NEW GEOCHEMICAL AND GEOCHRONOLOGICAL DATA FROM TWO GRANITIC ISLANDS OF DOWNEAST MAINE: MOUNT DESERT ROCK AND GREAT DUCK ISLAND
The entire ~7 acre island of MDR is underlain by a fine to medium-grained light gray, phaneritic A-type granite with less than ~5% mafic minerals. Conversely, GDI is composed of more diverse bedrock with a fine-grained, bright pink, phaneritic A-type granite, also quite low in mafic minerals, a welded flow-banded rhyolite, and a zone of altered metasedimentary rocks similar to the Bar Harbor Fm on MDI. Preliminary 238U/206Pb zircon ages of the granites from both islands suggest that they are significantly younger than the Silurian and Devonian granites of nearby MDI and Swans Island with GDI yielding a age of 313.3 +/- 3.0 Ma (Carboniferous) and MDR an age of 296.5 +/- 2.9 Ma (Permian). Bedrock fractures on MDR and GDI follow similar trends as other nearby coastal regions (oriented about 090 and 180) despite their younger ages. Taken together, these new geochronologic, geochemical, field, and remote-based spatial data enable us to refine the volcanic and tectonic history of this portion of the easternmost edge of North America.