MAGMA EMPLACEMENT DURING RIFTING AS EXEMPLIFIED BY THE EAST PACIFIC RISE AND THE BALD EAGLE INTRUSION IN THE MIDCONTIENT RIFT

The seismic anomalies along the East Pacific Rise are strikingly similar in shape and scale to the gravity and magnetic anomalies associated with the Bald Eagle intrusion in the Midcontinent Rift. Uniformly spaced low velocity closed seismic minima are centered on the smokers along the East Pacific Rise and are interpreted to define active magma flow through systems. Assuming that magma emplacement processes during continental and oceanic rifting share fundamental similarities, the closed gravity and magnetic maxima of the Bald Eagle intrusion may be interpreted to define the geometry of an eroded remnant of a solidified magmatic system analogous to the active magma flow through systems of the East Pacific Rise.

The Bald Eagle intrusion is an elliptical ~ 3 X 10 km, funnel-shaped intrusion at the base of the Duluth Complex. Surface exposures define an outer unit of troctolite and an inner unit of olivine gabbro. Both units exhibit only minor differentiation effects and have well-developed layering defined by mineral lamination and concordant, segregated lenses of plagioclase. These features can be modeled as products of crystal-melt separation in a dynamic flow-through magma chamber. The intrusion is connected to the South Kawishiwi intrusion (a gently dipping layered intrusion) by a vertical ~ 10 km long < 3 km wide macro-dike of well laminated troctolite and olivine gabbro similar to the Bald Eagle intrusion. Sulfide mineralization at its base can be modeled as a product of mixing magma from the Bald Eagle intrusion via the macro-dike with a hydrous sulfur bearing phase derived by heating of country rocks. The structural relationship of the connection of the Bald Eagle and the South Kawishiwi intrusions is similar to the ridge-off axis magmatic system (with its off-axis sulfide mineralization) of the East Pacific Rise.

Our initial examination of analogous rifting processes in the East Pacific Rise and the Midcontinent Rift suggest that further study will provide new insights into magma emplacement and sulfide mineralization processes. This conclusion finds further confirmation when one considers that the igneous rocks formed during the development of the Midcontinent Rift in Minnesota consist of a stratigraphic succession analogous to the lava flow - dike and sill - layered gabbro sequences of modern oceanic crust.