MIOCENE MAGNESIAN ANDESITES AND DACITES, EVIA, GREECE - ADAKITES ASSOCIATED WITH SUBDUCTING SLAB DETACHMENT AND EXTENSION

Mid-Miocene volcanic rocks are rare in the Aegean region, although ear ly Miocene and late Miocene-Quaternary volcanism is widespread. At Oxy lithos (island of Evia), 14 Ma dacites form a dome or subvolcanic comp lex. Phreatomagmatic eruptions formed nearby rhyolitic pyroclastic sur ge deposits. The calc-alkaline dacites contain bronzite, Mg-rich clino pyroxene, phlogopite and plagioclase phenocrysts. The magma results fr om mixing of a Mg-rich andesitic magma, similar to that in the nearby island of Skyros, with more felsic magmas represented by the rhyolitic pyroclastics. The dacites are geochemically similar to adakites, whic h are derived by partial melting of eclogitic subducted oceanic crust and have low Y and Yb and high Sr/Y ratio. Sr-87/Sr-86 approximate to 0.7095 is found in both dacite and rhyolite. Lead isotopic composition from the high-Mg andesite from Skyros, with Pb-207/Pb-204 = 15.70 and Pb-208/Pb-204 = 38.90, forms a linear trend with Evia dacite and rhyo lite with Pb-207/Pb-204 = 15.71 and Pb-208/Pb-204 = 39.05. The high te mperatures required to produce such magma resulted from decompression due to extension of the Aegean basin at the same time as the initial i ntrusion of the detached subducted slab in the western Aegean that has been imaged by seismic tomography. The Oxylithos rocks extend the kno wn occurrences of adakite series rocks: this series is not restricted to sites with subduction of young oceanic crust.