PRECAMBRIAN METAMORPHISM IN THE WYOMING PROVINCE: A PETROLOGICAL AND GEOCHEMICAL STUDY OF AMPHIBOLITES IN THE HIGHLAND MOUNTAIN RANGE IN SOUTHWESTERN MONTANA

This project analyzes a suite of Precambrian amphibolites collected from the flank of a metamorphic dome in the Highland Mountain Range in southwestern Montana. They are intercalated within a biotite – sillimanite — garnet schist unit and occur in concordant layers that range from 1-5 m thick and are laterally continuous to the extent of the outcrop.

These amphibolites have the mineral assemblage hornblende – plagioclase – quartz ± garnet ± clinopyroxene ± biotite ± ilmenite ± epidote ± sericite ± chlorite. Across all samples, hornblende is the major matrix mineral and makes up at least 55% of the mode. Quartzofeldspathic material makes up between 15-45%; where it is present, garnet does not exceed 10% of the bulk rock composition. Hornblende grains are generally <1-2 mm long, although some samples contain coarser grains up to 6 mm. Plagioclase and quartz commonly occur in 1-3 mm long, concentrated lenses and in some cases exhibit myrmekite texture. In some samples, garnet grains are rimmed and/or replaced by quartz and plagioclase. Mineral alignment fabric primarily defined by hornblendes and secondarily by quartzofeldspathic lenses is moderately well developed, although some samples present a more massive habit.

The mineral suite of these rocks indicates upper amphibolite facies metamorphism of Highland amphibolites. Where applicable, garnet – hornblende and garnet – clinopyroxene compositions analyzed under a SEM provide a thermometer and barometer, respectively. The rimmed habit of garnet is petrologic evidence for breakdown and decompression during metamorphism. Bulk rock geochemistry indicates that the protoliths of these amphibolites were sub-alkaline basalts to basaltic andesites. Major and trace element geochemistry have been used in order to assess the tectonic origin of the protolith.

Rocks similar to the amphibolites characterized in this project have been extensively studied in the nearby Tobacco Root Mountains. Both the Highland and Tobacco Root Mountains were subject to metamorphism during the Big Sky orogeny. Comparison of the P-T conditions of metamorphism and magmatic origin derived in this study to that of amphibolites from the Tobacco Root Mountains will assess whether or not these rock suites were cogenetic.