases that grade upward through gabbroic cumulates to hybrid rocks and granite. Deformation of underlying granite suggests up to 10’s of meters of weak crystal mush. Country rock blocks (up to 100m) occur extensively associated with gabbro. The overlying granite has arcuate schlieren and enclaves, which define aggrading magma chamber floors.

Mafic input heated and enhanced convection in felsic magma chambers and locally rejuvenated older cumulates, producing porphyry that crystallized at higher T and lower H₂O. Thermal arguments permit order-of-magnitude estimates for the volume of silicic magma chambers using estimates for the volume of mafic input and the resultant heating of silicic magma. Application of the Ti in quartz thermometer (Wark and Watson, 2006) to Ti zoning in quartz crystals in granite overlying mafic rocks indicates temperature increases of 20° to 120°C. These DTs 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 heat provided by the mafic injections. The thickness of silicic magma was at least 100s of meters, and perhaps as much as a kilometer. Since the extent of basaltic sheets approached 100 km², the volumes of eruptible silicic magma was 10s to 100 km³.