Abstract

Conglomerate tests are used to show that Late Precambrian Keweenawan conglomerates on the Keweenaw Peninsula, Michigan and at Mamainse Point, Ontario have suffered a partial chemical remagnetization that predated tectonic tilting. At Mamainse Point the overprint was acquired during the final normal polarity stage of Keweenawan volcanism, whereas on the Keweenaw Peninsula it occurred after cessation of volcanism and has a maximum Late Freda Sandstone age. The extent of overprinting on the Keweenaw Peninsula increases westwards along a 40 km strike length in the Copper Harbor Conglomerate, and may at least in part be related to secondary mineral zonation (including native copper) in the underlying Portage Lake volcanics. It is shown that neither of the two overprinting episodes can be responsible for the reversal asymmetry observed in Keweenawan igneous rocks.

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