Triassic shoshonites and andesites,Lakmon Mountains,western continental Greece: Differences in primary geochemistry and sheet silicate alteration products

Shallow marine lavas and pyroclastic rocks, several tens of metres thick, outcrop at the base of thrust slices in the Pindos nappe in the Lakmon Mountains of western continental Greece. These volcanic rocks are basalts and andesites formed during mid-Triassic subduction. Geochemically, on the basis of the potassium content, shoshonitic and calc-alkalic groups are distinguished. Principally the shoshonitic rocks contain primary K-feldspar. The calc-alkalic rocks contain complexly zoned feldspars and are depleted in intermediate REE: this depletion could reflect a small degree of partial melting of amphibole, or possibly phlogopite, during petrogenesis. These two characteristics are found in other examples of minor calc-alkalic rocks associated with shoshonites described in the literature. A petrogenetic model is developed involving deeper mantle enrichment in LILE and partial melting of this enriched mantle rock containing amphibole. Calc-alkalic rocks are the result of initial partial melting and may be mixed with small amounts of magma of deeper origin; further partial melting produces the more voluminous shoshonites. Celadonite developed as a secondary mineral during burial beneath about 1 km of Mesozoic to early Tertiary sediments. During nappe emplacement, vermiculite formed in the calc-alkalic rocks; in contrast, smectite developed in the shoshonites, because the potassium was sufficient to immobilize all available Fe in celadonite.