Geochemistry and origin of amphibolite and ultramafic rocks, Branham Lakes area, Tobacco Root Mountains, southwestern Montana

Abstract

An Archean amphibolite unit, composed of amphibolite, garnet amphibolite, and meta-ultramafic rocks, is exposed in the Branham Lakes area. The meta-ultramafic rocks in the amphibolite unit form either layers concordant or subconcordant to compositional layering in amphibolite, or pods where layering is disrupted by shear zones. Amphibolite from the amphibolite unit is geochemically and texturally distinct from thin amphibolite layers in another Archean unit, the intermediate gneiss and schist unit. The data distribution on Ti-Zr, Ti-Zr-Sr, and Ti-Zr-Y diagrams for amphibolite from the amphibolite unit are consistent with low-K or ocean-floor tholeiites, or calcalkaline basalts. On a Nb-Y-Zr plot, the data for amphibolites are consistent with N-type MORB or volcanic-arc basalts. The amphibolite unit may be a sliver of oceanic crust. -from Authors