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Stillwater Complex, Montana; rock succession, metamorphism and structure of the complex and adjacent rocks

1977; United States Government Publishing Office; Linguagem: Inglês

10.3133/pp999

2330-7102

Norman J Page,

Paleontology and Stratigraphy of Fossils

Along the northern border of the Beartooth Mountains in Stillwater, Sweetgrass, and Park Counties, Mont., a terrane of Precambrian rocks overlain by Paleozoic, Mesozoic, and Cenozoic sedimentary rocks contains the differentiated stratiform ultramafic and mafic Stillwater Complex.In addition to the complex, several groups of Precambrian rocks are distinguished by mineralogic, petrologic, chemical, structural, and age criteria as follows:(1) regionally metamorphosed rocks consisting of granitic gneisses, migmatites, biotite schists, and amphibolitic gneisses, (2) hornfelsed metasedimentary rocks that form the contact aureole of the Stillwater Complex, and (3) a sequence of quartz monzonites distinguished by grain size, mineralogy, and mutual intrusive relations.docks of the contact aureole of the complex are comparatively rich in MgO, total iron, Cr, and Ni and are depleted in K20 and N a20; they range in grade from pyroxene hornfels near the complex to albite-epidote hornfels away from it.They consist of (1) a finely layered sequence of metasedimentary rocks displaying relict small-scale crossbedding, cut-and-fill structures, and graded bedding that suggest a high-energy environment, (2) a diamictite unit of questionable glacial origin, (3) blue metaquartzites, possibly cherts, (4) an iron-formation that may have formed by chemical precipitation, (5) massive fine-grained metasedimentary rocks, and (6) layered rocks with no relict sedimentary features.The Precambrian geologic record in the vicinity of the Stillwater Complex shows a lengthy and complicated history of multiple deformation, contact and regional metamorphism, erosion, sedimentation, and several episodes of igneous intrusion.Interpretation of these events is hindered by effects of the Late Cretaceous to Eocene Laramide orogeny.In early Precambrian time, prior to about 3,140 m.y.(million years), erosion of a terrane consisting of rocks with ultramafic, mafic, and intermediate compositions provided clastic and chemical components to the sediments that accumulated in nearby basins.These MgO-and total iron-enriched sedimentary rocks were folded at least twice and were possibly involved in a low-grade regional metamorphism.Between 2,750 and 3,140 m.y., the Stillwater magma was intruded and fractionated, forming magmatic sediments, and was the heat source for the development of the contact aureole.During this time this terrane must have been a stable area.Later within the same time span, the block containing the Stillwater Complex and hornfelsed metasedimentary rocks was juxtaposed next to the early Precambrian regionally metamorphosed rocks, probably by wrench faults, one of which may be the Mill Creek-Stillwater fault zone.Along the eastern part of this zone at 2,750 m.y., a sequence of quartz monzonites was intruded and contact metamorphosed some of the older rocks.By the time of these intrusions the regionally metamorphosed rocks were probably already complexly folded and past the peak of the regional metamorphic event that formed the gneisses and migmatites.Between 1,600 and 1,800 m.y.mafic dikes were intruded, a penetrative deformation developed, and a low-grade regional metamorphic event affected the Stillwater Complex and adjacent rocks.Before the Cambrian, the terrane was faulted, rotated and tilted, uplifted, and eroded to form the ancestral Beartooth Mountains.By Middle Cambrian time the ancestral mountains had subsided and become the site for deposition of 8,000-10,000 ft (2,438-3,048 m) of marine and continental sedimentary rocks until the Late Cretaceous.Volcanism began in Late Cretaceous time, and the Laramide deformation began and continued through the early Eocene.Since then, the area has