BEDROCK GEOLOGY OF FOUR SMALL ISLANDS IN PENOBSCOT BAY, MAINE

Black Island, Phinney Island, Orono Island, and Round Island are four islands in Penobscot Bay off the coast of Maine, in an area of abundant and diverse igneous intrusions. Previous geologic mapping indicated that these islands were underlain by a single granitic pluton of Devonian age - the "Swans Island" pluton.

In August 1999, a day was spent on these islands mapping the distributions of lithologies, and collecting representative samples. The rock samples were analyzed petrographically and chemically to characterize the different lithologic units. Although granite is the dominant bedrock lithology on this group of islands, there is one large (15m wide) basaltic dike that cuts across Orono Island, and there are two outcrops of biotite-schist / amphibolite on Black Island.

The basaltic dike is a plagioclase-clinopyroxene-othopyroxene-phyric tholeiitic basalt. It is clearly related to the large Mesozoic dikes found throughout New England.

Although these four islands had been previously mapped as being underlain by the Swan's Island pluton, the petrography, mineral chemistry, and whole-rock chemistry of the granite samples indicate that two distinct granitic plutons exist in the area. One pluton is a medium-grained, equigranular biotite granite, and the other is a coarse-grained, porphyritic biotite-hornblende granite. Similar granites found on Swan's Island immediately to the south were determined to be of distinctly different ages (Carlson and Bailey, unpub. data): the porphyritic granite yielded a U/Pb zircon age of 370 Ma (+5 Ma), while the medium-grained granite yielded an age of 424 Ma (+2 Ma). Although the boundary between these two plutons is not distinct in the field, it cuts through the western side of Black Island.