ORIENTATION, SPATIAL DISTRIBUTION, AND CHEMICAL COMPOSITION OF PALEOZOIC DIABASE DIKES ON SWANS ISLAND AND NEIGHBORING ISLANDS, PENOBSCOT BAY, MAINE

The central Maine coast is underlain by an accreted terrane that is composed of over 100 plutons ranging in age from Silurian to Carboniferous. This region, referred to as the Coastal Maine Magmatic Province, has been the subject of numerous geological studies in recent years, yet the tectonic and magmatic history of the province is still poorly understood.

In the vicinity of Swans Island, late Silurian magmatism was bimodal, with excellent field evidence for the commingling and limited mixing of gabbroic and granitic magmas. Crosscutting the Silurian plutons are hundreds of diabase dikes that range in width from a few centimeters to over twelve meters. This study is the first to systematically examine the orientation and chemical composition of this diabase dike swarm in order to better understand the stress regime that existed, and the compositions of mafic magmas that were generated, during this period of bimodal magmatism.

Strike and dip measurements were made on 150 dikes in the field, and the trends of over 500 dikes were obtained from satellite and aerial imagery. Both data sets reveal that the dikes are strongly preferentially oriented, with strikes essentially due N-S, and with near vertical dips. Previous studies suggested that the dikes were oriented parallel to the dominant joints in the host granites; our studies indicate that this is not the case, with the two major sub-vertical joint sets in the host granite (and in the nearby Devonian granites) being nearly orthogonal at ~N25E and ~N75W.

Petrographic analysis reveals that all of the dikes have experienced greenschist facies metamorphism and are now composed almost entirely of amphibole, plagioclase feldspar, and Fe-Ti oxides; fresh groundmass clinopyroxene is uncommon. Preliminary whole-rock chemical data indicate that, despite the metamorphism, two distinct types of mafic magma were generated during this magmatic event (a high Ti-P and low Ti-P tholeiite); both with compositions more closely associated with a within-plate, extensional tectonic environment than with a convergent, subduction zone tectonic setting.