WHOLE-ROCK GEOCHEMISTRY AND U/PB ZIRCON AGE OF A GRANITIC PLUTON EXPOSED ON TWO SMALL ISLANDS IN PENOBSCOT BAY, MAINE

The geology of the central Maine coast is dominated by Silurian to Early Carboniferous plutons that intrude early Paleozoic metasedimentary and metavolcanic rocks. The plutons vary in composition, but most are either gabbroic or granitic, resulting in a strongly bimodal magmatic suite. The rocks that form the islands in Penobscot Bay are largely granitic: the eastern half of Swans Island, as well as nearby Frenchboro and Marshall Islands, are underlain by Silurian (~ 424 Ma) granite plutons, whereas the western half of Swans Island and many small islands to the north and west are underlain by younger (~ 375 Ma) Devonian plutons. The Silurian plutons are all cut by numerous small, north-trending, diabase dikes and, in many areas, exhibit extensive interaction and mingling with mafic magmas. In contrast, diabase dikes and / or evidence of coexisting mafic magmas are rare in the Devonian plutons.

The goal of our study was to determine the age and composition of granite bedrock exposed on the previously unmapped West Sister and East Sister islands, two small islands in Penobscot Bay. From aerial photographs, it appeared that neither island was cut by the diabase dikes characteristic of the surrounding Silurian plutons. Because of this, we hypothesized that the two islands may have formed during a younger magmatic event, and would therefore, have a different age and chemical composition.

Whole-rock chemical compositions of granites from the two islands were compared to those of other islands in the region. The results show that the granites on East and West Sister are similar in composition to the Silurian granites on nearby Swans Island and Frenchboro, and to some phases of the Devonian plutons: all are high silica (~ 75 wt. % SiO₂), metaluminous, A-type granites that cannot be distinguished easily on the basis of mineralogy or whole-rock geochemistry. An U/Pb zircon age of 397 +/- 9 Ma from the granite on West Sister indicates, however, that the islands did indeed form during a separate magmatic event that post-dated the bimodal, late Silurian magmatic event that formed most of the bedrock in this region. These new data indicate that granitic magmas of very similar composition were generated intermittently over a period of more than 50 Ma in the coastal Maine magmatic province.