MINERALOGICAL AND GEOCHEMICAL CHARACTERIZATION OF LINE CREEK PLATEAU PORPHYRY OUTCROPS (BEARTOOTH MOUNTAINS) AND FRONTIER FORMATION PORPHYRY COBBLES (BIGHORN BASIN), MONTANA AND WYOMING

The Line Creek Plateau in the southeastern Beartooth Mountains (Montana/Wyoming) exhibits outcrops of intermediate-composition porphyry. Outcrops consist of sills and dikes, small isolated and circular outcrops, and large irregular outcrops; samples from each of these outcrop types were collected for petrographic, geochemical, and geochronologic analysis. Noticeable mineralogical variation exists between outcrops of porphyry within the Beartooth Mountains. Hand samples from each site prominently feature plagioclase phenocrysts. Some outcrops near contacts with granitic/gneissic host rock contain well-formed euhedral quartz phenocrysts. Select outcrops feature orthoclase megacrysts. Petrographic analysis shows embayed quartz phenocrysts, sericitization rims on feldspars, and opacitic rims on biotites and amphiboles in all samples. Many larger phenocrysts consist of multiple crystals of plagioclase, orthoclase, and/or quartz, which may represent relic granitic texture (i.e., xenoliths). Embayed quartz and opacitic rims suggest decompression and dewatering, perhaps during magmatic ascent. Sericitization and embayment indicates disequilibrium conditions in the magma body. Sample alteration is exhibited by serpentine/chlorite and possible hematite; pervasive alteration may increase towards the Beartooth thrust. Early findings suggest a link between intermediate porphyry cobbles deposited in the ca. 95 Ma Frontier Formation in eastern Bighorn Basin (Wyoming) and the in-situ porphyry outcrops on Line Creek Plateau based on petrography and geochemistry. In-progress trace element geochemistry provides convincing evidence that the Beartooths area is the provenance of the Frontier Formation porphyry cobbles, while the geochemistry of coeval Idaho Batholith magmatics is too enriched to suggest an Idaho provenance. Pending geochemical, geochronological, and scanning electron microscope analysis is expected to clarify the nature of the relationship between Beartooth Mountains and Frontier Formation porphyry. Provenance of Frontier Formation cobbles and the nature of porphyry magmatism exposed on Line Creek Plateau may provide temporal constraints on igneous development within the northern Laramide foreland region.