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Sterling Igneous Suite - granite gneiss

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Pale pink to gray, medium-grained granite gneiss, commonly with small porphyroclasts of microcline/orthoclase. Similar to alaskite gneiss, but with more than 3% dark minerals. Composition is granite with generally less than 3% dark minerals. Compositions range from quartz monzonite to granite. Composed of sodic plagioclase, quartz, microcline, biotite, opaque minerals; minor muscovite common, garnet more rare; accessory apatite and zircon; sphene present in some rocks; secondary chlorite. Typically is compositionally homogeneous, with strong foliation and locally well-developed lineation defined by major minerals. Includes some rock mapped formerly as Ten Rod Granite Gneiss, Hope Valley Alaskite Gneiss, Potter Hill Granite Gneiss, and Scituate Granite Gneiss.
State Rhode Island
Name Sterling Igneous Suite - granite gneiss
Geologic age Late Proterozoic
Lithologic constituents
Major
Metamorphic > Gneissgranite gneiss - Pale pink to gray, medium-grained granite gneiss, commonly with small porphyroclasts of microcline/orthoclase. Similar to alaskite gneiss, but with more than 3% dark minerals. Composition is granite with generally less than 3% dark minerals. Compositions range from quartz monzonite to granite. Composed of sodic plagioclase, quartz, microcline, biotite, opaque minerals; minor muscovite common, garnet more rare; accessory apatite and zircon; sphene present in some rocks; secondary chlorite. Typically is compositionally homogeneous, with strong foliation and locally well-developed lineation defined by major minerals.
Comments Part of Hope Valley Subterrane. Age of 601 +/-5 Ma by U/Pb methods on zircon is reported by Hermes and Zartman (1985) from a sample in RI per RI005. Per printed map: age of rock based on radiometric data from RI005, RI009, and RI012.
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.

Hermes, O.D., and Zartman, R.E., 1985, Late Precambrian and Devonian plutonic terrane within the Avalon Zone of Rhode Island: Geological Society of America Bulletin, v. 96, p. 272-282.

Hermes, O.D., Gromet, L.P., and Zartman, R.E., 1981, Zircon geochronology and petrology of plutonic rocks in Rhode Island, p. 315-338: in Boothroyd, J.C., and Hermes, O.D., editors, Guidebook to Field Studies in Rhode Island and Adjacent Areas: 73rd New England Intercollegiate Geological Conference, University of Rhode Island, Kingston, 383p.

Zartman, R.E., Hermes, O.D., and Pease, M.H., Jr., 1988, Zircon crystallization ages, and subsequent disturbance events, in Gneissic rocks of eastern Connecticut and western Rhode Island: American Journal of Science, v. 288, p. 376-402.
NGMDB product
Counties Kent - Providence - Washington