Zoned pyroxenes from shoshonite lavas of Lesbos, Greece: inferences concerning shoshonite petrogenesis

Samples from several stratigraphic levels in the Miocene volcanic succession on Lesbos, Greece, contain complexly zoned clinopyroxene and feldspar phenocrysts. Electron microprobe analyses have been made of zoned clinopyroxenes and feldspars, and their inclusions; and phenocryst mineralogy is compared with host-rock chemical composition. The commonest pyroxene zoning sequence is cores of greenish salite or augite mantled by diopside which, in turn, may be rimmed by augite, with each type of pyroxene in optical and chemical discontinuity with the next. None of the pyroxenes has high pressure characteristics. The Ts content of phenocryst cores is correlated with the K-content of the host rock; and the most diopsidic overgrowths also occur in the most K-rich rocks. Augite rims occur only in Al-rich rocks. Most of the features of the pyroxenes can be accounted for by a hypothetical fractional crystallization scheme. However, two observations suggest that mixing of small magma batches may also be important. The earliest crystallized diopsides that rim salite are rich in Cr2O3, suggesting precipitation from an unfractionated magma. Anorthoclase (implying advanced crystallization of feldspars) occurs as cores to feldspar phenocrysts and as inclusions in clinopyroxene cores in several samples