GEOCHEMICAL AND PETROLOGIC ANALYSIS OF PALEOPROTEROZIC METAMORPHOSED MAFIC DIKES AND SILLS FROM THE HIGHLAND MOUNTAINS MONTANA

Paleoproterozoic metamorphosed mafic dikes and sills (MMDS) crosscut much of the suite of quartzofeldspathic gneisses that characterize the Precambrian core of the Highland Mountains in southwestern Montana. The Highland Mountains MMDS are garnet-clinopyroxene-hornblende-plagioclase-quartz amphibolites that may have a relict igneous fabric. The MMDS are recognized as dikes, although they crosscut the gneissic fabric with a very low angle of obliquity. In the field, the MMDS are commonly aphanitic and weathered to a red-brown color. The results of major and trace element geochemical analysis of the MMDS will be presented, as well as the results of garnet-hornblende thermobarometry. These results will be assessed in terms of tectonic setting and magma genesis. Similar MMDS’ in the neighboring Tobacco Root Mountains intruded at 2.1 Ga, and were metamorphosed to upper amphibolite grade about 1.77 Ga. In order to determine if the MMDS’ are cogenetic, the geochemistry of the Highland Mountains MMDS will be compared to that of the Tobacco Root Mountains MMDS (Brady et al. 2004). The emplacement of the Highland Mountains MMDS’ in the Montana Metasedimentary terrane (MMT) of the Wyoming province predates the 1.77 Ga Big Sky orogeny and its associated metamorphism by a few hundred million years. This orogeny is related to the Hearne-Wyoming province collision that formed a third of the North American continent. Understanding the character of these MMDS’ will help determine the paleoenvironment of the MMT before the Big Sky orogeny, and enrich our understanding of the accretion of the North American continent.