The Petrography and Geochemistry of Some Californian Arkoses with Application to Identifying Gneisses of Metasedimentary Origin

Abstract

Forty marine and continental Eocene and Oligocene arkosic sandstones and interbedded shales from the Santa Ynez Mountains, California and nine continental Holocene sands from the Salton Basin, California have been petrographically and chemically analyzed for 27 elements each. The arkoses and Holocene sands are composed of quartz and feldspar with minor amounts of clay, mica, and rock fragments. The shales are about 65% clay, 15% quartz, and 15% feldspar; Fe, Mg, Zn, K, Hb, Ti, Nb, Cr, and Ni are largely contained in the clay minerals with little detritai mafic rock or minerals. The data indicate a provenance of plutonio igneous and metamorphic rocks with minor volcanics; probably the Mojave Desert region. The composition of the sediments is roughly equivalent to an intermediate igneous rock minus dissolved material, particularly Na and Ca, plus diagenetically added sulfur. The sediments were derived by moderate weathering, in an area of high relief, as is shown by the abundance and relatively unaltered condition of the plagioclase and some of the unstable heavy minerals, but detrital basic rock is uncommon and the Fe, Mg, Ni, Co, Nb, and possibly Cr were contained in precipitated clay minerals derived from solution of basic rock. Chemical criteria are suggested that would enable a highly metamorphosed or migmatised arkosic sequence to be distinguished from primary injected granitic or partial melts of granitic composition. The diagnostic importance of Zr, Ni, Cr trends and abundances are emphasized; these elements, being relatively immobile, are likely to preserve original igneous or sedimentary trends.