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6: Keweenawan intrusive igneous rocks

By
Paul W. Weiblen
Paul W. Weiblen
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Published:
January 01, 1982

The Keweenawan intrusive rocks of the Lake Superior region may be conveniently divided into alkaline intrusions, layered tholeiitic intrusions, and quartz and olivine tholeiite dike and sill swarms. Within each group is a characteristic internal stratigraphic succession of rock types. The alkaline rocks are characterized by successions of fractionated, undersaturated to saturated gabbroic through alkalic differentiates. On the basis of data from the Duluth Complex, the layered tholeiitic intrusions have been subdivided into an early group of cumulates consisting of peridotite, anorthositic rocks and granophyre, and a later group of troctolitic through ferrogabbroic cumulates with minor granophyre. On the basis of crosscutting relations and paleomagnetic data, most quartz tholeiite dikes and sills are older (magnetically reversed) than olivine tholeiites (magnetically normal). However, some magnetically normal dikes and sills are quartz tholeiites. Age data and crosscutting relations do not provide a means for placing the alkaline rocks in a stratigraphic succession with the tholeiitic rocks; however, comparison with modern rift environments suggests the following succession of magma types: alkaline (CO2-rich to CO2-poor), quartz tholeiite (alkalic, H2O-saturated), olivine tholeiite (low alkalies and H2O). The distribution of the intrusive products of these magmas is consistent with a transition in the Keweenawan tectonic regime from northwest-southeast wrench faulting to rifting along northwest-southeast transform faults.

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GSA Memoirs

Geology and Tectonics of the Lake Superior Basin

Richard J. Wold
Richard J. Wold
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William J. Hinze
William J. Hinze
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Geological Society of America
Volume
156
ISBN print:
9780813711560
Publication date:
January 01, 1982

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