METAMORPHISM AND P-T PATHS OF K-FELDSPAR-GARNET-SILLIMANITE-BIOTITE BEARING ROCKS FROM THE HIGHLAND MOUNTAINS, SOUTHWESTERN MONTANA

The Tobacco Root and Highland Mountains in southwestern Montana are Laramide uplifts containing Precambrian rocks which record a 1.78-1.71 Ga metamorphic and deformational event known as the Big Sky orogeny. Previous work in the Tobacco Root Mountains (TRM) yielded a clockwise P-T evolution where the highest pressures and temperatures were followed by isobaric cooling and isothermal decompression.

The Highland Mountains, west of the TRM, are cored by a quartzofeldspathic gneiss unit cut by metamorphosed mafic dikes and sills. A thin and diverse unit (designated Xg) partially rims the core and is mostly garnet rich gneiss with marble, quartzite, iron formation, and amphibolite. These two units are similar to suites found in the TRM. The core is mantled by a unit containing biotite-sillimanite-garnet gneiss which is unlike any in the TRM. The core and mantle are juxtaposed across a zone of mylonitization and leucocratic melt.

The mantle gneiss and the Xg unit each contain rocks with the essential mineral assemblage sillimanite + garnet + K-spar + biotite with different textures. Samples from Camp Creek on the northwestern edge of the mantle contain no kyanite, sillimanite pseudomorphs after kyanite, cordierite or orthopyroxene, whereas abundant muscovite laths cut across the fabric, indicating nearly isobaric heating and cooling at P between 4 and 8 kbar and T between 650ºC and 800ºC. Samples from between Camp Creek and the Xg unit also have no evidence of kyanite but have less abundant and more skeletal back-reacted muscovite. Samples from the Xg unit have sillimanite pseudomorphs after kyanite and contain cordierite but have no retrograde muscovite, which indicates a clockwise P-T path with heating at P>8.0 kbar and cooling at P<3.5 kbar. The absence of orthopyroxene requires T<850ºC at P=6.5 kbars. ²⁰⁷Pb/²⁰⁶Pb spot ages on monazites from Camp Creek and the Xg unit were obtained from the ion microprobe at WHOI (Brown (2008) and Pearson (2008)). The majority of the Camp Creek rocks yielded an age range from 1870-1770 Ma and most of the Xg rocks yielded an age range of 1860-1760 Ma. The younger ages overlap with the Big Sky orogeny in the TRM while the older ages indicate the Highland Mountains were experiencing metamorphism necessary to grow monazites before the TRM.