Original paper

Shoshonites from Agios Nectarios, Lesbos, Greece: origin by mixing of felsic and mafic magma

Pe-Piper, Georgia; Matarangas, Dionisis; Reynolds, Peter H.; Chatterjee, Amalya K.


Miocene dykes and minor lavas at Agios Nectarios in southeastern Lesbos, of shoshonite (trachyandesite) composition, are texturally hybrid, containing felsic glass and minerals characteristic of primitive mafic magmas.The lava includes both felsic and mafic glass, now partly devitrified, and has phenocrysts of forsteritic olivine and clinopyroxene, with groundmass phlogopite. The olivine contains abundant chromite inclusions. The dyke rock has phenocrysts of phlogopite and clinopyroxene, both with chromite inclusions and the phlogopite is chemically similar to that in the lava. The nearby Skopelos ignimbrite has glass composition distinct from the more extensive Polychnitos Ignimbrite Formation, but similar to the felsic glass in the Agios Nectarios lava. Both lava and dyke rock appear to be hybrid, formed by mixing of basaltic crystal mush containing olivine, clinopyroxene and phlogopite phenocrysts with felsic magma of similar composition to glass from the Skopelos ignimbrite. New dates on the lava (K/Ar whole rock 17.9 ± 0.5 Ma) and the Skopelos ignimbrite (40Ar/39Ar biotite 18.1 ± 0.3 Ma) are essentially identical and allow the rocks to be placed in context within the volcanic stratigraphy of Lesbos. A new age of 17.9 ± 0.6 Ma on a dyke cutting the Sigri Pyroclastic Formation demonstrates that these older felsic pyroclastic rocks could be synchronous with the Skopelos ignimbrite. The occurrence of phlogopite phenocrysts with chromite inclusions and of pyrite inclusions in plagioclase and clinopyroxene phenocrysts indicates that the parent mafic magma was enriched in both S and K.


greecelesbosshoshonitechromiteolivineignimbritemagma mixing