REINTERPRETATION OF THE 1.1 GA CHENGWATANA VOLCANICS IN THE ST. CROIX HORST, MINNESOTA AND WISCONSIN

The St. Croix horst contains the most southerly exposure of volcanic and sedimentary rocks related to the 1.1 Ga Midcontinent rift system (MRS). To the north, rift-related rocks are well exposed around the margins of Lake Superior and detailed stratigraphic sections are well established. Outcrops within the St. Croix horst are sparse: the volcanic rocks are nearly all subaerial basalts and were previously assigned to the Chengwatana Volcanics. However, newly acquired high-resolution aeromagnetic data suggest the presence of several volcanic sequences. Cannon and others (2001) used this detailed aeromagnetic imaging and new age determinations to subdivide the former Chengwatana Volcanics into three units: from youngest to oldest, the units are the Minong Volcanics, the Clam Falls Volcanics, and a newly redefined Chengwatana Volcanics unit, all of which have normal magnetic polarity (with the possible exception of a few flows). The basalt flows of the Minong Volcanics (about 1094 Ma) unconformably overlie the Clam Falls Volcanics dated between 1102 and 1099 Ma. The Minong and Clam Falls Volcanics are separated from the redefined Chengwatana Volcanics (undated) by the Pine Fault.

A regional chemical stratigraphy has been recognized previously in MRS basaltic rocks around western Lake Superior. Comparison of the youngest basalts in the St. Croix horst with MRS chemical stratigraphy elsewhere suggests that the Minong basalts are most similar to the Portage Lake Volcanics (1096-1094 Ma). Depleted basalt compositions (similar to N-MORB) occur as flows towards the top of the Minong Volcanics as well as in dikes in the upper Portage Lake Volcanics. Some of the older Clam Falls and Chengwatana basalts show compositions similar to the Upper Kallander Creek Volcanics (about 1098 Ma). Thus, the basalts in the St. Croix horst may record the transition from localized volcanic sources to the voluminous flood basalts characteristic of the main stage of rifting of the MRS.