| Popis: |
Over ninety Pre-Cambrian argillaceous rocks (mainly Canadian greywackes, argillites and a few low-grade schists) were studied petrographically and analysed spectrochemically for the following trace elements: B, Cr, V, Ni, Cu, Mn and Ba. The petrographic study indicated that greywackes are argillaceous sandstones, commonly with a microbreccia texture, as pointed out in several recent papers and textbooks, rather than, as often held, the basic equivalent of an arkose, consisting largely of relatively undecomposed particles of basic igneous rocks or their metamorphosed equivalents. Only volcanic greywackes approach the latter definition. The trace element composition of the greywackes and argillites suggests that they are marine rather than freshwater deposits. It also suggests that the oldest sediments yet recognized in the Pre-Cambrian were accumulated in waters which differed little in composition from the seas of the present day. The trace element content of the Pre-Cambrian argillaceous rocks appears to be normally distributed (statistically), and is similar for Archean and Proterozoic as well as later rocks. The range in composition for any element extends from zero to about twice its mean value. The probability that a rock will violate this distribution law is small. |
