Geochemistry of quartzofeldspathic gneisses and metamorphic mafic rocks of the Indian Creek and Pony–Middle Mountain Metamorphic Suites, Tobacco Root Mountains, Montana
Quartzofeldspathic gneisses and metamorphic mafic rocks are the dominant lithologies in the Indian Creek and Pony–Middle Mountain Metamorphic Suites of the Tobacco Root Mountains. Field relations, geochemical discriminant analysis, and isotopic systematics indicate that these rocks derive from a bimodal volcanic suite ca. 3.3 Ga. The quartzofeldspathic gneisses contain sodic rocks of the tonalite-trond-hjemite-granodiorite suite as well as potassic varieties. This suite of rocks most likely contains some lithologies derived from sedimentary or volcaniclastic sources, and there is evidence that alkali metasomatism occurred prior to or during subsequent major tectonothermal events. The entire suite of gneisses and metamorphic mafic rocks has geochemical characteristics that are indicative of an active continental arc setting, with deposition most likely in an extensional, backarc setting similar to the Mesozoic through Tertiary rocks of the eastern Sierra Nevada Mountains or Mojave Desert. The formation of these rocks represents an early, distinct stage of crustal evolution that preceded the (unconformable?) deposition of one or more platform-type sedimentary sequences (e.g., marbles, pelitic schists, quartzites, banded iron formations). All primary lithologic contacts and textures or structures indicative of possible protoliths have been largely obliterated due to transposition during Archean and Paleoproterozoic (ca. 2.4 and ca. 1.8 Ga) deformation and metamorphism.