AGE AND EMPLACEMENT DEPTHS OF GRANITIC PLUTONS, SWANS ISLAND, MAINE: ZIRCON GEOCHRONOLOGY, HORNBLENDE GEOBAROMETRY, AND MAGMATIC EPIDOTE

Swans Island lies along the southern Maine coast, in the center of the Coastal Maine Magmatic Province (CMMP). The island is underlain by two distinct plutonic suites: a bimodal gabbro - granite complex on the eastern portion of the island, and a rapakivi granite / granodiorite on the western portion. Recent U-Pb zircon ages reveal that the gabbro – granite complex is late Silurian in age (423.2 +/- 1.7 Ma), similar to the Cadillac Mountain granite ~7 km to the northeast (Wiebe et al., 1997), while the rapakivi granite is mid-Devonian in age (375.7 +/- 2.4 Ma).

The Silurian pluton intrudes a bimodal assemblage of greenschist facies metavolcanic rocks (basalts, rhyolites, and pyroclastic rocks), similar to the Silurian Cranberry Island volcanic sequence to the north (Seaman, 2010). The Silurian pluton also locally exhibits a miarolitic texture, suggesting a fairly shallow emplacement depth. No contacts of the Devonian pluton with the surrounding country rock are exposed; large xenoliths of various amphibolite facies meta-sedimentary rocks (e.g. biotite schists, calc-silicates, and biotite – amphibole grits) are, however, locally abundant.

Granodiorites from the Devonian pluton have been found to contain, albeit rarely, crystals of epidote with textures indicative of primary magmatic crystallization (e.g. intergrown with apatite, quartz, and plagioclase feldspar). The apparent presence of magmatic epidote in the Devonian pluton, along with the high-grade metamorphic rocks found as large xenoliths along the margin of the pluton, suggest a greater emplacement depth than for the Silurian gabbro – granite complex to the east. We hope to test this hypothesis by obtaining high quality mineral phase compositions in order to apply a number of proposed hornblende geobarometers. We also hope to be able to confirm the presence of magmatic epidote by observing compositional differences between the clearly secondary epidote present in many samples, and the few grains that appear to be texturally magmatic.