Collinsville Formation

Mixture of rock types as described for the two members; in many areas felsic and mafic striped metavolcanic rocks predominate.
State Connecticut
Name Collinsville Formation
Geologic age Middle Ordovician
Lithologic constituents
Major
Metamorphic > Amphibolite
Metamorphic > Gneiss
Minor
Metamorphic > Metaigneous > Metavolcanicin many areas felsic and mafic striped metavolcanic rocks predominate
Incidental
Metamorphic > Granulite
Comments Part of Central Lowlands; Iapetus (Oceanic) Terrane - Connecticut Valley Synclinorium; Gneiss Dome Belt. Hawley Formation and equivalent formations (includes Collinsville Formation) (Middle Ordovician). Two members referred to in primary description are units: Ocs and Ocg. Secondary unit description per CT008. Secondary unit description from USGS Geologic Names lexicon (ref. CT017): Metamorphic strata in the report area that lie above the Bristol Gneiss (as it is here revised) and below The Straits Schist are assigned to the Collinsville Formation. Unit is divided into a lower unnamed hornblende gneiss member (was uppermost part of Stanley's (1964) Bristol Member) approximately 400 ft thick, an unnamed metaquartzite member that averages 100 ft, and the upper Sweetheart Mountain Member adopted here as defined by Stanley. Inferred age is Middle Ordovician. Correlates, at least in part, with the Hawley Formation of MA (Simpson, 1990). Also from ref. CT017: Part of Bronson Hill anticlinorium, not part of Rowe-Hawley zone, but discussed in this report to emphasize importance of junction between Rowe-Hawley zone and Bronson Hill anticlinorium. Collinsville is exposed in Granville and Shelburne Falls domes, and to a limited extent in Goshen dome (Hatch and Warren, 1982). Consists of various light-colored, plagioclase-rich gneisses and interlayered amphibolite and hornblende gneiss. Upper part is amphibolite-rich; lower part is predominantly feldspathic gneiss. In places, thin aluminous feldspathic schist, locally containing coticule and amphibolite, forms an upper unnamed member of formation in Granville dome, or is Sweetheart Mountain Member of Stanley (1964) in Collinsville and Bristol domes in CT. Most complete sequence is in Shelburne Falls dome where L.M. Hall (1977, written commun.) has recognized seven mappable subdivisions. With minor modification, they are (ascending) 1) amphibolite with thin felsic gneiss layers; 2) rusty-weathering massive granulites; 3) very homogeneous garnetiferous biotite gneiss; 4) interlayered amphibolite and felsic gneiss; 5) felsic gneiss with scattered biotite +/-magnetite, garnet, and hornblende; 6) interbedded amphibolite and white felsic gneiss; and 7) gray, tan-weathering granulites containing thin coticule layers. Members 4, 5 and 6 are approximately equivalent to Ammonoosuc Volcanics. Member 3 is lithically similar to Monson and Fourmile Gneisses. Age in report is Ordovician. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.]
References
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Counties Fairfield - Litchfield - New Haven