THE NATURE OF TRIASSIC EXTENSION-RELATED MAGMATISM IN GREECE - EVIDENCE FROM ND AND PB ISOTOPE GEOCHEMISTRY

The widespread Triassic volcanic rocks of Greece, dismembered during t he Hellenide orogeny, are used to interpret the nature of Triassic rif ting. Four assemblages of volcanic rocks are distinguished on geochemi cal criteria: (1) a predominant subalkaline basalt-andesite-dacite ser ies with a high proportion of pyroclastic rocks; (2) minor shoshonites ; (3) alkali basalt and (4) MORE. The stratigraphic and palaeogeograph ic distribution of these rock types is synthesized. New Pb and Nd isot opic data are used to discriminate between hypotheses suggesting that either subduction or extension was responsible for the Triassic volcan ism. In the subalkaline basalt assemblage, epsilon(Nd) is negative wit h depleted mantle model ages >1.5 Ga. Pb isotopic compositions are mos tly close to the very distinctive compositional field of Cenozoic exte nsional rocks of the Aegean area, with very high Pb-207/Pb-204 for rel atively low Pb-206/Pb-204 ratios. These isotopic data confirm interpre tations based on trace elements that subalkaline basalts were predomin antly derived from melt-depleted peridotite in the sub-continental lit hospheric mantle as a result of extension. Small areas of enriched hyd rous mantle partially melted to yield shoshonitic magmas. Nd and Pb is otopic compositions of the alkali basalts are quite different from tho se in other rock types and suggest a HIMU mantle source component deri ved from a small plume, which also influenced MORE compositions. Distr ibution of these various rock types is used to constrain palaeogeograp hic reconstruction of Triassic micro-continental blocks.