Hebron Gneiss

Interlayered dark-gray, medium- to coarse-grained schist, composed of andesine, quartz, biotite, and local K-feldspar, and greenish-gray, fine- to medium-grained calc-silicate rock, composed of labradorite, quartz, biotite, actinolite, hornblende, and diopside, and locally scapolite. Local lenses of graphitic two-mica schist.
State Connecticut
Name Hebron Gneiss
Geologic age Silurian and Ordovician
Lithologic constituents
Major
Metamorphic > SchistInterlayered dark-gray, medium- to coarse-grained schist, composed of andesine, quartz, biotite, and local K-feldspar; local lenses of graphitic two-mica schist
Metamorphic > Metasedimentary > Calc-silicate-rockgreenish-gray, fine- to medium-grained calc-silicate rock, composed of labradorite, quartz, biotite, actinolite, hornblende, and diopside, and locally scapolite
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 Hebron Formation is here restricted to rocks in the area south and west of the Willimantic dome, south of the Wangumbaug Lake fault, and west of a fault in the Fitchville quad delineated by the Lebanon Gabbro. It includes the type area of the Hebron. Other rocks formerly mapped as Hebron are here assigned to the Oakdale, Dudley, and Southbridge Formations. It is separated from all other stratigraphic units by fault or intrusive contacts across which no stratigraphy can be correlated. Age is Late Proterozoic(?) based on correlation with part of the Paxton Group (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 Hartford - Middlesex - New London - Tolland - Windham