Porphyritic phase [of Potter Hill Granite Gneiss]

Light- to medium-gray, fine- to medium-grained, porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline (much of it as megacrysts up to 4 cm long), quartz, oligoclase, biotite and magnetite. Sterling Plutonic Suite is here restricted to the Hope Valley terrane. (The Hope Valley together with the Esmond-Dedham terrane make up the Avalon superterrane of this report.) The Ponaganset Gneiss and the Ten Rod Granite Gneiss lie within the Esmond-Dedham terrane and are therefore removed from the Sterling. The Ponaganset is interpreted as an intensely deformed phase of the Esmond Plutonic Suite. The Sterling, as defined here, includes only the Hope Valley Alaskite Gneiss and the Potter Hill Granite Gneiss. These rocks intrude the Plainfield Formation and the Waterford Group in southeastern CT and western Rhode Island and are Late Proterozoic in age. The gneiss of the Potter Hill is mainly weathered, deeply stained, somewhat crumbly, and generally slabby. Contains xenoliths of the Plainfield Formation. Is distinguished from Hope Valley by its higher biotite content (Skehan and Rast, 1990).
State Connecticut
Name Porphyritic phase [of Potter Hill Granite Gneiss]
Geologic age Proterozoic Z?
Lithologic constituents
Major
Metamorphic > GneissLight- to medium-gray, fine- to medium-grained, porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline (much of it as megacrysts up to 4 cm long), quartz, oligoclase, biotite and magnetite.
Comments Part of Eastern Uplands; Avalonian (Continental) Terrane; Avalonian Anticlinorium; Sterling Plutonic Group (Proterozoic Z?). Secondary unit description per CT016.
References

Rodgers, John, compiler, 1985, Bedrock Geological Map of Connecticut: Connecticut Geological and Natural History Survey, Hartford, Connecticut, 2 sheets, scale 1:125,000.

Skehan, J.W. and Rast, Nicholas, 1990, Pre-Mesozoic evolution of Avalon terranes of southern New England, IN Socci, A.D., Skehan, J.W., and Smith, G.W., eds., Geology of the composite Avalon terrane of southern New England: Geological Society of America Special Paper, 245, p. 13-53.

NGMDB product
Counties New London