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

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.