Major- and selected trace-element analyses of 123 samples of plagioclase gneisses of the Killingworth dome, south-central Connecticut, show it to be more complicated than previous field work and mapping has indicated. Chemical and areal trends define distinct types of gneiss; four are of trondhjemitic to tonalitic composition (referred to as the “central, southern, eastern, and western plagioclase gneisses”), and a fifth is granitic gneiss. The smooth variation in concentrations of all elements with respect to silica in each gneiss easily is interpreted as igneous in origin and also indicates that metamorphism was isochemical for the most part. The pluton that is represented by the central tonalitic gneiss (65%–74% SiO2) is characterized by unusually high Sr (350–670 ppm) and Ba (400–1,500 ppm), high Al2O3 (14.5%–17.5%), and low Fe2O3 (total) (2.1%–4.3%) and displays reverse compositional zoning (least siliceous in the center). The southern gneiss (62%–73% SiO2), distinctive in its relatively high hornblende content, has intermediate A12O3 (14.2%–17.5%) and Fe2O3 (total) (3.4%–7.4%) and has lower average Na2O (2.6%–3.5%) and somewhat higher average K2O (1.2%–3.3%) than do the other plagioclase gneisses. The peripheral western gneiss (69%–77% SiO) is distinguished from the eastern gneiss (72%–77% SiO2) by its higher Fe2O3 (total) (2.8%–5.6% versus 1.1%–3.8%) and lower A12O3 (11.7%–14.5% versus 12.0%–15.0%). Both have low K2O, but the western gneiss is commonly <1.0%, whereas the eastern gneiss is commonly >1.0%.

The protolith of the central gneiss was a high-Al tonalite which intruded high-Si dacites represented now by the eastern and western gneisses (the western gneiss probably overlying the eastern). The southern unit is a metatonalite to metagranodiorite. The central and eastern gneisses are correlated with Monson Gneiss, whereas the western gneiss is compositionally similar to felsic Ammonoosuc Volcanics. The southern gneiss has no known counterpart among Bronson Hill rocks but is similar in chemical, mineralogical, and physical characteristics to upper Proterozoic Rope Ferry Gneiss of the Selden Neck dome.

The low-Al eastern unit was probably generated by partial melting of amphibolite with residual plagioclase. The central high-Al tonalite may have a similar origin but formed under high PH2O Possibly somewhat deeper than the eastern unit, where hornblende was probably the predominant residual phase. Compositional trends of the tholeiitic western gneiss compare well with those of dacites of the island-arc tholeiitic series. Collectively, these rocks probably constituted part of a continental margin volcanic arc, which is consistent with current interpretations of the Ordovician (or older) development of the Bronson Hill anticlinorium rocks in central New England. The similarity of the southern gneiss to the Rope Ferry Gneiss supports its correlation with the older Avalon terrane and suggests that a tectonic boundary may separate the southern gneiss from the Bronson Hill rocks.

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