MIDDLE DEVONIAN VOLCANIC AND GRANITIC ROCKS FROM THE GULF OF MAINE

Samples were collected from bedrock outcrops over an across-strike distance of more than 100 km in the central Gulf of Maine using the submersible Alvin in 1971-1972. Many of the samples come from an area associated with an elliptical positive magnetic anomaly that is part of the Central Plutonic Zone identified from aeromagnetic data. However, the sample locations span the location of the Fundy Fault, the likely boundary between Ganderian and Avalonian terranes of the northern Appalachian orogen. To further investigate these strategically located rocks, we obtained samples from the archives at the Woods Hole Oceanographic Institute. Initial work during the 1970's had suggested that the samples are of Ordovician age; however, 2 volcanic samples have yielded U-Pb ages of 393.8 ± 1.3 Ma and 393.0 ± 2.6 Ma, and preliminary results from 3 plutonic samples are similar, indicating that the suite is mid-Devonian. Based on petrochemical characteristics, plutonic samples from both east and west of the Fundy Fault are A-type, within-plate alkali-feldspar granite and syenogranite. They contain iron-rich clinopyroxene, amphibole, and biotite. Most are peralkaline, with high concentrations of SiO₂, Y, Zr, Nb, and REE. They show strong negative Eu anomalies. The dated samples all have positive epsilon Nd values (1.8 to 3.7). The volcanic samples are more altered than the plutonic samples, but are chemically similar, consistent with their similar age. A plutonic sample from the location farthest west of the Fundy Fault differs from the other samples in being fine-grained, recrystallized monzogranite with garnet. That sample is undated but resembles Silurian granite which characterizes the Kingston arc terrane in Ganderia in southern New Brunswick. The sample from the location farthest east is also different, being a granophyric monzogranite with higher Fe, Mg, and Ca than the peralkaline suite. Reliably dated felsic plutonic or volcanic rocks with ages of ca. 400 Ma are relatively rare in the onshore part of the northern Appalachian orogen. The New Hampshire Plutonic Suite may be of that age, but is dominantly mafic to intermediate. The tectonic implications of these rocks, for which we suggest the name Cashes Ledge Igneous Suite after the prominent bathymetric feature from which some of the samples were collected, is uncertain.