Ophiolitic source rocks for Taconic-age flysch: Trace-element evidence

Abstract

Flysch generated along the Appalachian orogen during the Ordovician Taconic Orogeny ranges in age from late Arenig to early Caradoc (allochthonous flysch) and from late Llanvirn to Ashgill (autochthonous flysch). Sandstones in these units are dominantly lithic arenites, with a heavy mineral suite containing common to abundant Mg-rich chromite north of the Canada–United States border. The only reasonable sources for the chromite were the lower ultramafic units of obducted ophiolites. Trace-element analyses (Cr, Ni, V, Cu, Ti, Zr, Y) of flysch sandstones indicate a striking increase in Cr and Ni concentrations from the United States Appalachians to the Canadian Appalachians. In the United States units, Cr is in the range of 10 to 50 ppm; Canadian flysch units average an order of magnitude more Cr. These results suggest that ophiolite obduction either did not occur or was of minor importance during the Taconic Orogeny in the United States Appalachians. The difference in ophiolite abundance between the Canadian and United States Appalachians suggests a fundamental difference in tectonic style along the orogen, a feature that must be accommodated by plate-tectonic models. New models will probably need to consider both obliquity of subduction and geological complexity of the Ordovician collision zone in order to explain the spatial distribution of obducted ophiolite sheets.