THE TUNK LAKE PLUTON REVISITED

The Devonian (~360 Ma) Tunk Lake pluton of coastal Maine, USA is a concentrically zoned granitic body that grades from an outer hypersolvus granite to a middle subsolvus rapakivi granite zone, and to an inner subsolvus non-rapakivi granite. The pluton has been the subject of earlier studies by Karner (1968; 1971), Gylling (2001) and Seaman and Gylling (2003, 2005). Models previously suggested by Seaman and Gylling, focused on the thermal effects of a large underplate of basaltic magma, must be modified in light of recent compositional data and further field work. In the pluton, each zone grades into the next over several meters. The granite is zoned from the rim to the core in terms of mineral assemblage, mineral composition, zircon crystallization temperature, and major and trace element concentrations. The pluton is partially surrounded on the north and south by a contact zone of chaotic mingling of basaltic and/or gabbroic enclaves and xenoliths in a high-silica granitic matrix. Gradations in mineral assemblage and textural characteristics in the Tunk Lake pluton are consistent with gradients in temperature and water concentration in a cooling and crystallizing granitic body, independent of the effects of any underplating basaltic magma. The overall compositional similarity of the granite, despite differences in mineral assemblage, suggests that the granites are comagmatic, and that the gradients in temperature and water concentration across the magma chamber developed as the magma cooled and crystallized. Slight variations in initial Nd isotopic ratio in granites from different zones of the pluton suggest that contamination of the granitic magma by coexisting basaltic magma played little role in generating the granite zones of the pluton. The high-silica granite that surrounds the Tunk Lake pluton on the northern and southern boundaries and hosts chilled magmatic mafic enclaves and blocks of mafic rocks is interpreted as a late intrusion of felsic melt from a deeper source related to the Tunk Lake pluton. Passage of the high-silica magma through a basaltic layer either within a deeper reservoir, or below the Tunk Lake pluton, would have resulted in incorporation of basaltic enclaves and gabbroic cumulate crystal mush in the felsic magma.