Fractionation between TiO2 and Zr as a measure of sorting within shale and sandstone series (Northern Portugal)
Garcia, Daniel; Coelho, Joao; Perrin, Michel
European Journal of Mineralogy Volume 3 Number 2 (1991), p. 401 - 414
published: Apr 18, 1991
manuscript accepted: Oct 19, 1990
manuscript received: Aug 17, 1990
ArtNo. ESP147050302011, Price: 29.00 €
Abstract The mineralogy and the geochemistry of clastic sedimentary rocks are basically determined by the nature, the maturity of the source area and by sorting processes. The present contribution investigates the influence of these factors on the distribution of several "inert" elements (Al, Ti, Zr, Nb, Y, Ce) between shales and the associated sandstones in detrital series from northern Portugal. The contrasting mechanical properties of zircon and Ti-bearing clay minerals at the sedimentation stage result in a systematic but variable fractionation of Ti and Zr in shale-sandstone pairs. TiO2/Zr correlates well with the Al2O3 content of clastic sediments and the range' of TiO2/Zr variations within a given lithological association is proposed as a chemical indicator of sorting efficiency. Weathering tends to concentrate Al, Ti, Zr and several other "inert" elements in clastic sediments without significant fractionation ; ternary diagrams between "inert" elements hence eliminate weathering effects. Such diagrams may be used to constrain the composition of the source and the chemistry of the sorted end-members. On a Al2O3-TiO2-Zr diagram, the slope of the sorting-related trend is shown to vary with the abundance of detrital feldspar in sandstones. As the above-mentioned elements are usually immobile during metamorphichydrothermal processes, the observed regularities have many applications in the fields of lithostratigraphy, hydrothermal and metamorphic petrology.