TECTONIC INVESTIGATION OF PRECAMBRIAN ROCKS IN THE GRAVELLY RANGE, MONTANA

Amphibolites, metagabbros and other mafic metamorphic rocks form part of a north-dipping section of Precambrian basement exposed in the central Gravelly Range of SW Montana and are the basis for further investigation of the Big Sky orogeny, a 1.78 to 1.71 Ga collisional event involving the northern margin of the Wyoming province. Extensive evidence for upper amphibolite grade metamorphism from the Big Sky orogeny, followed by isobaric cooling and tectonic unroofing, has been found in the Tobacco Root Mountains some 30 miles north of the study area.

The study area contains low-grade metamorphic rocks juxtaposed structurally above and north of rocks that have mineral assemblages and fabrics indicative of significantly higher grade metamorphism. Garnet is present in three amphibolite samples south of this contact, allowing for thermobarometric analysis of the metamorphic conditions of mineral growth. These results, together with an ⁴⁰Ar/³⁹Ar amphibole date obtained from a garnet-bearing amphibolite, constrain the extent of the Big Sky orogen in the study area.

Metagabbros crop out as discontinuous bodies on both sides of the contact, and lack the persistent planar foliation of the rocks surrounding them. Analyses of bulk chemistry and mineral assemblages enable a comparison of the two metagabbro locations. These analyses, along with bulk chemistry and mineral assemblages of the other sample rocks, constrain the possible protoliths and the tectonic history of the area.