Narragansett Pier Plutonic Suite - porphyritic granite

Similar to Png, except contains phenocrysts of microcline and plagioclase up to 1.5 cm in a medium-grained groundmass. 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).
State Rhode Island
Name Narragansett Pier Plutonic Suite - porphyritic granite
Geologic age Permian
Lithologic constituents
Igneous > Plutonic > Granitic > GraniteSimilar to Png, except contains phenocrysts of microcline and plagioclase up to 1.5 cm in a medium-grained groundmass.
Comments Part of Avalon Terrane Narragansett Pier Plutonic Suite - intrudes both Hope Valley and Esmond-Dedham subterranes (e.g., stitching pluton). Contains abundant veins of pegmatite too small to be mapped individually. Secondary unit description per RI003.

Hermes, O.D. (comp.), Gromet, L.P. (comp.), Murray, D.P. (comp.), Hamidzada, N.A., Skehan, J.W., and Mosher, S., 1994, Bedrock geologic map of Rhode Island: Rhode Island Map Series No. 1, Kingston, Rhode Island, scale 1:100,000.

Hozik, M.J., 1992, Tectonic implications of the brittle fracture history of the Permian Narragansett Pier Granite, Rhode Island, IN Bartholomew, M.J., and others, eds., Basement tectonics 8; Characterization and comparison of ancient and Mesozoic continental margins; proceedings of the eighth international conference on basement tectonics: International Conference on Basement Tectonics Proceedings, no. 8, 8th International Conference, Butte, MT, August, 1988, p. 503-525.

NGMDB product
Counties Washington