THE ORIGIN OF XENOLITHS WITH CUMULUS TEXTURES FOUND ABOVE THE SUBSURFACE EXTENSION OF THE STILLWATER COMPLEX, MONTANA

The Stillwater Complex (~2.71 Ga) is a layered mafic-ultramafic intrusion exposed in the Beartooth Mountains of south-central Montana. Interpretations of gravity measurements suggest that the north-dipping complex extends under cover at depth. Younger Cretaceous stocks (Susie Peak and Sliderock Mountain), diorite sills, and andesite dikes of Cretaceous age are exposed to the north, overlying the area of the gravity anomaly, which may represent a subsurface extension of the Stillwater Complex. In the summer of 2013, samples of the two Cretaceous stocks with xenoliths were collected for study. Xenoliths with cumulus textures resembling those of Stillwater Complex were chosen for further investigation. The andesitic host rocks exhibit porphyritic textures—phenocrysts of plagioclase, amphibole ± biotite occur in a finer-grained groundmass of the same minerals. Electron microprobe analyses of amphibole grains from the host and xenoliths are comparable in composition. Electron microprobe analyses for two samples of coexisting xenolith amphibole (Tschermakitic hornblende, magnesio-hastingsite) and plagioclase (rim An_83-91Ab_17-9) constrain temperature conditions at various pressures (P at 3, 5, and 6 kb; T = 500-656°C, respectively. These conditions are consistent with amphibolite facies metamorphism. Few of the sampled xenoliths retain the original igneous mineralogy (with the exception of plagioclase) but one sample contains relict “Stillwater-like” clinopyroxene and another contains chromite. Plagioclase and relict clinopyroxene compositions are somewhat comparable to Stillwater mineral compositions. Metamorphism and intense alteration of the xenoliths suggest that either they were metamorphosed prior to their inclusion in the melt or were metamorphosed or hydrothermally altered as a result of incorporation into the melt.