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, Tobacco Root Mountains, southwestern Montana. These lithologies are separated from other Archean lithologic associations by shear zones and an inferred thrust fault system and they are intruded by the Cretaceous Tobacco Root Batholith. The meta-ultramafic rocks in the amphibolite unit form either layers concordant to subconcordant to compositional layering in amphibolite, or pods where layering is disrupted by shear zones. Hornblende-rich blackwalls occur at the contacts between the amphibolite and meta-ultramafic layers and along shear zones that disrupt layers. Amphibolite from the amphibolite unit is geochemically and texturally distinct from thin amphibolite layers in another Archean unit, the intermediate gneiss and schist unit. Meta-ultramafic rocks contain 45–55% SiO2, 3–7% A12O3, 0.19–0.31% TiO2, 22–32% MgO, 0.01–0.54% P2O5, and 1300–4000 ppm Cr; CaO/MgO = 0.11–0.33, La/Ce = 0.14–0.56; La/Sm = 2.88–6.02. 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 tectonically juxtaposed against shelf sediments and the protolith of the intermediate gneiss and schist unit.