NEW CONSTRAINTS ON THE GEOMETRY OF MAFIC SHEETS AND DIKES FROM A GRAVITY SURVEY OF THE VINALHAVEN PLUTON, MAINE

The Vinalhaven pluton is part of the coastal Maine magmatic province. Field relations indicate that the pluton was constructed by multiple replenishments of mafic and felsic magma into an active, late Silurian magma chamber. Although the pluton is mostly granite, mafic rocks (both gabbros and basalts) are common and occur in vertical dikes, shallowly-dipping sheets, and balls of pillows. Vertical basalt dikes record intrusion of mafic magma into solid-state granite; shallowly-dipping gabbro sheets and balls of pillow basalts record intrusion of mafic magma into an active, liquid chamber. Here we use the geometry of mafic bodies to constrain the spatial and temporal evolution of this predominantly felsic magma chamber. A better understanding of the three-dimensional geometry of mafic bodies provides insight into these questions: What was the shape and size of the active (liquid) magma chamber? How did the geometry of the active magma chamber evolve through time?

The density contrast between mafic and felsic rocks (2.96 and 2.63g/cm^3) makes this pluton an excellent subject for a detailed gravity survey. Here we present data and models from a summer 2004 survey involving 170 stations spaced 0.5 to 3 km apart. Bouguer gravity values range from 7 to 19 milligals. Lower values correlate with map scale exposures of coarse- and fine-grained granite. Higher Bouguer gravity values correlate with: (1) map-scale exposures of gabbro in the S and E parts of the pluton; (2) adjacent exposures of coarse-grained granite; and (3) an ENE-trending band of porphyry exposed near the center of the pluton. Although modeling of the data is somewhat complicated by heterogeneous and poorly-exposed (i.e. underwater) country rock, preliminary results indicate that the pluton extends to about 2 km depth, consistent with previously published results for nearby plutons. Gabbro sheets, interpreted to represent the evolving floor of the magma chamber, are discontinuous and non-planar. In addition, a steep mafic dike associated with the porphyry is much more extensive at depth than surface exposures suggest.