MOLAR RATIOS IN THE STUDY OF UNALTERED AND HYDROTHERMALLY ALTERED GREYWACKES AND SHALES

Molar ratio plots involving SiO2, Al2O3, Na2O and K2O constitute a ver satile method of determining both the chemical and mineralogical compo sition of arenite and argillite end-members of heterogeneous greywacke -argillite sequences and the mineral composition and mineral proportio ns in very fine-grained unaltered and hydrothermally altered pelitic r ocks. Comparison of arenite end-member compositions from turbidites of different ages and/or from different areas provide some indication of the composition of different crustal sources. Hyperbolic sorting curv es generated from the end-member compositions of unaltered samples ena ble calculations of gains and losses of oxides such as K2O in hydrothe rmally altered greywacke-argillites. The proportions of secondary mine rals such as albite, sericite and K-feIdspar in hydrothermally altered pelites can be determined readily from molar ratio plots and provide insight into the nature of the hydrothermal fluid.