GEOCHEMICAL ANALYSIS AND ALONG-STRIKE DISCRIMINATION OF TACONIAN ARC REGIONS ALONG THE ROWE-HAWLEY ZONE OF WESTERN NEW ENGLAND

The complex pre-Ordovician rock sequences of the New England Appalachians containing arc-related volcanics formed in peri-Laurentian, Iapetan, and peri-Gondwanan settings and can preserve polyorogenic metamorphism among Taconian, Salinic, Acadian, and Alleghanian events. These sequences record the orogen’s original architecture of reentrants and promontories, and the style of their accretion and structural evolution influence post-Ordovician tectonics. Within New England, there is tectono-stratigraphic, geochemical, and geochronological evidence for multiple island arc terranes that sequentially accreted to the Laurentian margin. The Shelburne Falls Arc, located within the Rowe-Hawley Zone (RHZ), has been cited as the colliding terrane during the Taconian Orogeny, but RHZ volcanics have not been definitively correlated along-strike through western VT, MA, and CT. Geochemical analysis of mappable metabasalt and meta-andesite units collected along the RHZ in all three states is being used to test correlations among them. MORB-normalized multi-element plots of minor and trace element data differentiate subalkaline metabasalts of the Hawley Fm of western MA (large negative Nb, large positive P, flat Zr to Yb <1) and Collinsville Fm of western CT (small negative Nb, flat Zr to Yb within the range 10 < x > 0.1). Collinsville meta-andesites and dacites have flat to negatively sloping HFSE with values ≤1. These data and Pearce element ratios indicate that the Hawley and Collinsville rocks could have formed in the forearc and backarc basins, respectively, of one arc complex similar to modern rifted supra-subduction arcs of the western Pacific (e.g., Bonin). Moretown Fm subalkaline basalt samples from western VT also display backarc basin geochemistry (small negative Nb, Zr to Yb ~1, relatively flat HFSE ~1). The Taconian Orogeny could have been the result of a sequence of accretionary events involving backarc, arc, and forearc regions of one composite arc. Rocks not part of this arc are meta-andesites exposed in the core of the Waterbury dome structurally below the RHZ and separated from it by a major thrust fault. These rocks have Zr, Hf, and Sm >1, positive Y, negative Yb <1, and Zr/Y vs. Zr shows a within plate setting.