CONTROLS ON GARNET GROWTH IN METABASALT; MINNESOTA RIVER VALLEY, SW MN

This research investigates the controls on garnet growth in basalt during metamorphism. The study area is a 50-meter outcrop of metabasalt interlayerd with pelitic schist located near Memorial Park in Granite Falls, MN, within the Minnesota River Valley subprovince. In this outcrop, metamorphic garnet is present in some metabasalt layers in the eastern end of the outcrop, but garnet is absent in all metabasalt layers in the western portion. This research investigates if chemical differences between layers are controlling the garnet growth during metamorphism, since it is unlikely the P-T conditions vary significantly within the outcrop. This study incorporates petrographic, geochemical, and electron microprobe analysis to identify the variability in mineral phases present for each rock, along with the chemical variability in bulk-rock and mineral compositions to identify chemical trends associated with the presence or absence of garnet (Grt) in each metabasalt layer .

Petrographic analysis identified 2 Grt-bearing (MVB-05A & B) and 2 Grt-absent (MVB-05C & F) samples suitable for electron microprobe analysis to compare coexisting mineral compositions and determine P-T conditions for different layers. Mineral phases orthopyroxene (Opx), plagioclase (Pl), biotite (Bt), and quartz (Qtz) are present in all 4 samples with clinopyroxene (Cpx) present only in Grt-absent samples. Mineral compositions in Grt-bearing samples are similar to each other, as are those in Grt-absent samples; however, X_Ca in Pl, X_Fe in Bt, and X_Mg in Opx are all greater in Grt-absent samples, and X_Al in Opx is greater in Grt-bearing samples.

When assemblages from each sample are plotted on an A-C-F diagram, the Grt-bearing assemblages Opx-Grt-Pl and Opx-Cpx-Pl can occur in equilibrium allowing the variation be a result of varying bulk-rock chemistry only. The bulk-rock variation necessary for the above, lower Ca and/or higher Al in Grt-bearing samples, is realized when comparing bulk-rock analyses of Grt-bearing to Grt-absent samples, mostly through an increase in Wt% CaO. This, combined with P-T calculations that overlap for all 4 samples, support the interpretation that in this case it is bulk chemistry and not P-T conditions that lead to the presence of garnet in some basalt layers but not all.