NEW GEOCHEMICAL AND GEOCHRONOLOGICAL DATA FROM TWO GRANITIC ISLANDS OF DOWNEAST MAINE: MOUNT DESERT ROCK AND GREAT DUCK ISLAND

Great Duck Island (GDI) and Mount Desert Rock (MDR) are two remote islands off the southern coast of Mount Desert Island (MDI), Maine. Both islands are working field stations maintained by College of the Atlantic. Each island has been host to many ecological research projects during the last decade, although none have specifically focused on the bedrock and surficial geology. Maine Geological Survey maps show both islands are composed predominantly of Devonian granite, similar to nearby islands including MDI. Building on new preliminary surficial and bedrock maps of the islands, here we present new geochemical and geochronological data for the granites of each island. Beyond the range of available high-resolution aerial satellite and LiDAR data, MDR currently lacks a high-resolution topographic base-layer. Thus, by processing new drone imagery using Agisoft Photoscan, we produced a georeferenced orthomosaic and digital surface model for the entire 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.