The nature of Triassic extension-related magmatism in Greece: evidence from Nd and Pb isotope geochemistry

Abstract

The widespread Triassic volcanic rocks of Greece, dismembered during the Hellenide orogeny, are used to interpret the nature of Triassic rifting. Four assemblages of volcanic rocks are distinguished on geochemical criteria: (1) a predominant subalkaline basalt–andesite–dacite series with a high proportion of pyroclastic rocks; (2) minor shoshonites; (3) alkali basalt and (4) MORB. The stratigraphic and palaeogeographic distribution of these rock types is synthesized. New Pb and Nd isotopic data are used to discriminate between hypotheses suggesting that either subduction or extension was responsible for the Triassic volcanism. In the subalkaline basalt assemblage, ε_Nd is negative with depleted mantle model ages >1.5 Ga. Pb isotopic compositions are mostly close to the very distinctive compositional field of Cenozoic extensional rocks of the Aegean area, with very high ²⁰⁷Pb/²⁰⁴Pb for relatively low ²⁰⁶Pb/²⁰⁴Pb ratios. These isotopic data confirm interpretations based on trace elements that subalkaline basalts were predominantly derived from melt-depleted peridotite in the sub-continental lithospheric mantle as a result of extension. Small areas of enriched hydrous mantle partially melted to yield shoshonitic magmas. Nd and Pb isotopic compositions of the alkali basalts are quite different from those in other rock types and suggest a HIMU mantle source component derived from a small plume, which also influenced MORB compositions. Distribution of these various rock types is used to constrain palaeogeographic reconstruction of Triassic micro-continental blocks.