Precambrian basement geologic map of Montana– An interpretation of aeromagnetic anomalies

Abstract

Volcanic rocks (Cenozoic) MESOPROTEROZOIC SEDIMENTARY ROCKS Belt Supergroup (Mesoproterozoic)-Area containing major bodies patterned Yellowjacket Formation and associated metasedimentary rocks (Mesoproterozoic)-Area containing major bodies patterned TRANS-HUDSON OROGEN Volcanic-plutonic arc complexes (Paleoproterozoic)-Includes Archean gneisses TRANS-MONTANA OROGEN (ACCRETED TERRANES) Wallace terrane (Paleoproterozoic)-Covered Biotite-quartz-feldspar gneiss and amphibolite of Wallace terrane (Paleoproterozoic)-Exposed in Bitterroot Range Granite, diorite, and gneiss (Paleoproterozoic)-Crops out in Little Belt Mountains. U-Pb zircon age of 1,860-1,880 Ma (Mueller and others, 2002) Medicine Hat block (Archean)-Covered CONTINENTAL-MARGIN ASSEMBLAGE (FOLD-AND-THRUST BELT) Imbricately intercalated rocks (Paleoproterozoic and Archean)-Covered Meta-sandstone, shale, iron-formation, and graphitic shale intruded by gabbro dikes (Paleoproterozoic)-Crops out in Gravelly Range. Interpreted as foreland basin deposit Marble, quartzite, and schist (Paleoproterozoic)-Inferred to have formed on a rifted, passive margin (about 2.0 Ga). Formerly regarded as Archean Massive to weakly foliated granite to granodiorite (Late Archean) Foliated and gneissic granitoid rocks (Late Archean)-About 2.74-2.79 Ga (Wooden and others, 1988) Mafic to ultramafic rocks in Spanish Peaks area and Tobacco Root Mountains (Archean) WYOMING PROVINCE (CRATON) Stillwater Complex (Late Archean)-2.7 Ga (Wooden and others, 1988). Exposed Foliated and gneissic granitoid rocks (Late Archean)-About 2.74-2.79 Ga (Wooden and others, 1988). Exposed Granitic rocks, undivided (Archean)-Magmatic domain. Covered Gneissic rocks, undivided (Archean)-Gneiss domain. Covered

Proterozoic evolution of the western margin of the Wyoming craton: implications for the tectonic and magmatic evolution of the northern Rocky Mountains

“…The distribution of crustal age provinces shown on Fig. 1 is derived from exposures of mainly Archean rocks from the Wyoming craton (Mogk et al 1992;Chamberlain et al 2003), Paleoproterozoic and less abundant Archean rocks in the Great Falls tectonic zone Kellogg et al 2003;Vogl et al 2003Vogl et al , 2004aVogl et al , 2004b, along with evidence for 2.4-1.6 Ga basement beneath the Belt basin, and the Idaho batholith (Mueller et al 1995Foster et al 2002;Sims et al 2004). The following section summaries the basement terranes west and northwest of the Wyoming craton.…”

“…1). Sims et al (2004) separate the Medicine Hat block from the basement in the Priest River complex, based on magnetic anomalies. The southeastern border of the Medicine Hat block in central Montana is the Great Falls tectonic zone (O'Neill and Lopez 1985;O'Neill 1998) and it is bounded on the east by the Trans-Hudson orogen.…”

“…This is consistent with the interpretation that the western edge of the thick Wyoming craton is within this area. Magnetic anomaly patterns also suggest the presence of a major basement boundary along the edge of the thrust belt (Sims et al 2004).…”

“…Isotopic tracer data and inherited zircons from the Cretaceous plutons indicate that Proterozoic lower crust west of this area includes rock-forming events of ca. 1.8-1.6 Ga. High-grade metamorphism and partial melting associated with orogenic events that overprint Archean rocks in the northwestern Wyoming craton (e.g., Tobacco Root and Highland Mountains) indicate orogenic events between -2.4 and -1.77 Ga (Mueller et al 1994(Mueller et al , 2005Roberts et al 2002;Sims et al 2004). Because of the limited and discontinuous exposure of the Paleoproterozoic rocks and the currently limited geochronologic data, it is not possible to decipher the structural relationships between different segments of the material accreted to Wyoming or the number of orogenic events involved.…”

Crust and upper mantle structure in central Brazil derived by receiver functions and SKS splitting analysis

“…This border region, now represented by the outcrops of the geologic units of the Tocantins province in central Brazil, can have its geologic past pictured as in Fig. 14, where we used the same model of Sims et al (2004) for North America (on the origin and evolution of the Wyoming province). In this case, it is clear that, in addition to a possible influence of the asthenosphere found, in central Brazil, when we consider the region affected by the former foredeep and fold-and-thrust belt of the Tocantins province, there is a clear and strong influence of the lithosphere superimposed to the mantle influence on the observed anisotropy results.…”