Narragansett Pier Plutonic Suite - porphyritic granite (RIPnpg;0)

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	Major 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.
References	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	NGMDB product page for 19719
Counties	Washington

State

Rhode Island

Name

Narragansett Pier Plutonic Suite - porphyritic granite

Geologic age

Permian

Lithologic constituents

Major

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.

References

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

NGMDB product page for 19719

Counties

Washington