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

Miocene to Pleistocene calc-alkaline volcanism in the East Carpathian are 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-crystal 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 are. The most primitive isotopic compositions are found in the low-K dacites of the northern Calimani volcanic centre and are interpreted as largely mantle derived. A second possible mantle reservoir of lower Nd-143/Nd-144 and lower Pb-206/Pb-204 is identified from back-arc basic calc-alkaline rocks in the south of the are. 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.