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Geological Society of America


Just over two billion years ago, basaltic magma intruded rocks of the Pony– Middle Mountain Metamorphic Suite and the Indian Creek Metamorphic Suite that now crop out in the Tobacco Root Mountains of Montana near the northwestern margin of the Wyoming province. Numerous examples can be found of mafic dikes that crosscut layering and gneissic banding, demonstrating that the host rocks were metamorphosed to form the gneissic texture prior to intrusion of the dikes. Although many of the intrusions appear to be sills that followed compositional layering, close inspection reveals low-angle discordance in nearly every case, consistent with rotation of dikes by shearing into nearly layer-parallel orientations. The mafic dikes do not intrude the adjacent Spuhler Peak Metamorphic Suite, which we interpret to mean that the Spuhler Peak Metamorphic Suite was not present at the time of intrusion. These dikes and sills were metamorphosed along with their host rocks during the Big Sky orogeny, a major orogenic event at 1.77 Ga that is documented in this volume. The fine-grained, garnet-bearing, rusty-weathering metamorphosed mafic dikes and sills (MMDS) generally have a distinctive appearance in the field. Some MMDS were clearly folded or boudinaged during metamorphism, but many show only weak foliation and still have sharp contacts with their host rocks. A decrease in grain size commonly occurs on the margins of the MMDS and is believed to be a metamorphic texture that developed from the chilled margins of intrusions into cold rocks. Chemically, the MMDS are subalkaline tholeiites that have been modified by significant fractional crystallization, as evidenced by molar Mg/(Mg + Fe) values that vary from 0.6 to 0.3 and TiO2 values that vary from 0.6 to 3.0 wt%. The ratio of TiO2 to P2 O5 is virtually constant, indicating a common origin for the MMDS and that these two components were similarly incompatible with the fractionating minerals. The weight percentages of both change by a factor of five, which would require crystallization and removal of 80% of the liquid if there were a single magma source. Based on the major element chemistry, plagioclase, orthopyroxene (or pigeonite), and clinopyroxene were the major fractionating phases, with little involvement of olivine. Trace element variations are consistent with this interpretation. For example, there is no clear correlation between Ni and MgO, which would be expected if there were significant olivine fractionation. Measured rare earth element (REE) data for the MMDS are all at least ten times chondrite REE values. In most cases, light rare earth elements (LREEs) are enriched relative to heavy rare earth elements (HREEs). These data are consistent with a model of intrusion of the MMDS into cool, metamorphic rocks at 2.06 Ga during continental rifting of the Wyoming province. The resulting ocean basin closed at 1.77 Ga during the Big Sky orogeny, emplacing rocks of the Spuhler Peak Metamorphic Suite and slivers of an ultramafic, orthopyroxene cumulate, deforming all units together, and metamorphosing all rocks to at least upper amphibolite facies conditions.


fractional crystallization, subalkaline tholeiites, Wyoming province

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© 2004 Geological Society of America


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