GEOCHEMICAL ANALYSIS OF PRECAMBRIAN IGNEOUS AND METAMORPHIC ROCKS OF THE HIGHLAND MOUNTAINS, SW MONTANA

Precambrian pelitic schists and gneisses containing probable in situ partial melt are widespread throughout the Highland Mountains of southwest Montana while metamorphosed intrusive granitoids are much less so, but are well exposed along Camp Creek. Geochemical analysis forms the basis for a characterization of these suites of rocks and trace element geochemistry functions as a genetic fingerprint for determining relationships between meta-intrusives and felsic melt layers within the migmatites. Both suites of rocks – intrusives and migmatites – are uniformly peraluminous which suggests a crustal source of melt, but mineral assemblages vary greatly among the meta-intrusive granitoids. Both two mica and hornblende-bearing foliated felsic sills as well as mafic hornblende-rich tonalites occur and garnet is a common accessory mineral in all intrusives. Plagioclase is present in all rocks, but feldspar compositions are also highly variable. Many rocks altogether lack alkali feldspar whereas others contain as much as 50% K-feldspar. Where possible, intrusive ages of these rocks based on zircon U-Pb analysis will be presented. Research in the adjacent Tobacco Root Mountains to the east has provided evidence for a significant regional metamorphic and collisional event at 1.77 Ga, which has been called the Big Sky orogeny, along the northwestern edge of the Archean Wyoming province. These data help to constrain the series of thermotectonic events experienced by the Highland Mountains during the Big Sky orogeny.