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The Archean Terranes of Minnesota (cont.)

4.3 Morton Block: Lithography

The Morton block, south of the Montevideo block, is separated from the latter by the Yellow Medicine Shear Zone (figures 2-1 and 4-1). Its boundary with the Jeffers block, the Brown County geophysical lineament (Southwick and Chandler, 1996), is only visible on the aeromagnetic map (figure 4-1). The outcrops appear in the Minnesota River valley from an area midway between New Ulm and Franklin up to the Redwood County-Yellow Medicine County border, south of the town of Sacred Heart. The most famous rock is a migmatite commonly designated as the Morton gneiss, from the name of the town where it has been quarried since the 19th century. It is a hybrid rock with contorted banding, composed of gray and pink gneiss, interlayered or intruded by dark mafic rocks.

archean morton gneiss

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Figure 4.3-1. Morton gneiss. Light-gray tonalite gneiss and dark-gray amphibolite are folded.

archean interlayered tonalitic and granitic morton gneiss

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Figure 4.3-2. Morton gneiss. Light-gray tonalite gneiss layers and pink granite gneiss layers are both intruded by dark amphibolite.

Morton gneiss thin section-PPL-plane polarized light Morton gneiss thin section-XPL-crossed nichols

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Figure 4.3-3. Morton gneiss thin section: tonalite gneiss (quartz, plagioclase, biotite).

Left: plane polarized light. Quartz and plagioclase in white, biotite in yellow-green and dark-brown. Right: crossed polarizers. Field of view: 2.3 mm. More microphotographs next page.

As in the Montevideo block, Lund (1956) considered the mafic rocks composed a "basic complex" of gabbro and diorite gneisses which the more recent gneisses intruded into . He called the younger gneiss quartz-monzonite gneiss and distinguished several facies (mineral percentages from his samples):

1-gray facies of tonalite: 50-60% oligoclase , ~25-35% quartz, ~5-15% biotite, 0-2% K-feldspar

2-pink facies of leucogranite: 50% K-feldspar, ~12% oligoclase, ~35% quartz

3-pink facies of quartz-monzonite: equal amounts of K-feldspar, oligoclase and quartz

4-pink & gray facies of quartz-monzonite: as the preceding facies, with less quartz and ~5% biotite.

Lund used Johannsen's (1939) classification. In the IUGS classification currently in use, the quartz-monzonite would be a granite.

Two less abundant rocks were identified by Lund in the Morton block: the Sacred Heart granite (a massive medium-grained pink-gray to red granite), visible south of the town of Sacred Heart, and the Fort Ridgely granite (a pink-gray porphyritic granite) southeast of Franklin. The Sacred Heart granite (~40% K-feldspar, ~30% oligoclase, ~25% quartz, some biotite) was described as an adamellite (Johannsen's classification; a granite in IUGS'). The Fort Ridgely only sample showed more K-feldspar and much less oligoclase and was classified as a leucogranite. Lund noticed the close relationship of these granites with the Morton gneiss and Goldich (1961) showed the granites intruded the gneiss, even if their Rb-Sr and K-Ar ages were similar, which could be due to the recrystallization of biotite.

minnesota river valley morton block sacred heart granite

Figure 4.3-4. Sacred Heart Granite

Reprinted from Bickford, M.E., Wooden J.L., Bauer R.L., Schmitz M.D., 2007, Chapter 6.1 Paleoarchean Gneisses in the Minnesota River Valley and Northern Michigan, USA, Developments in Precambrian Geology, Volume 15, Pages 731–750, Copyright (2007), with permission from Elsevier.

As in the Montevideo block, Lund found many "post-granitic intrusives": numerous basaltic dikes intrude the Morton gneiss, peculiarly near Franklin and the Cedar Mountain complex, 2 miles southwest of Franklin. This "mountain" is a round-shaped hill, about 1/2 mile (800 m) across, raising about 100 feet (~30 m) above the Minnesota River valley bottom; it consists of 3 rock types: from the outside to the center, a fine-grained olivine trachybasalt porphyry, a granophyre gabbro and granite (Goldich et al., 1961).

Grant (1972) noticed a kind of stratiformity in the gneisses exposed in the valley, with four layers, each several thousand feet thick, corresponding to Lund's gray tonalitic facies: from bottom to top, three layers of quartzofeldspathic gneiss with a decreasing amount of amphibolite intrusions (that he called "rafts") and a fourth topmost layer of biotite-rich gneiss. He referred to quartz-monzonite as the other dominant rock of the Morton block, mainly represented in the Sacred Heart granite, which intrudes the gneisses.

Goldich and Wooden (1980), after Goldich et al.'s (1970) work on the age of the rocks of the MRV, emphasized the distinction between the paleosome & the neosome of the Morton gneiss. The paleosome consists of tonalitic to granodioritic gneisses with amphibolite intrusions. The neosome is made of different granitic gneiss varieties which are, from oldest to youngest: pegmatite & microcline granite, fine-grained adamellite ("adamellite-1"), agmatic granodiorite, and "adamellite-2", itself comprising two subtypes, a microadamellite porphyry and a gneissic fine-grained adamellite. Goldich and Wooden were using Johannsen's classification (1939); the IUGS now discourages the use of the term "adamellite" , which describes a granite with equal amounts of plagioclase and K-feldspar. The rock they called adamellite-1 is the commonest granitic phase of the Morton gneiss, while the adamellite-2 is similar to the Sacred Heart granite.

Next: More microphotographs of Morton gneiss

1 Introduction

2 Geological setting
3The four suprovinces

4 The Minnesota River Valley Suprovince
4-1 Benson block

4-2 Montevideo Block: Lithography

4-3 Morton Block: Lithography (this page)

4-3 Morton Block: Lithography (cont.)

(to be continued)

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