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The Highline Trail Lakes area lies within the Archean basement complex in the Beartooth Mountains of Montana and Wyoming. Eighteen small, pod-shaped, ultramafic bodies associated with granitic gneiss, migmatite, amphibolite, and pegmatite are distributed along a narrow, slightly arcuate zone within the area.

The ultramafic rocks are mineralogically complex, containing olivine, ortho-pyroxene, hornblende, anthophyllite, phlogopite, chlorite, serpentine, talc, magnetite, and green spinel in almost every combination in the three dominant rock types: olivine-rich, hornblende-rich, and hornblende-anthophyllite rocks. All textures are crystalloblastic; relict igneous textures were not found. Hornblende-anthophyllite rocks show pronounced lineation of mineral grains and appear to be derived from sheared hornblende-rich rocks. Strongly lineated rocks at the margins of some bodies consist almost entirely of anthophyllite.

Closely spaced, highly planar orthopyroxene layers (2 to 3 mm thick) produce rhythmic orthopyroxene-hornblende layering in a few hornblende-rich rocks. These layers, commonly one grain-diameter thick, are coincident with nearly planar surfaces defined by magnetite trails, which appear to represent fractures healed by recrystallization. Locally, less planar layers of identical orthopyroxene traverse the more planar set at high angles. Orthopyroxene-hornblende layering is not associated with any other distinctive fabric element. Later shearing produced a parallel set of serpentinized layers containing fragments of hornblende and orthopyroxene, and bordered by intensely fractured rock. Both types of layering are best explained as resulting from the structural control of metasomatic processes.

The bodies are structurally and chemically similar to alpine ultrabasic rocks, and appear to be metamorphic derivatives of a strongly differentiated picrite. The sequence of events following consolidation is as follows: (1) recrystallization under high amphibolite facies conditions to olivine-rich and hornblende-rich rocks; (2) development of orthopyroxene along shear fractures near granulite facies conditions; (3) tectonic breakup of parent body and movement of fragments through plastic country rocks; (4) growth of clinochlore, phlogopite, and anthophyllite at temperatures below about 600°C penecontemporaneously with feldspathization of of country rocks; and (5) serpentinization and steatitization, probably representing several later events. The observed nonequilibrium mineral assemblages resulted from incomplete replacement of earlier assemblages. Most replacements appear to involve metasomatism.

Age relationships and geographic distribution suggest that these masses are parts of a disrupted, metamorphosed, satellite intrusion of the Stillwater igneous complex.

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