LEAD ISOTOPIC COMPOSITIONS OF NEOGENE VOLCANIC-ROCKS FROM THE AEGEAN EXTENSIONAL AREA

Lead isotope composition has been determined for a diverse suite of Ce nozoic igneous rocks in the central and north Aegean Sea. These includ e voluminous Miocene shoshonitic and calc-alkali volcanic rocks and co rrelative plutonic phases; middle Miocene adakitic (high Mg) andesite and associated rhyolite; and middle Miocene to Quaternary ne-normative basalt, trachyte and K-rich andesite. The trace-element contents of m ost of the studied rocks suggests that Pb-isotopic composition is not a result of upper-crustal contamination, but rather is a feature of th e mantle source for K-rich rocks. This mantle source is represented by primitive mafic to intermediate volcanic rocks with Sr-87/Sr-86 appro ximate to 0.7095, high Pb-207/Pb-204 and Pb-208/Pb-204, > 1000 pp, Sr and Ba, and > 35 ppm Pb. The high Pb-207/Pb-204 requires a metasomatic origin that lowered U/Pb and increased Th/Pb, followed by a second-st age residence time of > 1 Ga. This K-rich magma was generated as a res ult of late Tertiary extension of the Aegean. In the South Aegean Arc, there is more evidence for crustal contamination, either from present crust or from subduction of Nile-derived sediment.