PRELIMINARY GEOCHEMISTRY OF SAMPLES FROM YELLOWSTONE NATIONAL PARK

Precambrian terrains within Yellowstone National Park consist largely of granitic orthogneisses that underlie an area of approximately 250 km² and are exposed in the northern portions of the park. Previous mapping by others has grouped these units as one, despite variations that exist in their mineralogy. Pb isotopic ratios for Lamar Canyon gneisses have yielded intrusive ages of 2625-2700 Ma, which coincide with igneous activity throughout the region in areas such as the Eastern Beartooth Mountains to the north. Exact ages for other units in the area have yet to be determined and relative timing relies on structural and contact relationships. Preliminary data from whole rock major chemical analyses of samples collected from Lamar Canyon (L.C.), Hellroring Creek (H.C.) and Slough Creek (S.C.) indicate covariations that display smooth trends suggestive of fractional crystallization while samples collected from the Yellowstone River Trail (Y.R.T.) area display dissimilar compositions. Plotting samples from each location on an AFM diagram indicate a tholeiitic trend with SiO₂ values ranging from 53-73 wt%. Alumina saturation index values vary from peralkaline to peraluminous between samples from L.C. and S.C. but are consistently peraluminous for H.C. and Y.R.T. samples. Trace element plots indicate similarities between the L.C. and S.C. gneisses but little to no relationship with the H.C. or Y.R.T. gneisses. These initial results suggest that the H.C., L.C. and S.C. may have evolved from fractional crystallization of a single magma while the Y.R.T. originated from a separate source. However, trace element plots indicate differences between the H.C. samples and the L.C. and S.C. These may be the result of incorporation of different country rock during emplacement or a desperate emplacement altogether.