SHALLOW BIMODAL PLUTONISM IN THE GOULDSBORO COMPLEX, COASTAL MAINE MAGMATIC PROVINCE

The Gouldsboro pluton on the Schoodic peninsula preserves a cross-section showing mafic-felsic hybridization and fractional crystallization. The Gouldsboro pluton intrudes an undated greenstone overlain by the 477 ± 18 Ma (Metzger, 1977) Bar Harbor Formation. 10cm-2m diameter blocks of both greenstone and Bar Harbor Formation are found in granite. The Gouldsboro granite is the most voluminous unit: fine to medium grained alkali feldspar, quartz, plagioclase, hornblende. Biotite concentration is variable but increases to the south (where highest levels are exposed). Miarolitic cavities are common at higher levels, along with felsic enclave-rich zones (Wiebe and Adams, 1995). Other units, from base to top, are: hybrid diorite-gabbro, granodiorite, granite with basalt pillows, and a previously unrecognized pyroclastic unit that is a volcanic breccia similar to the lowest member of the Cranberry Island Series (Seaman et al., 1999) and is intruded by the Gouldsboro pluton. The location of volcanic rocks support our interpretation that the southern part of the system reflects solidification near the roof of the chamber. In the northeast, closer to the pluton base, evidence of felsic-mafic mechanical interaction, as well as stoping and buckling of country rock exists. The gabbro-granite contact can be gradational or brecciated. Enclave rich zones are associated with latest mafic intrusions. South of the gabbro-granite contact are pillowed mafic sheets striking east-west and dipping shallowly north. Pillow and enclave rich zones suggest multiple episodes of mafic injection that manifested differently due to thermal conditions of silicic magma at the location/ level of injection. Mass movement analogs eg. (Harper et al.,2004) may also be applicable to production of breccia-like features at the floor of the system. The similarity of volcanic breccias in the Gouldsboro complex to the "shatter-zone" described around the Cadillac granite (Gilman et al., 1988) suggest that the Gouldsboro may have erupted. Reconnaissance mapping east of Schoodic peninsula shows isolated packages of volcanic rock identical to exposures described here. Late mafic injections into the sheet-like Gouldsboro magma chamber appear to have been the heat source for melt segregation, extraction and eruption during regional extension.