Abstract

The igneous and metamorphic rocks of southeastern Massachusetts and Rhode Island occupy one of the most poorly understood geologic terranes in the northern Appalachians. Interest in this terrane has been renewed with the realization that it may contain importtant evidence concerning the opening and closing of the Atlantic Ocean and its predecessors (Wilson, 1966, 1973) and that some of the rocks in this region may be similar to the Avalon zone of Newfoundland (Williams, 1976; Robinson, 1976; Rast and others, 1976). In this paper, we describe the mineralogy and petrology of some Precambrian (?) granites in Rhode Island. Despite superimposed deformation and metamorphism, it is possible to identify relict igneous textures and to show that these granites formed from iron-enriched, alkaline magmas at a high level in the crust. High-grade meta-morphism and deformation has caused complete recrystallization of the feldspars in parts of the granite. Some of this metamorphism occurred during Permian time, but the effects of earlier episodes cannot be evaluated.

Geologic Setting

We have studied the Scituate Granite Gneiss (SGG), Hope Valley Alaskite Gneiss (HVA), and the Ten Rod Granite Gneiss (TRG) in southern and central Rhode Island (Quinn, 1971). The age of these granites is probably upper Precambrian or lower Paleozoic based on several whole-rock Rb-Sr isochrons. Day (1968 and unpub. data) has made preliminary analyses that suggest an age of about 600 m.y. for the three units. These preliminary data are supplemented by reports of granites with similar ages from nearby areas (Fairbairn and others, 1967; Hills and Dasch, 1972; Smith and Giletti, 1977; Kovach and others, 1977).

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