TRACE-ELEMENTS AS SOURCE INDICATORS IN CRATONIC SEDIMENTS - A CASE-STUDY FROM THE EARLY PROTEROZOIC LIBBY CREEK GROUP, SOUTHEASTERN WYOMING

To understand how grain size and sorting effect trace element distributions in sediments, we compare the compositions of closely associated quartzites, pelites, and glacial diamictites from the 2-Ga Libby Creek Group in SE Wyoming. We found similar REE distributions (LREE enriched, negative Eu anomalies), and that most compositional differences can be explained by variable quartz dilution. Trace element distributions from mineral separates in a fuchsite quartzite show that fuchsite contains most of the Th, U, REE, Sc, Co, and Ni and controls the whole-rock REE distributions. Zircon contains most of the Zr and Hf, but <5% of the HREE in the pelites and <20% in the quartzites. Other than Zr and Hf, most trace elements in Libby Creek sediments are chiefly in micas and were probably in clays in the original sediments. Increased Eu anomalies in upper Libby Creek sediments signal a sudden increase in Eu-depleted granitoids in sediment sources, which reflects late tectonic uplift. Mixing models favor granite, basalt, and TTG (tonalite-trondhjemite-granodiorite) sources with TTG and basalt dominating in lower-sediment sources and granite in upper-sediment sources. The presence of glacial diamictites and Ca, Na, Sr, and CIW paleoweathering indices suggest that climates changed from relatively cold during deposition of lower Libby Creek sediments to mild and finally to humid (hot?) with decreasing age. With a few notable exceptions (Ca, Sr, Ni, and Cr), the matrix composition of the Early Proterozoic Libby Creek and Gowganda glacial diamictites, including REE distributions, agrees well with the average composition of Late Archean upper continental crust. Thus glacial diamictites may provide excellent indices of both relative and absolute major and trace element distributions in average upper crust.