The new Bedford area; A preliminary assessment

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The New Bedford area is among the least studied in eastern New England, yet an understanding of its tectonic history is essential to the development of a viable paradigm for the assemblage of Avalonian terranes in New England. We review previous work and present the preliminary results of an ongoing program whose goal is to systematically study the structural, petrographic, and geochemical relations in the New Bedford area of southeastern Massachusetts. The region can be divided into two suites of rocks. Group I includes variably deformed granitoids that bear close resemblance to the Proterozoic- to Devonian-aged granitic basement of the Esmond-Dedham terrane. Of particular interest is a newly recognized unit of slightly deformed ∼600-Ma, alkalic granite plus diorite, in which contact relations imply comingling of magmas of contrasting composition. In contrast, Group II consists of alaskitic and banded gneisses and schist that are reminiscent of gneissic rocks in western Rhode Island and adjacent Connecticut. Trace-element analyses for both groups are presented, and form the basis for discussion of their possible correlation with other granitoids in southeastern New England. The dominant structural feature in the New Bedford area is a steeply dipping to vertical, east-west-trending foliation that becomes more pervasively developed to the south. This tectonic fabric is strongly oblique to the northeast-trending schistosity of the southern Narragansett Basin, and geometrically more comparable to the eastnortheast- trending folds found in the Carboniferous rocks of the northern Narragansett Basin. At present, the geometric and temporal relations among structural features in the New Bedford area and the adjoining Carboniferous rocks remain unclear. Although several interpretations of the regional setting of the lithologies of the New Bedford area are possible, currently the most plausible considers the lithologies of Group I to be a continuation of the Esmond-Dedham terranes, while Group II may be correlative with either the Hope Valley terrane or the Bass River Complex.

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Special Paper of the Geological Society of America