Ammonoosuc Volcanics

[1] The Middle Ordovician Ammonoosuc Volcanics are a component of the Bronson Hill Arc, which is approximately 400 km (250 mi) long and reaches from the Quebec border down to southern Connecticut.

The Bronson Hill Arc is made up of both volcanic and plutonic sequences of felsic and mafic rocks, whose ages range from the Cambrian into the Lower Devonian.

[3] Petrochemical analysis of the unit concluded that the mafic rocks in the area showed island-arc tholeiite and backarc basin signals, which would support this idea.

[2] The origin of the Ammonoosuc Volcanics can be traced back to the Taconic orogeny, an Ordovician mountain-building event in the New England area that occurred after the Bronson Hill Arc accreted to the existing continent.

The felsic material that makes up some of the Ammonoosuc Volcanics is not close to this fault, and is typically granofels, composed primarily of quartz, Na-rich plagioclase, biotite, and sometimes contains garnets, magnetite, or epidote locally.

[2] A great deal of the felsic material in the Ammonoosuc Volcanics is found in the form of trondhjemite, a type of tonalite in which the majority of the plagioclase is oligoclase.