Crustal assimilation as a major petrogenetic process in the East Carpathian neogene and quaternary continental margin arc, Romania

Abstract

Miocene to Pleistocene calc-alkaline volcanism in the East Carpathian arc of Romania was related to the subduction of a small ocean basin beneath the continental Tisza-Dacia microplate. Volcanic products are predominantly andesitic to dacitic in composition, with rare basalts and rhyodacites (51-71% SiO2; mg-number 0·65-0·26) and have medium- to high-K calcalkaline and shoshonitic affinities. Mg, Cr and Ni are low in all rock-types, indicating the absence of primary erupted compositions. Detailed trace element and Sr, Nd, Pb and O isotope data suggest that magmas were strongly crustally contaminated. Assimilation and fractional crystallization (AFC) calculations predict the consumption of 5-35% local upper-crustal metasediments or sediments from the palaeo-accretionary wedge. Variations in the isotopic composition of the contaminants and parental magmas caused variations in the mixing trajectories in different parts of the arc. The most primitive isotopic compositions are found in low-K dacites of the northern Cãlimani volcanic centre and are interpreted as largely mantle derived. A second possible mantle reservoir of lower 143Nd/144Nd and lower 206Pb/204Pb is identified from back-arc basic calc-alkaline rocks in the south of the arc. Both magmatic reservoirs have elevated isotopic characteristics, owing either to source bulk mixing (between depleted or enriched asthenosphere and <1% average subducted local sediment) or lower-crustal contamination.