AN EARLY RECORD OF RHEIC OCEAN FORMATION IN THE NORTHERN APPALACHIANS: THE NORTH HAVEN FORMATION IN PENOBSCOT BAY, MAINE

Fault-bounded volcanic rocks on North Haven Island in Penobscot Bay, Maine have been largely unstudied, leaving their age and the tectonic environment responsible for their eruption unclear. The North Haven Formation is pre-Silurian in age and dominated by weakly metamorphosed basaltic rocks (>90%) with lesser amounts of felsic volcanics, gabbro, and sedimentary rocks. Detailed petrographic analysis, whole rock geochemistry, and U-Pb geochronology of the volcanic rocks provide new insights into the origin of the formation.

In the field, many of the basaltic rocks display pillow structures and a porphyritic texture dominated by fine-grained chlorite and phenocrysts of plagioclase. Felsic rocks show considerable variability and include both flows and tuffs. Whole rock geochemical analysis reveals the mafic rocks (n=8) are high TiO₂ sub-alkaline basalts and the felsic rocks (n = 5) are rhyolites and dacites. Chondrite-normalized REE plots for the mafic rocks reveal flat patterns, whereas the felsic samples show LREE enrichment relative to HREE. Basaltic rock compositions on tectonic discrimination diagrams consistently plot in MORB or within-plate tectonic settings. This combined with the high TiO₂ contents and the absence of Nb anomalies suggests rocks of the North Haven Formation were not generated through subduction processes.

U-Pb zircon ages (LA-ICP-MS) were obtained from two felsic samples from North Haven Island. A sample collected from the eastern end of the island gave a weighted mean age of 519 ± 12 Ma (n=11). This Cambrian age is consistent with geological relationships and is interpreted to represent the eruptive age of volcanic rocks in the North Haven Formation. A second sample, collected from the central area of the island, gave an anomalously young Silurian-Devonian age and we suggest these rocks may represent a faulted inlier of younger volcanic rocks exposed to the south.

Similarities in the age and geochemistry of volcanic rocks on North Haven Island with those in the Ellsworth Terrane support previously proposed correlations between these units. Following the tectonic model of Schulz et al. (2008) for the Ellsworth Terrane, we suggest the North Haven Formation formed during the rifting of microcontinents off the Gondwanan margin as part of the formation of the Rheic Ocean in the Middle Cambrian.