Pre-Mesozoic evolution of Avalon terranes of southern New England

With the recognition of the Hope Valley shear zone (HVSZ) as a terrane boundary, the Esmond-Dedham terrane (EDT) was subdivided, and the western division was named the Hope Valley terrane (HVT). The oldest rocks of the HVT consist of schist, gneiss and quartzite (Plainfield Formation), and metavolcanic and metaplutonic gneisses and amphibolites (Waterford Group), some of the latter yielding a radiometric age of 620 Ma. Members of the Sterling Plutonic Suite, consisting of granite gneiss and alaskite gneiss, intrude these older units. An exact radiometric age could not be determined for the alkaline pluton, Joshua Rock Granite Gneiss, but is assigned to the broad age range from c. 380 to 280 Ma. The Narragansett Plutonic Suite yields a radiometric age of c. 273 Ma, is a terrane-linking plutonic sequence cutting through the HVSZ, and links the HVT to the EDT. The EDT has a stratigraphic sequence that in many respects is similar to that of HVT, but has pronounced differences that mainly consist of a wider range of rock units and ages represented. Additionally, the rocks of HVT, and especially those near coastal Connecticut, have been elevated more generally to higher metamorphic grades than the EDT. The Harmony Complex and the Blackstone Group predominantly consist of plutonic and volcanic rocks, and schist, quartzite, and basaltic volcanics, respectively, into which have been intruded members of the Esmond Plutonic Suite or rocks correlated with them. The Price Neck Formation, of the Newport Basin, contrasts notably with the Harmony and Blackstone, but is intruded by the Cliff Walk Granite, similar in age and composition to the Esmond, and consists predominantly of fine-grained graded sedimentary rocks with volcanogenic beds. Fossiliferous limestone, phyllite, and siltstone make up Lower and Middle Cambrian rocks of the Pirate Cave Formation and the Conanicut Group of the Newport Basin, rocks unknown in HVT. A large part of the EDT is underlain by alkaline plutonic and volcanic rocks of the Scituate Plutonic Supersuite, whose radiometrically determined age is c. 373 Ma. Fluvial coal-bearing sedimentary rocks (Rhode Island Group) of the Narragansett and related basins contain a rich floral assemblage, which permits accurate dating to Westphalian and Stephanian stages of the Carboniferous. These rocks are unrepresented in the HVT. On the basis of structural and metamorphic data for the above stratigraphic units, a pre-Mesozoic evolutionary history has been outlined from late Proterozoic through Permian events. Compressional tectonic events within the Avalon superterrane and the composite Avalon terrane include the late Proterozoic Avalonian orogeny and the Alleghanian orogeny; mid-Paleozoic rifting events are interpreted for the alkaline plutonic rocks. Collisions involving the Avalon composite terrane with terranes farther to the west were responsible for Acadian and possibly late-stage Taconian orogenic events elsewhere in southern New England.