Assessing Differences in Composition between Low Metamorphic Grade Mudstones and High-Grade Schists Using Logratio Techniques

Abstract

Based on stratigraphic and petrologic similarities, it has been proposed that rocks exposed in roof pendants of the central Sierra Nevada, California are part of a tectonically displaced fragment of the Cordilleran miogeocline. We have identified significant geochemical similarities between metamudstones making up this miogeoclinal fragment and rocks comprising its proposed parents in the Mojave Desert-southern Great Basin region by geochemical analysis of samples collected from Snow Lake and Boyden Cave roof pendants, and from the Precambrian to Cambrian section of the Cordilleran miogeocline exposed in the Nopah Range, southeastern California. In order to circumvent the constant-sum problem inherent in geochemical data, the data were transformed into a continuous-variable format using logratio techniques. When the differences in means of aluminum-normalized logratioed data derived from the Snow Lake-Boyden Cave data set and the Nopah Range samples were calculated on an element by element basis, ~86% of the differences were not statistically significant at the 95% confidence level. The large number of compositional similarities identified between Snow Lake, Boyden Cave, and Nopah Range samples are consistent with the idea that the metasedimentary rocks of the central Sierra Nevada batholith are composed of material shed from the western North American interior and that roof pendants in the central Sierra Nevada are part of a displaced miogeoclinal fragment. This work demonstrates that rigorous statistical analyses of geochemical data transformed into a continuous-variable format can be a useful tool in evaluating the plausibility of lithostratigraphic correlations between metamorphosed, complexly deformed, and displaced metasedimentary rocks.