GEOLOGIC, GEOCHEMICAL, AND GEOCHRONOLOGIC EVIDENCE FOR THE EXISTANCE AND TIMING OF A SIGNIFICANT MAFIC UNDERPLATED REGION BENEATH THE MIDCONTINENT RIFT IN THE LAKE SUPERIOR REGION

In the late 80’s, the acquisition of deep seismic reflection images across the 1.1 Ga Midcontinent Rift (MCR) in the Lake Superior region, integrated with potential field, geologic, geochemical and geochronologic data, greatly enhanced our understanding of the MCR’s structural and tectonomagmatic evolution. In 1997, Bill Hinze and coworkers summarized that understanding in a seminal paper on the MCR in GSA Special Paper 312. In addition to providing a clear picture of the asymmetric synformal structure of volcanic and sedimentary units across the MCR, modelling of seismic and potential field data summarized by Hinze strongly indicates the presence of a significant mafic underplated region beneath the MCR in the Lake Superior region.

The recent acquisition of passive seismic and magnetotelluric data from the Earthscope survey of the central US has inspired a renewed interest and reassessment of the MCR’s 3-D structure and evolution. Modelling of the seismic structure of the MCR from the Earthscope data have verified the existence of an underplated region in the Lake Superior part of the rift (Shen et al., JGR, 2013). This talk will summarize the geological, geochemical and geochronological data from volcanic and intrusive rocks in the Lake Superior region that strongly support the existence of this deep crustal feature and address the timing of its development. The main empirical evidence to be presented include:

- Felsic magma generated by lower crustal anatexis;

- Anorthositic rocks generated by plagioclase flotation at lower crustal depths;

- Chemostratigraphy of volcanic sequences indicating varying degrees of crustal contamination and polybaric fractionation; and

- A 5 Ma hiatus in mafic magmatism that may correspond to the growth phase of the mafic underplate.

The segregation of plagioclase from mafic phases (olivine and pyroxene) in lower crustal chambers and the anatectic melting of the lower crust should create an underplated area that is density stratified, though at a scale that is hard to predict. It will be interesting whether some of the passive seismic imaging of the EarthScope data can discern this stratification.