Eugene I. Smith, 1983. "Geochemistry and evolution of the early Proterozoic, post-Penokean rhyolites, granites, and related rocks of south-central Wisconsin, U.S.A.", Early Proterozoic Geology of the Great Lakes Region, L. G. Medaris, Jr.
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Four chemically and mineralogically distinct rock suites formed in south-central Wisconsin during a major anorogenic intrusive-extrusive event 1.76 b.y. ago. This anorogenic event followed by at least 60 m.y. the terminal calc-alkalic plutonism and volcanism of the Penokean orogeny. The 1.76-b.y.-old rocks may represent the only surface expression in the Lake Superior area of a 1.69 to 1.78-b.y.-old felsic terrain that can be traced in the subsurface from Wisconsin into northern Illinois and possibly as far as western Arizona. The magma types formed during the anorogenic event are (1) a peraluminous suite of texturally variable ash-flow tuffs; (2) a meta-luminous suite containing quartz- and orthoclase-bearing rhyolites and closely associated granophyric granites; (3) a suite of low SiO2, high Sr, and REE depleted granites at Baxter Hollow and chemically similar rhyolite dikes on Observatory Hill and at Marquette (Baxter Hollow suite); and (4) a suite containing a diorite intrusion and numerous dikes of tholeiitic basalt and andesite (Denzer suite).
Major and trace element modeling studies suggest the following scenario for the formation of these post-Penokean rocks. The peraluminous and metaluminous suiteswere probably derived by partial melting (16%) of different crustal sources of tonalitic composition having slightly different residue mineralogies. These source rocks may have formed earlier by igneous activity related to the Penokean orogeny. Compositional variation within the two suites may be related to feldspar-dominated fractional crystallization. The Baxter Hollow suite was probably generated by large-scale (66%) partial melting of a crustal source of dioritic composition. Heat for such a major melting episode may have been provided by magma of the Denzer suite rising from a mantle source. Modeling studies suggest that 10% melting of either garnet peridotite, or eclogite will provide liquid of the composition of the Denzer suite. Fractional crystallization is most probably responsible for the chemical variation observed within both the Denzer and Baxter Hollow suites.