GEOCHEMICAL CHARACTERIZATION OF THE EAST CENTRAL MINNESOTA BATHOLITH

The East Central Minnesota Batholith (ECMB) is a series of igneous plutons exposed along the Sauk River valley near St. Cloud in central Minnesota. The current model for the emplacement of the ECMB relies on U-Pb age dates and the geographic distribution of the batholith with respect to other Yavapai-era granitoids. Holm et al. (2005) published U-Pb ages for batholith emplacement between 1.77-1.79 Ga. He describes the distribution of the ECMB as a series of southward migrating granitoid intrusions north of the Yavapai accretionary boundary. He proposes this distribution, and the post-Penokean ages for the ECMB are the result of slab rollback during the Yavapai orogeny. The research presented here uses whole-rock analysis for major, trace, and rare-Earth elements to interpret the tectonic environment for the emplacement of the ECMB. Outcomes of this research will better constrain the model for Yavapai subduction.

Whole rock geochemistry for major elements (ICP-AES), REE and trace elements (ICP-MS) was determined for 15 samples collected from 7 distinct intrusions across the batholith. These data were compared using several classification plots including Harker diagrams, MALI, Fe-index, aluminum saturation index, alkalinity index, feldspar silica saturation index (Frost et al., 2001; Frost and Frost, 2008),. And the trace element discrimination diagrams (Pearce et al., 1984). Petrographic analysis on thin sections from 10 of these granite samples was performed to determine mineral assemblage and modal abundance.

To date, whole-rock data from 10 of 15 samples indicate that the suite range from quartz monzonite to granite, are calc-alkalic to alkalic, largely ferroan and metaluminous with Fe-index and ASI increasing with silica content. Alkalinity index and feldspar silica saturation index classify the preliminary samples as being metaluminous and silica saturated. The geochemical patterns of the ECMB appear most similar to Caledonian style post orogenic granites (Frost and Frost, 2008). Geochemical data from five additional samples from the ECMB are currently being analyzed to create a more robust data set.