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
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.
Beltrame, R.J.; Chandler, V.W.; Gulbranson, Brian L..
RI-27 Geophysical Investigation of the Cedar Mountain Complex, Redwood County, Minnesota.
Minnesota Geological Survey.
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