Mixed felsic and mafic rocks

Heterogeneous composite intrusive well-layered unit consisting of biotite and hornblende metatrondhjemite, garnet-hornblende-plagioclase amphibolite, and metadiabase dikes; locally called Ruger Hill facies (Onr) in the Spring Hill syncline (Ratcliffe and Armstrong, 2001). Layering thickness ranges from 5 cm to 1 m. Unit may be in part metavolcanic as well as intrusive. Resembles well-layered undated felsic and mafic volcanics intercalated with metasediments of the Cram Hill Formation (Ochv). North River Igneous Suite (502±4 Ma to 471.4±3.7 Ma): collection of metatonalite, metatrondhjemite, and metabasalt occurring as intrusive dikes, sills, and small stocks, and possibly meta-andesite and metadacitic tuffs. Correlative with extrusive dacitic metavolcanic and meta-andesitic rocks of the Moretown and Cram Hill Formations. Coextensive in part with igneous rocks of the Hawley Formation of Massachusetts. Part of the Volcanic-arc intrusive and volcanic rocks of the North River Igneous Suite of the Rowe-Hawley zone. Rocks of the Early to Late Taconic Accreted Terrane of the Rowe-Hawley Zone. Eastern allochthonous sequence, oceanic and accretionary realm, ultramafic inclusions, volcanic-arc intrusives, and volcanic rocks.
State Vermont
Name Mixed felsic and mafic rocks
Geologic age Ordovician and Late Cambrian
Lithologic constituents
Metamorphic > AmphiboliteGarnet-hornblende-plagioclase amphibolite.
Metamorphic > Metaigneous > Metaintrusive > Metatonalite > MetatrondhjemiteBiotite and hornblende metatrondhjemite.
Metamorphic > Metaigneous > Metaintrusive > MetadiabaseMetadiabase dikes.
Comments Original unit label is Onbm; but age is listed as Ordovician and Late Cambrian. Age confirmed by U-Pb age in unit description. Changed unit label to OCAnbm.
NGMDB product
Counties Windsor