|Name||Perry Mountain Formation, Sedimentary and subordinate distal felsic and mafic volcanic facies in Piermont allochthon|
|Geologic age||Lower?- Middle? Silurian|
Metamorphic > Metasedimentary > Metaclastic
Metamorphic > Metaigneous > Metavolcanicfelsic and mafic volcanic facies in Piermont allochthon
|Comments||Part of the Central Maine Composite Terrane (Central Maine Trough) - Variably metamorphosed sedimentary and volcanic rocks of greenschist to granulite facies, locally migmatized. Area includes structural belts between the Monroe fault on the west and the Campbell Hill fault on the east; that is, the Bronson Hill anticlinorium, Piedmont allochthon, Kearsarge-central Maine synclinorium, central New Hampshire anticlinorium, and Rochester-Lebanon (Maine) antiformal synclinorium. Perry Mountain Formation = 414+/-4 U/Pb (tuff bed), 412+/-2 U/Pb (tuffisite sill), <430 and >410 U/Pb (rhyolite intrusive unit) per J.N. Aleinikoff, oral commun., June8, 1990. Felsic metatuff = 418+/-4 U/Pb per NH046. Secondary unit description from USGS Geologic Names Lexicon (ref. NH047): Mapped as one undivided unit and four unnamed subdivisions. Undivided part consists of sharply interbedded quartzites, light-gray nongraphitic metapelite, and "fast-graded" metaturbidites; common coticule layers. Subdivisions include a metadacite and metarhyolite unit in southern NH; a proximal bimodal volcanic facies in Piermont allochthon; a sedimentary and distal felsic and mafic volcanic facies in Piermont allochthon; and metabasalt lenses in Piermont allochthon. Mapped undivided with Rangeley Formation. Age of Early(?) and Middle(?) Silurian supported by isotope ages (sample localities shown on accompanying 1:500,000-scale map): 418 +/-4 Ma using U/Pb methods for felsic metatuff, and 418 +/-10 Ma using U/Pb methods for felsic dike from possible feeder system for the felsic metatuff (Lyons and others, 1983); and 414 +/-4 Ma for tuff bed, 412 +/-2 Ma for tuffisite sill, <430 and >410 Ma for rhyolite intrusive, all using U/Pb methods (J.N. Aleinikoff, oral commun., 1990).|
Lyons, J.B., Bothner, W.A., Moench, R.H., and Thompson, J.B., Jr., 1997, Bedrock Geologic Map of New Hampshire: Reston, VA, U.S. Geological Survey Special Map, 2 sheets, scale 1:250,000.
Lyons, J.B., Zartman, R.E., and Aleinikoff, J.N., 1983, U-Pb ages of zircons from the Ordovician Highlandcroft Plutonic Suite and from Silurian intrusives [abs.]: Geological Society of America Abstracts with Programs, v. 15, p. 187.
|Counties||Coos - Grafton|