Geochemistry of the Archean rocks in the Morton and Granite Falls areas, southwestern Minnesota

Abstract

Abstract The Archean of the Minnesota River Valley, southwestern Minnesota, includes a variety of rock types that range in age from approximately 3600 to 2550 Ma. In the Morton area a complex migmatite (Morton Gneiss) is composed of a paleosome of tonalitic to granodioritic gneiss and amphibolite and a neosome of granodiorite, adamellite, and granite pegmatite. The leucotonalitic gneiss in the paleosome is trondhjemitic, with potassium largely in biotite. The tonalite, and closely related granodiorite, originated 3500 to 3600 Ma ago. These rocks, together with basaltic rocks, were deformed to form a metamorphic complex prior to the emplacement of granitic pegmatite and adamellite (adamellite-1) of the neosome 3050 Ma ago. The major deformation that produced the convoluted and variegated present-day Morton Gneiss occurred 2600 Ma ago. Some granitic rock (adamellite-2) was emplaced during or in late stages of the deformation. Chemically, adamellite-2 is similar to the aplite dikes that cut across the structure of the Morton Gneiss and to a phase of the Sacred Heart Granite. These rocks were formed 2550 to 2600 Ma ago. The Archean rocks in the Granite Falls area are divided into two major groups: mafic and felsic gneisses which are interlayered on a large scale. The mafic gneisses, which have not been recognized to the southeast in the Morton area, are of two principal types: hornblende-pyroxene gneiss and biotite-pyroxene gneiss. Some of these rocks are metasedimentary with graywacke percursors. Closely associated are amphibolites of tholeiitic to basaltic komatiite composition and metagabbro. Like the Morton Gneiss, the gneiss in the Granite Falls area is a hybrid rock. The older phase or paleosome is granodioritic with minor tonalitic gneiss and amphibolite. The granodioritic gneisses date back to 3600–3700 Ma ago. These rocks were deformed to a metamorphic terrane that was invaded by granite pegmatite and adamellite 3050 Ma ago. The major deformation that produced the present-day structure occurred 2600 Ma ago. Proterozoic diabase dikes and high-level adamellite plutons were emplaced in the Archean gneisses of the Minnesota River Valley 1800 Ma ago.