Origin of the Morton Gneiss, southwestern Minnesota: Part 2. Geochemistry
J. L. WOODEN, S. S. GOLDICH, N. H. SUHR, 1980. "Origin of the Morton Gneiss, southwestern Minnesota: Part 2. Geochemistry", Selected Studies of Archean Gneisses and Lower Proterozoic Rocks, Southern Canadian Shield, G. B. Morey, Gilbert N. Hanson
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The paleosome of the hybrid Morton Gneiss consists of tonalitic and granodioritic gneisses with enclaves or clasts of amphibolite. The neosome contains granitic gneisses ranging from granodiorite to K-rich granite. The major rock groups are differentiated in plots of the normative orthoclase:plagioclase: quartz ratios and of K2O versus Na2O. These groups are further subdivided on the basis of major, minor, and trace elements in discrimination diagrams in which both concentrations and ratios of elements are considered. All the principal rock types were originally of igneous origin.
The tonalite gneisses are subdivided into Fe-rich and Fe-poor varieties; both are biotite-quartz-oligoclase assemblages with minor microcline and hornblende and are classed as trondhjemitic gneiss. With increase in microcline, the tonalite gneiss grades to granodiorite gneiss that cannot be differentiated in the field but is a distinct petrographic and chemical type. Locally, shearing and recrystallization of layered tonalite gneiss formed veins of granoblastic granodioritic gneiss. Relatively large amounts of Rb were lost during the shearing and recrystallization.
The basic chemical patterns of the amphibolites indicate that they were derived from basaltic precursors of two different types. A hornblende-plagioclase amphibolite is tholeiitic, and a hornblende-rich, low-alumina variety is komatiitic in composition. Both are Fe-rich, and within each group there are variations, which are assigned to magmatic processes, but also some that are related to secondary metasomatic processes. Relatively large amounts of K and Rb (and, in some samples, also Ba and Sr) were introduced. The amphibolite also must be regarded as the source of some contamination by physical mixing during deformation of the closely associated tonalite gneiss.
Microcline pegmatitic granite, granodiorite, and adamellite are recognized in the granitic gneiss phase of the Morton Gneiss. Two groups of adamellite are distinguished: an older gneiss called adamellite-1 and a younger gneiss called adamellite-2. Adamellite-2 is further subdivided into microadamellite porphyry that occurs at Morton and a gneissic fine-grained adamellite at localities northwest of Morton. The microadamellite porphyry at Morton chemically resembles a phase of the Sacred Heart Granite, but the gneissic adamellite to the northwest resembles in composition some of the aplite dikes that traverse the structure of the Morton Gneiss. The aplites, however, show the effects of strong fractionation, and the average aplite composition differs from that of adamellite-2 in the relative enrichment in Rb and impoverishment in Sr and Ba.