ABSTRACT

Pre-metamorphic parentage and depositional environment are poorly understood for Archean supracrustal sequences exposed in the cores of uplifted blocks in southwestern Montana. Recent workers have abandoned inconsistent genetic/stratigraphic designations (i.e. Pre-Cherry Creek Group) and suggest the Montana basement is structurally and IithologicaIly continuous This study represents the first attempt to correlate Archean lithologic sequences across range boundaries.

Field studies in the Copper Mountain area of the southern Tobacco Root Mountains and Kelly area of the northern Ruby Range have shown that distinctive and consistent lithologic packages occupy tight refolded synforms in both areas. The sequence upwards in both areas consists of quartzo-feldspathic gneiss, dolomitic marble, amphibolite, quartzite/chert, garnet-biotite-silliminite schist, and banded iron-formation. Small tectonically emplaced ultramafic fragments are present in both locales Foliation parallels lithologic contacts. Upper amphibolite to lower granulite-grade metamorphism, and at least two periods of folding around N-NE axes make strict stratigraphic correlations difficult.

Petrology and sequence are strikingly similar for both areas and are interpreted as representing deposition in a remnant Archean marginal basin. Active basaltic volcanism accompanied deposition of carbonate (marble), pelitic (schist), mafic volcaniclastic (banded amphibolite), and chemical (chert/quartzite and banded iron-formation) sediments. Mafic flows and tuffs were the progenitors of most amphibolite. Quartzofeldspathic gneiss most probably represents a mix of intrusive igneous rocks and arkose-shale sequences which floored the basin.