Metamorphism and P-T Path of Garnet-Sillimanite-Biotite Migmatites, Highland Mountains, Montana

Precambrian basement rocks of the northwestern Wyoming Province underlying southwest Montana form the metamorphosed core of an arc-continental collision known as the Big Sky orogen (~1.78 – 1.71 Ga). Previous studies have shown that the Big Sky orogeny affected Precambrian rock in the Tobacco Root Mountains along a clockwise P-T path, first at a high temperature and high pressure stage during burial followed by partial melting during decompression.

The adjacent Highland Mountains make up the westernmost part of the Wyoming province; basement exposure consists of well-foliated to massive quartzofeldspathic gniess intruded by mafic dikes and biotite gneiss rich in garnet, sillimanite and K-feldspar. Precambrian rocks of the Highland Mountains have a core of quartzofeldspathic gneiss with mafic dikes similar to the Tobacco Root Mountains, flanked by an overlying domain of biotite sillimanite schist and gneiss that is unlike the Tobacco Root suites. This schist unit outcrops extensively in Camp Creek. Camp Creek is therefore a lithologically distinct terrane of massive to well-foliated rocks containing mineral assemblages that include both biotite-rich gneiss and migmatitic gneiss rich in K-feldspar and randomly oriented clusters of sillimanite. The absence of kyanite or sillimanite pseudomorphs of kyanite is a striking contrast to similar rocks in the Tobacco Roots.

Twenty-six ion microprobe 207Pb/206Pb spot ages on monazite from three Camp Creek samples have been obtained from the IMS 1280 at WHOI.

Although two of these spot ages fall within the Big Sky array (1780-1790 Ma) most are slightly older ranging from 1810-1890 Ma. This age array is consistent with electron microprobe ages from one Camp Creek sample reported by Gerwin (2006). That sample has two age domains with mean ages of 1819 ± 28 Ma and 1737 ± 69 Ma, i.e. an older core and an age consistent with the close of the Big Sky orogenic event. Further dating and examination of samples from Camp Creek to determine a possible P-T path will provide insight as to the role of biotite gneiss terrane played in the building of the Highland Mountains, and possibly the extent to which the Big Sky orogeny affected the Highland Mountains.