RI-27 Geophysical Investigation of the Cedar Mountain Complex, Redwood County, Minnesota

Abstract

The Cedar Mountain Complex is a roughly circular composite intrusion, 500 to 600 m in diameter, in the Archean gneiss terrane of southwestern Minnesota. Emplaced during early Proterozoic time, the complex consists of a central core of monzonite surrounded by slightly older, compositionally layered dioritic rocks. Existing geophysical and geologic data indicate that the complex may reflect a widespread igneous event that also formed the granite-rhyolite terrane of central Wisconsin. A gravity and magnetic survey was conducted over the Cedar Mountain Complex to (1) delineate the contacts which are nowhere exposed; (2) interpret the configuration of the intrusion at depth; and (3) determine what relationships the complex may have with similar intrusions in the area and determine how they might be detected geophysically. Magnetic anomaly and rock property data indicate that the dioritic rocks are the principal sources of anomalies and that they are polarized near or along the present earth's field. Magnetic anomaly data indicate an angular and irregular configuration for the diorite-gneiss contact and a somewhat rectangular configuration for the monzonite-diorite contact. Model studies of the magnetic data imply that some contacts may dip steeply outward, but most are nearly vertical. The gravity anomaly data indicate that the core monzonite may be in part underlain by dioritic rocks. The configuration of the outermost contact and the density constraints indicate forceful injection as the mechanism for emplacement of the dioritic rocks. Similar emplacement is possible for the core monzonite, but the regular configuration of the inner contact, density constraints, and gravity interpretation favor stoping out of large blocks of diorite. Upward continuation of the magnetic data indicates that gross anomaly attributes of the complex and related intrusions would be detectable by aeromagnetic surveying using a flight-line spacing of 400 m and terrain clearance of 150 m.