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Nature and origin of the Triassic volcanism in Albania and Othrys: a key to understanding the Neotethys opening?

Philippe Monjoie¹, Henriette Lapierre¹, Artan Tashko², Georges H. Mascle¹, Aline Dechamp¹, Bardhyl Muceku² and Pierre Brunet³

¹ Laboratoire de Géodynamique des Chaînes Alpines, UMR-CRNS 5025, OSUG, Université Grenoble I, BP 54, 38041 Grenoble cedex (France). Contact: gmascle{at}ujf-grenoble.fr
² Faculté de Géologie et Mines, Université Polytechnique de Tirana, Rruga El Basani 12, Tirana (Albanie)
³ Laboratoire Mécanismes de Transfert en Géologie, UMR-CNRS 5563, Université Paul Sabatier, OMP-DTP, 14 Avenue E. Belin, 31400 Toulouse (France)

Triassic volcanic rocks, stratigraphically associated with pelagic or reef limestones, are tectonically juxtaposed with Mesozoic ophiolites in the Tethyan realm. From the central (Dinarides, Hellenides) and eastern Mediterranean (Antalya, Troodos, Baër Bassit) to the Semail nappes (Oman), they occur either associated to the tectonic sole of the ophiolitic nappes or as a distinct tectonic pile intercalated between the ophiolites and other underthrust units. In the Dinaro-Hellenic belt, the Pelagonian units represent the lower plate, which is underthrust beneath the ophiolites. Middle to Late Triassic volcanic sequences are interpreted as the eastern flank of the Pelagonian platform and are therefore considered as a distal, deep-water part of the Pelagonian margin.

The Triassic volcanics from Albania and Othrys are made up of basaltic pillowed and massive flows, associated locally with dolerites and trachytes. New elemental, Nd and Pb isotopic data allow to recognize four types of volcanic suites: (1) intra-oceanic alkaline and tholeiitic basalts, (2) intra-oceanic arc-tholeiites, (3) back-arc basin basalts, (4) calc-alkaline mafic to felsic rocks. Nd and Pb isotopic initial ratios suggest that the within-plate volcanic rocks were derived from an enriched oceanic island basalt type mantle source, devoid of any continental crustal component. The lower Nd value of the trachyte could be due to assimilation of oceanic altered crust or sediments in a shallow magma chamber. Island arc tholeiites and back-arc basin basalts have a similar wide range of Nd. The absence of Nb negative anomalies in the back-arc basin basalts suggests that the basin floored by these basalts was wide and mature. The high Th contents of the island arc tholeiites suggest that the arc volcanoes were located not far away from the continental margin.

Albania and Othrys volcanics contrast with the Late Triassic volcanism from eastern Mediterranean (SW Cyprus, SW Turkey), which displays solely features of oceanic within plate suites. The presence of back-arc basin basalts associated with arc-related volcanics in Central Mediterranean indicates that they were close to a still active subduction during the Upper Triassic, while back-arc basins developed, associated with within-plate volcanism, leading to the NeoTethys opening.

Key Words: Triassic volcanism • Albania • Greece • Geochemistry • Isotopes • Geodynamic setting • Island-arc • Back-arc basin • Oceanic island