Geology and geochemistry of the Spuhler Peak Metamorphic Suite
H. Robert Burger, William H. Peck, Kathleen E. Johnson, Kara A. Tierney, Chris J. Poulsen, Pamela Cady, Josh Lowell, William A. MacFarlane, M. Jennifer Sincock, LeAndra L. Archuleta, Ann Pufall, M. Jason Cox, 2004. "Geology and geochemistry of the Spuhler Peak Metamorphic Suite", Precambrian Geology of the Tobacco Root Mountains, Montana, John B. Brady, H. Robert Burger, John T. Cheney, Tekla A. Harms
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The Spuhler Peak Metamorphic Suite consists of a thick sequence of Archean metamorphic rocks, dominated by amphibolite, orthoamphibole-garnet gneiss, and hornblende gneiss and containing minor quantities of quartzite, aluminous schist, quartzofeldspathic gneiss, and meta-ultramafic rocks. Although the Spuhler Peak Metamorphic Suite represents less than 3% of Precambrian exposures in the Tobacco Root Mountains, it has been recognized as a unique suite in the region, due in large part to the unusually abundant amphibolite and the spectacular appearance of the orthoamphibole-garnet gneisses.
An analysis of mafic rocks within the Spuhler Peak Metamorphic Suite, using classification diagrams, tectonic discrimination diagrams, and rare earth element plots, provides results consistent with primary igneous processes and pre-metamorphic hydrothermal alteration. Spuhler Peak Metamorphic Suite amphibolites compare well to typical Archean tholeiitic basalts with arc-related affinities. The major element geochemistry of the Spuhler Peak Metamorphic Suite orthoamphibole-garnet gneisses is within the range of the suite's amphibolites, excepting CaO and MgO contents. This supports the hypothesis that the amphibolites represent metamorphosed equivalents to the unaltered orthoamphibole-garnet gneiss protolith. When the effects of hydrothermal alteration are taken into account, the geochemistry of the Spuhler Peak Metamorphic Suite orthoamphibole-garnet gneisses is consistent with a volcanic protolith, genetically related to the protolith of its amphibolites. Outcropping within the suite are a number of small bodies of meta-ultramafic rocks, but these rocks are chemically distinct from the suite. We believe these bodies possibly are tectonically emplaced and not directly related to the Spuhler Peak Metamorphic Suite protolith. This protolith, therefore, is interpreted as largely mafic volcanic rocks (90% of the total package) deposited in a marine environment and intercalated with minor amounts of sedimentary material.
The Spuhler Peak Metamorphic Suite is distinct from the other Archean suites in the Tobacco Root Mountains—the Indian Creek Metamorphic Suite, which consists of quartzofeldspathic gneisses and metasedimentary rocks including marble, and the Pony–Middle Mountain Metamorphic Suite, which is composed primarily of quartzofeldspathic gneisses with subsidiary hornblende gneiss. Field relationships and radiometric dating demonstrate that the Spuhler Peak Metamorphic Suite was brought into fault contact with the Indian Creek and Pony–Middle Mountain Metamorphic Suites during the Big Sky orogeny, a major tectonothermal event at 1780–1720 Ma.