Narragansett Bay Group - Rhode Island Formation (Pennsylvanian)
Narragansett Bay Group - Rhode Island Formation - In northern Rhode Island, consists of gray to black, fine- to coarse-grained quartz arenite, litharenite, shale, and conglomerate, with minor beds of anthracite and meta-anthracite. In southern Rhode Island, consists of meta-sandstone, meta-conglomerate, schist, carbonaceous schist, and graphite. Plant fossils are common.
Narragansett Pier Plutonic Suite - leucocratic granite (Permian)
Narragansett Pier Plutonic Suite - leucocratic granite - White to gray leucocratic granite composed of microcline, oligoclase, and quartz. Up to 10% accessory muscovite and garnet, with lesser apatite, zircon, and monazite. Biotite and opaque minerals notably sparse or absent. Mainly massive, but locally exhibits flow foliation caused by alternating layers of pegmatite, aplite, and medium-grained equigranular granite; pegmatitic and aplitic material of mineralogy similar to the host granite. The granite contains widespread xenolithic inclusions and screens of metasedimentary rock, some of which are PAnbr; bedding and fabric in the screens generally is oriented parallel to that in the host metasedimentary country rock. Unit is a 5 km by 40 km batholith that underlies the southern coast of RI. Intrusion during the latter part of Alleghanian deformation of the Narragansett Basin is indicated by its Permian age, rapid cooling, and field relations. The age of the Narragansett Pier is well constrained by several factors. Brown and others (1978) report a Pennsylvanian fossil contained in an inclusion in the pluton. Radiometric ages of 276 Ma by Kocis and others (1978), 272+/-4 Ma by Hermes and others (1981), and 273+/-2 Ma by Zartman and Hermes (1987) are consistently Early Permian. Structural studies by Mosher (1983) and Reck and Mosther (1988) indicate that intrusion began during the third period of deformation in the Narragansett Basin. Argon release patterns (Dallmeyer, 1982) indicate that the granite and intruded sediments had cooled below argon retention temperatures of hornblende by the Late Permian and of biotite by the Early Triassic. Intruded by the Westerly Granite, which has been radiometrically dated at 276+/-7 Ma. The two granites are probably genetically related based on similarities in age, mineralogy, and geochemistry (Hozik, 1992).