PETROLOGY, AGE, AND GEOCHEMISTRY OF THE YARMOUTH ISLAND FORMATION, CASCO BAY, MAINE: INSIGHTS INTO THE PALEOZOIC TECTONIC EVOLUTION OF MID-COASTAL MAINE

The Yarmouth Island Formation of the East Harpswell Group, exposed in northern Casco Bay, Maine, is a deformed assemblage of metamorphosed volcanic and sedimentary rocks. Due to a lack of previous detailed study, the original depositional age, the tectonic setting of deposition, and relationship to surrounding rocks has been unclear.

Detailed petrologic and geochemical studies of the meta-volcanic rocks in the Yarmouth Island Fm. indicate a near continuous range of sub-alkaline compositions (SiO₂ = 47.9 to 77.5 wt.%). Trace element studies show relatively flat chondrite-normalized REE distributions (10-60x chondritic abundances), and MORB-normalized spider plots reveal a prominent negative Nb anomaly. The depositional age of the Yarmouth Island Fm. is interpreted to be 472 ± 5 Ma based on new U-Pb zircon dates obtained from two felsic meta-volcanic rocks in the unit. This age is significantly older than previously reported and requires revisions to the current tectonic model for the region. New detrital zircon dates (n=121) collected from the overlying Sebascodegan Fm. in the East Harpswell Group indicate detritus derived from terranes of a peri-Gondwanan origin. Collectively, the new U-Pb data indicate formations in the East Harpswell Group are similar in age (Early to Middle Ordovician) and tectonic affinity (peri-Gondwanan) to adjacent rocks of the Casco Bay Group.

The wide range of volcanic rock compositions, the negative Nb-anomaly, and trace element geochemistry suggest volcanic rocks of the Yarmouth Island Fm. were erupted in an active volcanic arc tectonic setting with some continental crustal influence. Because marine sedimentary rocks are located both within and stratigraphically above the unit, the data are most consistent with a transitional setting between continental arc subduction and subsidence, perhaps due to the onset of back-arc rifting. In contrast with prior findings, these data indicate the East Harpswell and Casco Bay groups are contemporaneous and collectively likely represent along-arc variations in deposition in Early to Middle Ordovician time. These peri-Gondwanan volcanic terranes were initially accreted to the ancient North American margin in the Silurian, and subsequently folded, metamorphosed to amphibolite facies, and intruded by granitic rocks during the Devonian.