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The Morton Gneiss of southwestern Minnesota is a migmatitic hybrid rock. The paleosome consists of tonalitic to granodioritic gneisses and amphibolites; the neosome, of a variety of granitic gneisses. The older rocks are largely biotite-quartz-oligoclase gneisses with minor microcline and hornblende and are tonalitic in composition grading to granodiorite. Locally, shearing and recrystallization of layered tonalitic gneiss developed a granoblastic granodioritic gneiss with idioblastic crystals of hornblende in a granular matrix of microcline, quartz, and oligoclase.

Amphibolite is closely related to the tonalitic gneisses and occurs as pieces or clasts that range in size from small pods or schlieren to large angular blocks. Some of the amphibolite is younger than the tonalitic gneisses and represents fragmented dikes or sill-like masses; however, some may be of the same age or older than the tonalitic gneisses. Occurrence and mineralogy indicate that the amphibolites are all of igneous precursors with compositional variations suggestive of tholeiitic to basaltic komatiite parentage.

The neosome is composed of granitic varieties from oldest to youngest: (1) pegmatite and microcline granite, (2) fine-grained adamellite (adamellite-1), (3) granodiorite and (4) microadamellite porphyry at Morton and gneissic fine-grained adamellite to the northwest (adamellite-2).

At least two major periods of deformation are recognized. The older involved the tonalitic gneisses and the amphibolites and was developed before the emplacement of the granitic phases. The tonalitic gneiss-amphibolite complex was disrupted at the time of the intrusion of the pegmatitic and adamellitic magmas, but it is not clear how much of the present-day structure of the Morton Gneiss was developed at that time. The second major deformation affected the granitic phases as well as the older tonalitic gneiss-amphibolite complex and was responsible for the highly contorted structure of the Morton Gneiss. The rocks included in adamellite-2 are least deformed, and these appear to have been emplaced in the late stages of the second major deformation.

Straight, relatively undeformed dikes of aplite, aplite-pegmatite, and pegmatite cut the Morton Gneiss. They represent the last Archean igneous activity in the region. A poorly developed foliation in the aplite dikes is attributed to stresses generated during the uplift of the region. In the southeastern part of the Morton area, between Franklin and New Ulm, there are Proterozoic diabase dikes and small plutons containing granitic phases. These rocks were intruded approximately 1,800 m.y. ago.

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