CONSTRAINING THE 3-D STRUCTURE OF THE VINALHAVEN INTRUSIVE COMPLEX, ME, USING OUTCROP AND GRAVITY DATA

The ~420 Ma Vinalhaven Intrusive Complex (VIC) on Vinalhaven Island, ME, is part of the coastal Maine magmatic province, and records intrusion of a multi-compositional pluton. Previous studies have constrained the compositional nature and the general surface structure of the VIC, indicating that the pluton was likely constructed by multiple replenishments of mafic and felsic magma into an active magma chamber. At the surface the pluton is mostly granite, with mafic rocks (gabbros and basalts) occuring in vertical dikes, shallowly-dipping sheets, and balls of pillows. However, little is known about the sub-surface structure of the VIC. The large density contrast between felsic and mafic units makes it feasible to conduct a detailed gravity survey on the pluton. To visualize the VIC at depth, gravity data were compiled from a 2004 survey involving 170 stations spaced 0.5 to 3 km apart, with 168 of the stations being used in this study. A previous gravity inversion on this data yielded 2-dimensional results; this study presents 3-dimensional results. Modeling results suggest that the VIC volume might exceed 50 km^3, which is much more voluminous than outcrops indicate. Also, although exposures are largely felsic, results suggest that the VIC is likely composed primarily (>70%) of mafic material. Also, the preferred gravity inversion model indicates that a large, NW dipping mafic dike cross-cuts the felsic rocks, suggesting at least partial late-stage mafic intrusion.