Trondhjemite and metamorphosed quartz keratophyre tuff of the Ammonoosuc volcanics (Ordovician), western New Hampshire and adjacent Vermont and Massachusetts

The Ammonoosuc Volcanics and equivalent rocks of Ordovician age are exposed in the Oliverian domes along the Bronson Hill anti-clinorium (BHA) between northern New Hampshire and southern Connecticut. In western New Hampshire and adjacent Vermont and Massachusetts, the Ammonoosuc lithology consists of a lower, mainly mafic unit of homblende-plagioclase amphibolite, and an upper, mainly felsic, metamorphosed quartz keratophyre tuff. These lithologies are locally interlayered, and both are intruded by sills, dikes, and plugs of trondhjemite. Trondhjemite also constitutes the interior gneissic “core” of several small domes or plutons. The trondhjemite is highly siliceous (SiO₂ = 73%–81%), low in A1₂O₃ (11.3%–13.5%), generally contains < 1% K₂O, and thus resembles some trondhjemites in island-arc or continental-margin settings. Chemical trends of both trondhjemite and Ammonoosuc Volcanics (felsic and mafic) are essentially calc-alkaline.

Variations in both major and trace elements of trondhjemites in several of the domes suggest several somewhat different sources along the BHA. Overall, however, the major- and minor-element chemistry of the trondhjemites is closely similar to that of the Ammonoosuc quartz keratophyre tuff. These rocks could have been produced either by partial melting or by fractional crystallization of basaltic source rocks. The partial-melting model is preferred because of the largely bimodal basalt-quartz keratophyre Ammonoosuc assemblage in which andesitic and other intermediate compositions are virtually lacking. The relatively thin Ammonoosuc section appears to preclude generation of trondhjemite at the presently exposed base of an island arc, as has been postulated for very similar trondhjemite-amphibolite assemblages (Twillingate trondhjemite, Little Port Complex) in Newfoundland. Instead, generation of the felsic Ammonoosuc rocks more likely occurred at deeper levels along a subduction zone dipping eastward under the BHA, as postulated in current plate-tectonic models. The close juxtaposition in space and time of sialic crust and Ammonoosuc Volcanics may explain the calc-alkaline trends of the latter and suggests a paleotectonic environment of convergent oceanic-continental plate margins, possibly with significant crustal shortening across the arc.