Mid-Miocene shoshonitic volcanism in Lesbos is preceded and followed by minor calcalkaline activity that probably terminated in the Pliocene. Two differentiation trends are recognised in the shoshonites. The older shoshonites include voluminous acid rocks (mostly ignimbrites) and show normal shoshonite REE patterns. They were probably derived from partial melting of garnet peridotite enriched in LILE, with later fractionation controlled by anhydrous phases. Cooling units in the ignimbrites show upward increase in the K content of glass, suggesting K-feldspar fractionation in a high level magma chamber. Later shoshonites and the calc-alkali rocks have HREE enrichment and contain hydrous cognate xenoliths. Partial melting of a rising periodite diapir which had been relatively enriched in HREE within the garnet stability field and then entered the spinel stability field, would produce the observed HREE enrichment pattern. Later fractionation of these younger shoshonites and calc-alkali rocks was controlled largely by hydrous phases.