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Northeastern Section - 36th Annual Meeting (March 12-14, 2001)

WHITE, Chris E., Department of Natural Resources, P.O. Box 698, Halifax, NS B3J 2T9, Canada and BARR, Sandra M., Department of Geology, Acadia Univ, Wolfville, NS B0P 1X0, Canada, whitece@gov.ns.ca

Over the past 15 years, various models for the evolution of the northern Appalachian orogen have suggested that Silurian volcanic and sedimentary units in southern New Brunswick, southern Nova Scotia, and the Antigonish and Cape Breton Highlands of northern Nova Scotia represent a Silurian overstep sequence. Although recent geochronological work has confirmed that these units are of similar early Silurian age (ca. 440 Ma - 430 Ma), mapping and petrochemical studies show that they have different petrochemical characteristics, and do not constitute an overstep sequence.

The Kingston terrane of southern New Brunswick consists of mainly felsic tuffaceous rocks intruded by comagmatic high-level granitic rocks. Both have yielded U-Pb (zircon) ages of ca. 438-435 Ma. Their chemical compositions indicate calc-alkalic affinity and emplacement in a continental margin volcanic arc. These rocks are similar in age, petrologic features, and inferred tectonic setting to volcanic and granitic rocks of the central Aspy terrane in Cape Breton Island, and direct correlation between these now widely separated units is proposed here. They are interpreted to represent a Silurian ocean-closure event between Late Proterozoic-Early Paleozoic peri-Gondwanan terranes. U-Pb dating has shown that volcanic rocks in the White Rock Formation of the Meguma terrane in southern Nova Scotia are of similar age, but they are a bimodal alkalic basalt-rhyolite suite formed in a within-plate continental extensional setting, and differ chemically from the Kingston-Aspy volcanic sedimentary suites. Silurian volcanic and fossiliferous sedimentary sequences in the Arisaig Group of the Antigonish Highlands are located in the Avalon terrane. Although petrochemical features suggest that they may have formed in a continental extensional setting like that of the White Rock Formation, some chemical characteristics are distinct from those of the White Rock Formation, and lithologically, the two units show little similarity.

Hence, we suggest that it is not valid to use these widespread units to infer that the various outboard terranes of the orogen were together by the Silurian.

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