GEOCHEMICAL DISCRIMINATION BETWEEN SHOSHONITIC AND POTASSIC VOLCANIC-ROCKS IN DIFFERENT TECTONIC SETTINGS - A PILOT-STUDY

Potassic volcanic rocks (PVRs), such as shoshonites, are an important constituent of young volcanic suites in five main tectonic settings: Continental ares (e.g. Andes); Post-collisional arcs (e.g. Alps); Oceanic arcs, subdivisible into initial (e.g. Marianas) and late (e.g. Sunda Arc); and Within-Plate (e.g. inland N. American Cordillera). Since some PVRs are associated with world-class gold and base metal deposits, and can be important in tectonic reconstruction of ancient terranes, new diagrams based on immobile elements (Al, Ti, P, Y, Zr, Nb, La, Ce, Hf) are presented for discriminating the tectonic setting of PVRs from their geochemistry; spidergrams and previously published discrimination diagrams based on 'normal' basalts proved unsuitable. Diagrams were derived, in part via multigroup linear discriminant analysis, using a database of fresh, primitive, Cenozoic PVRs (497 analyses), itself carefully screened from a newly erected database of 2,222 PVRs of all ages and compositions. To maximise discrimination, the diagrams are hierarchical: the first applies to all five settings and removes the most distinctive group (Within-Plate), while the remainder target more subtle distinctions. Overall, concentrations of most LILE, LREE and HFSE, and ratios such as Zr/Al and Ti/Al, tend to be highest in Within-Plate PVRs, intermediate in Continental and Postcollisional Arc PVRs, and lowest in Oceanic Arc PVRs. In detail, Postcollisional are distinguished from Continental Arc PVRs by higher Ce/P, and lower Zr/Ce, Ti/Nb ratios of the latter; late from initial Oceanic arc PVRs by higher Hf, La and P of the former. These geochemical differences have a firm theoretical foundation in petrogenetic processes during magma emplacement and/or in source heterogeneities related to subduction or mantle metasomatism.