Abstract

The East Central Minnesota Batholith (ECMB) is a series of granitic plutons exposed along the Sauk river valley near St. Cloud Minnesota. The currently accepted understanding of the processes that generated these plutons relies on the location and age of emplacement. The location of the ECMB is within a terrain that was accreted onto the North American continent during the Penokean orogeny between 1875 to 1835 Ma (Van Schmus, 1976). However the granites within the ECMB are younger than that having ages between 1772 Ma and 1783 Ma (Holm et al., 2005), similar to ages for the Yavapai orogeny occurring approximately 150 km to the south. To explain how plutons of this age could be intruding so far north of the deformational boundary, Holm et al. (2005) propose an emplacement model where the cold and dense tectonic plate subducting under the continent during the Yavapai orogeny sank causing decompression of the crust and magma generation. The research presented here uses whole-rock geochemical analysis for major, trace, and rare-Earth elements to interpret the tectonic environment for the emplacement of the ECMB in hopes to better constrain the model for tectonics in this region at the time of the Penokean and Yavapai Orogenies.

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.

Whole-rock geochemical data indicate that the samples of 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 samples as being metaluminous and silica saturated. The geochemical patterns of the ECMB appear most similar to the Caledonian style post orogenic granites seen in Scandinavia (Frost and Frost, 2008).

College

College of Science & Engineering

Department

Geosciences

Location

Winona, Minnesota

Breakout Room

29

Start Date

4-14-2021 1:00 PM

End Date

4-14-2021 1:45 PM

Presentation Type

Video (Live-Zoom)

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Apr 14th, 1:00 PM Apr 14th, 1:45 PM

Geochemical Classification of the East Central Minnesota Batholith

Winona, Minnesota

The East Central Minnesota Batholith (ECMB) is a series of granitic plutons exposed along the Sauk river valley near St. Cloud Minnesota. The currently accepted understanding of the processes that generated these plutons relies on the location and age of emplacement. The location of the ECMB is within a terrain that was accreted onto the North American continent during the Penokean orogeny between 1875 to 1835 Ma (Van Schmus, 1976). However the granites within the ECMB are younger than that having ages between 1772 Ma and 1783 Ma (Holm et al., 2005), similar to ages for the Yavapai orogeny occurring approximately 150 km to the south. To explain how plutons of this age could be intruding so far north of the deformational boundary, Holm et al. (2005) propose an emplacement model where the cold and dense tectonic plate subducting under the continent during the Yavapai orogeny sank causing decompression of the crust and magma generation. The research presented here uses whole-rock geochemical analysis for major, trace, and rare-Earth elements to interpret the tectonic environment for the emplacement of the ECMB in hopes to better constrain the model for tectonics in this region at the time of the Penokean and Yavapai Orogenies.

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.

Whole-rock geochemical data indicate that the samples of 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 samples as being metaluminous and silica saturated. The geochemical patterns of the ECMB appear most similar to the Caledonian style post orogenic granites seen in Scandinavia (Frost and Frost, 2008).