Genetic Relationships Between Lithospheric Mantle, Alkaline and Calc-Alkaline Basic Volcanoes in the Eastern Carpathians (Romania): Evidence from Trace Element and Nd-Sr Isotope Geochemistry

Abstract

The inner part of the Carpathian arc is the site of an intense alkaline (Dowries et al., 1995 and references therein) and calc-alkaline (Mason et al., 1996 and references therein) basic volcanism since the Miocene times. Both types of volcanoes are closely associated in space and in time and it is difficult to reconcile the two distinct models for magma petrogenesis, plumeand subduction-related, to explain the synchronous occurrences of both types of volcanoes. The geological data of Csontos (1995) contradict the possibility of an oceanic crust subduction between the European and the Moesian plates since or even before the eruptions occurred. Together with extensive geophysical data (Mocanu and Radulescu, 1994), they provide evidence for the existence of at least two major tectonic sutures along which different continental crust blocks are suboverthrusted relative to each other and significantly subducted into the lithospheric mantle. Xenoliths of this mantle were collected by some recent alkaline volcanoes (Persani Mts) closed to calc-alkaline ones (South Harghita Mts), both being active since around 2 Ma (P6cskay et al., 1995). Thus the eastern area of the Transylvanian Basin (Romania) is suitable for exploration of the possible relationships between the subcontinental mantle and basic volcanic rocks of different affinities, and the reasons for such volcanic rock associations. This assumes an exhaustive study of the xenoliths in order to decipher the history of the subcontinental mantle. Based on the results of Vaselli et al. (1995) from which major element data on mineral separates and whole-rocks, and Nd-Sr isotopic data on clinopyroxene separates were taken, a petrographical and trace element study was performed on the various mantle lithologies. Our interpretation (Chalot-Prat and Boullier, 1997) makes the assumption that the metasomatism of the subcrustal mantle, initially harzburgitic, was not University Henri Poincar&Nancy, BP 239, F-54506 Vandoeuvrelbs-Nancy, France and CNRS-CRPG, BP 20, 54501 VandoeuvreIbs-Nancy, France