Abstract

Isolated exposures (inliers) of Precambrian rhyolites and granites (1,765 m.y. old) crop out in the Fox River valley and in the Baraboo area of south-central Wisconsin. The geochemical characterization of rock units, in addition to field and petrographic studies, was used to unravel the complex geology of two of these inliers, the Marquette and Marcellon rhyolites, and to determine in a preliminary fashion the geology of the Precambrian igneous terrain between exposures, which is covered by a section of Paleozoic sedimentary rocks and Pleistocene glacial deposits of varying thickness.

The rhyolite at Marquette consists of a series of ash-flow tuffs, interbedded with mud-flow breccia, which are broadly folded into a series of normal and overturned asymmetric folds. The rhyolite is cut by a 100-m-thick andesite dike that intruded along a northeast-trending normal fault. The top of the exposed section is a porphyritic rhyolite containing quartz, alkali feldspar, and plagioclase phenocrysts. This unit is underlain by ash-flow tuffs ranging from porphyritic quartz, plagioclase, alkali feldspar rhyolite to rhyolite with only plagioclase phenocrysts. Plagioclase units are phenocryst poor and have Rb/Sr ratios greater than 1; the three mineral units are phenocryst rich and have Rb/Sr ratios less than 1.

The rhyolite at Marcellon consists of four ash-flow tuffs folded into a northeast-trending asymmetric antiform. Lithologically, the Marcellon units are spherulitic, flow-banded, brecciated, and massive. Mineralogically, they vary from porphyritic quartz, plagioclase, alkali feldspar rhyolite to plagioclase-bearing rhyolite. Geochemical correlation was used to relate units from one part of the inlier to the other and thus establish the existence of the antiform. This exposure is cut by andesite and basalt dikes.

Forty new major- and trace-element analyses for the Fox River valley and Baraboo rhyolites and granites were used to subdivide these rocks into four chemical groups: (1) fine-grained granite at Baxter Hollow and coarse-grained rhyolite dikes at Observatory Hill are characterized by high CaO and low Rb/Sr; these rocks intrude rhyolite flows and may represent an intrusive event that occurred after the major extrusive episode; (2) fine-grained and porphyritic rhyolite at the Marquette exposure and the fine-grained plagioclase-bearing rhyolite at Marcellon have intermediate CaO and Rb/Sr; (3) porphyritic rhyolite and granophyric granite are characterized by low CaO and high Rb/Sr; similarities in chemistry between these two rock types suggest that the granophyric granites are the subvolcanic equivalents of the rhyolites; and (4) rhyolites at the Marcellon and Baraboo exposures are intermediate in chemistry between groups 2 and 3. The four groups are chemically related and show calc-alkalic affinities.

Chemical correlation and geologic mapping show that the chemical groups occur geographically as northeast-trending bands across south-central Wisconsin. Granophyric granite lies to the northwest of group 3 porphyritic rhyolites; the porphyritic rhyolites lie in general to the northwest of the texturally variable rhyolites of groups 2 and 4. Structural trends within exposures parallel contacts between chemical groups, thus strengthening the thesis that the chemical trends reflect the geology of the large area of buried Precambrian rock in south-central Wisconsin.

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