CORRELATION AND STRATIGRAPHY OF EXOTIC BLOCKS IN THE VINALHAVEN PLUTON, MAINE, TO THE SEAL COVE FORMATION

Vinalhaven Island, located in lower Penobscot Bay, Maine, is a bimodal pluton rimmed by stratified sedimentary and volcanic rocks. The island is ideal for exploring questions related to magma chamber dynamics, because its coastal exposures and granite quarries offer a nearly complete cross section from the bottom of the chamber in the south to the top of the chamber in the north. To better define the chamber dynamics, exotic blocks in the pluton were examined and compared to the stratified rocks that rim the island. Blocks of metamorphosed bedded tuffs in the pluton correlate well with the Seal Cove Formation exposed on the north-central coast of Vinalhaven. The exposed Seal Cove Formation is composed primarily of 0.1-2.0 meter thick layers of quartzite and calc-silicate, and also includes a garnet-rich rock (80% garnet) and tuff breccia. The exposure is cut by a granitic sill. Blocks of the Seal Cove Formation, sized from a few meters to tens of meters, are located at Round Neck, on the south-central coast, and on Greens Island, southwest of Vinalhaven. Similar to the in-place exposure, these blocks contain 0.1 to 1.7 meter thick layers of quartzite and calc-silicates, and are locally intruded by granite from the surrounding pluton. Both the blocks and the exposed Seal Cove Formation show evidence of contact metamorphism. Garnet compositions with a high andradite component (and45-68, gro41-18) provide supporting evidence to suggest that the protolithic tuff was calcic, possibly andesitic in nature, and underwent metasomatism resulting from thermal metamorphism related to the emplacement of the Vinalhaven pluton. The position and attitudes of the exotic blocks seem to support suggestions that the magma chamber underwent convection; however, the large, coherent nature of the blocks provides convincing evidence that the magma in the chamber was not turbid