Geochemistry of early Precambrian graywackes from Wyoming

Abstract

Twenty-three samples of slightly metamorphosed early Precambrian graywackes from the southern Wind River Mountains in Wyoming have been analyzed for Si, Al, Ti, Fe, Mg, K, Ca, Rb, Sr, Zr and Ni by X-ray fluorescence and for Na and Mn by neutron activation; modal analyses have also been made of these samples. These graywackes, which possess well preserved primary textures and structures, are composed chiefly of quartz and biotite with lesser amounts of plagioclase. Most of the matrix minerals appear to have formed during metamor- phism. Significant compositional differences, especially in Rb concentration and in Rb:Sr and Na:K ratios, occur between individual graywacke beds. The original chemical composition of the graywackes appears not to have been greatly affected by diagenesis and metamorphism. The rock and mineral fragments in the graywackes indicate a source area composed chiefly of quartz-rich metasedimentary rocks with lesser amounts of granitic rock. The chemical composition of the graywackes indicates that the source area was predominantly quartz diorite to granodiorite in composition. Relatively large amounts of Fe, Mg and Ni and a high Mg: Fe ratio in some of the graywackes suggest a minor mafic component in the source area. The relatively low Ca and high Na contents of many of the graywackes may have resulted from the reaction of calcic plagioclase with sea water producing a more sodic plagioclase and releasing Ca 2+ ions. The composition of the Wyoming Precambrian graywackes is not unlike that of most pre- Cretaceous graywackes. If diagenetic and low-grade metamorphic compositional changes have been small, pre-Cretaceous graywackes may represent nearly unmodified samples of continental crust over the last 3.0−3.5 × 10 9 yr.