Porphyritic member [of Southbridge Formation]

Light- to medium-gray, fine-grained porphyritic massive to layered gneiss, composed of quartz, oligoclase, microcline, and biotite, with megacrysts 1 to 2 cm long of microcline.
State Connecticut
Name Porphyritic member [of Southbridge Formation]
Geologic age Silurian or Ordovician or both
Lithologic constituents
Major
Metamorphic > GneissLight- to medium-gray, fine-grained porphyritic massive to layered gneiss, composed of quartz, oligoclase, microcline, and biotite, with megacrysts 1 to 2 cm long of microcline.
Comments Part of Eastern Uplands; Iapetus (Oceanic) Terrane - Merrimack Synclinorium; Hebron Gneiss and equivalent formations (including equivalents of Paxton and Oakdale Formations of Massachusetts) (including Bigelow Brook and Southbridge Formations) (Silurian and Ordovician). The Southbridge Formation here includes strata formerly mapped in CT as the Hebron Formation and in adjacent MA as the Paxton Formation. It is assigned to the Paxton Group. A sillimanite schist member is described in CT; it is conformably overlain by biotite schist and gneiss. Age is Late Proterozoic(?) based on intrusion of 440 m.y. Hedgehog Hill gneiss into overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989) per CT013.
References

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

Pease, M.H., Jr., 1989, Correlation of the Oakdale Formation and Paxton Group of central Massachusetts with strata in northeastern Connecticut: U.S. Geological Survey Bulletin, 1796, 26 p.

https://pubs.er.usgs.gov/publication/b1796

NGMDB product
Counties Windham