Archean Tectonics of the North Snowy Block, Beartooth Mountains, Montana

Abstract

The North Snowy Block (NSB), northwestern Beartooth Mountains, Montana, is composed of seven major lithologic units of Archean age: (1) Mount Cowen granitic augen gneiss; (2) a paragneiss unit that consists of a variety of quartzofeldspathic gneisses, psammitic schists, amphibolites, and iron formation; (3) an augen-textured granitic sill; (4) the phyllitic Davis Creek Schist; (5) a trondhjemitic gneiss-amphibolite complex; (6) the Pine Creek Nappe complex, a regional isoclinal structure with an amphibolite core and symmetrically disposed marble and quartzite; and (7) a supracrustal-migmatite complex that consists of a quartzite-amphibolite unit and a heterogeneous gneiss with lit-par-lit injections of granitic material into the supracrustal rocks. Each unit has its own distinct lithologic assemblage, metamorphic grade, and structural style. The contacts between most units are marked by mylonitic rocks (which are interpreted as the products of ductile faulting) and abrupt changes in metamorphic grade. In ...