Plainfield Formation plus Potter Hill Granite Gneiss plus Narragansett Pier Granite

Plainfield Formation - Interlayered light-gray, thin-bedded quartzite, in places with feldspar, mica, graphite, or pyrite, light- to medium-gray gneiss composed of quartz, oligoclase, and biotite (rarely microcline), medium- to dark-gray schist composed of quartz, oligoclase, biotite, sillimanite, and garnet, dark-gray or green gneiss composed of plagioclase, quartz, biotite, and hornblende (commonly with diopside), amphibolite, diopsite-bearing quartzite, and calc-silicate rock. In places contains quartz-sillimanite nodules. Potter Hill Granite Gneiss - Light-pink to gray, tan-weathering, fine- to medium-grained, rarely porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline, quartz, oligoclase (or albite), biotite, and magnetite, minor muscovite, and local garnet. Narragansett Pier Granite - Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite, and accessory muscovite and magnetite. Considerable associated pegmatite.
State Connecticut
Name Plainfield Formation plus Potter Hill Granite Gneiss plus Narragansett Pier Granite
Geologic age Proterozoic Z? and Permian
Lithologic constituents
Major
Metamorphic > Metasedimentary > Metaclastic > QuartzitePlainfield Formation - Interlayered light-gray, thin-bedded quartzite, in places with feldspar, mica, graphite, or pyrite; diopsite-bearing quartzite.
Igneous > Plutonic > Granitic > GraniteNarragansett Pier Granite - Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite, and accessory muscovite and magnetite.
Metamorphic > GneissPlainfield Formation - light- to medium-gray gneiss composed of quartz, oligoclase, and biotite (rarely microcline), dark-gray or green gneiss composed of plagioclase, quartz, biotite, and hornblende (commonly with diopside). Potter Hill Granite Gneiss - Light-pink to gray, tan-weathering, fine- to medium-grained, rarely porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline, quartz, oligoclase (or albite), biotite, and magnetite, minor muscovite, and local garnet.
Minor
Metamorphic > SchistPlainfield Formation - medium- to dark-gray schist composed of quartz, oligoclase, biotite, sillimanite, and garnet.
Incidental
Igneous > Plutonic > Granitic > Leucocratic-granitic > PegmatiteNarragansett Pier Granite - Considerable associated pegmatite.
Metamorphic > AmphibolitePlainfield Formation
Metamorphic > Metasedimentary > Calc-silicate-rockPlainfield Formation - calc-silicate rock
Comments Part of Eastern Uplands; Avalonian (Continental) Terrane. Units are described seperately on printed map; combined in digital map.
References

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

NGMDB product
Counties New London