Journal of Petrology

Crustal Assimilation as a Major Petrogenetic Process in the East Carpathian Neogene and Quaternary Continental Margin Arc, Romania.

Paul R.D. Mason, Hilary Downes, Matthew F. Thirlwall, Ioan Seghedi, Alexandru Szakacs, David Lowry, David Mattey

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#=0.65-0.26) and have medium to high-K calc-alkaline 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. 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 Calimani 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, due either to source bulk mixing (between depleted or enriched asthenosphere and <1% average subducted local sediment) or lower crustal contamination.

Keywords:

Carpathians;assimilation;calc-alkaline;Sr-Nd-Pb-O isotopes;laser fluorination

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