ENIGMATIC AMPHIBOLITE LOCATED NORTH OF THE STILLWATER COMPLEX, MONTANA

The Stillwater Complex (~2.71 Ga) is an Archean layered mafic-ultramafic intrusion in southwest Montana. Layering of the complex dips to the north and appears to dip into the Dry Fork Thrust. A gravity anomaly north of the fault may be the complex under cover. Xenoliths with cumulus textures similar to complex are found in Tertiary stocks to the north of the fault. The only outcrop of Precambrian age north of the fault is an amphibolite. We collected eight samples of this amphibolite approximately 2 km north of the complex.

Previous workers described these rocks as amphibolite schist, composed of amphibole and clinozoisite with minor quartz, calcite, chlorite, and epidote. The amphibolite schist may represent: (1) country rocks below the complex, (2) metamorphosed roof rocks above the complex, or (3) metamorphosed cumulates of the complex.

Our samples are mafic meta-igneous rocks of lower amphibolite to greenschist-facies with relict igneous textures to mylonitic fabrics. Electron microprobe analyses of coexisting amphibole and plagioclase were used to constrain the temperature conditions at various pressures for three samples (P at 3, 5, and 6 kb; T = 417-441°C). These conditions are consistent with greenschist facies metamorphism.

Bulk rock geochemistry was used to test the origin hypotheses. Our data were compared to existing geochemical data from country rocks below the complex, Stillwater cumulates, and mafic dikes cutting the complex. No known roof rocks have been identified. The samples plot in the iron-rich basalt field using the Jensen classification and are dissimilar from the greywacke and iron formation found below the complex. Primitive-mantle-normalized spider diagrams of incompatible elements suggest that the samples are not cumulates. REE distributions of the samples resemble the liquid-like patterns of Precambrian dike rocks (~2.65 Ga) which cut the complex. The amphibolite unit (~4.5 km²) is much larger than the linear dikes (10 m in width) that cut the complex. Our results provide constraints to the origin of these enigmatic metamorphic rocks.