2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 228-11
Presentation Time: 11:30 AM

ROLE OF FELSIC AND FELDPATHIC ROCKS IN TRIGGERING SUBVOLCANIC EMPLACEMENT OF MAFIC INTRUSIONS: EVIDENCE FROM THE MIDCONTINENT RIFT IN NORTHEASTERN MINNESOTA


MILLER, James D., Department of Earth and Environmental Sciences, University of Minnesota Duluth, 230 Heller Hall, Duluth, MN 55812

Subvolcanic mafic intrusions make up over 40% of the igneous rocks of NE Minnesota associated with the 1.1Ga Midcontinent Rift (MCR). The greatest concentration of mafic intrusions comprise the enormous (~20,000 km3) Duluth Complex, which was emplaced into the basal section of a 5-10 km thick edifice of comagmatic volcanics. Most Duluth Complex intrusions, and mafic intrusions emplaced higher in the volcanic pile, occur as sheet-like bodies that commonly underlie either felsic rocks (rhyolite flows or granophyre intrusions) or feldspathic rocks (gabbroic to troctolitic anorthosite). In all cases, field relationships indicate that the felsic/feldspathic rocks are older than the mafic rocks that underlie them.

Three styles of mafic underplating are recognized in the MCR of NE Minnesota – 1) mafic layered intrusions beneath large granophyre bodies; 2) mafic layered intrusions beneath anorthositic bodies; and 3) diabase sheets beneath thick rhyolite flows.

Empirical field evidence and density/viscosity/thermal considerations suggest that the felsic/feldspathic rocks served as density barriers to mafic magmas, which had risen into the upper crust to the point of neutral bouyancy. The felsic/feldspathic rocks not only triggered underplating of the mafic magmas, but also commonly served as thermal insulators to the underplated mafic bodies, thus resulting in slow cooling and crystallization differentiation.