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Crustal Assimilation as a Major Petrogenetic Process in the East Carpathian Neogene and Quaternary Continental Margin Arc, Romania |
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| Authors: | MASON PAUL R D; DOWNES HILARY; THIRLWALL MATTHEW F; SEGHEDI IOAN; SZAKCS ALEXANDRU; LOWRY DAVID; MATTEY DAVID |
| Institution: | 1RESEARCH SCHOOL OF GEOLOGICAL AND GEOPHYSICAL SCIENCES, BIRKBECK AND UNIVERSITY COLLEGES GOWER STREET, LONDON WC1E 6BE, UK 2DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY UNIVERSITY OF LONDON EGHAM HILL, EGHAM, SURREY TW20 0EX, UK 3GEOLOGICAL INSTITUTE OF ROMANIA STR. CARANSEBES 1, 78S44 BUCHAREST 32, ROMANIA |
| Abstract: | Miocene to Pleistocene calc-alkaline volcanism in the East Carpathianarc of Romania was related to the subduction of a small oceanbasin beneath the continental Tisza–Dacia microlate. Volcanicproducts are predominantly andesitic to dadtic in composition,with rare basalts and rhyodacites (51–l71% SiO2; mg-number0.65–0.26) and have medium- to high-K calcalkaline andshoshonitic affinities. Mg, Cr and Ni are low in all rock-types,indicating the absence of primary erupted compositions. Detailedtrace element and Sr, Nd, Pb and 0 isotope data suggest thatmagmas were strongly crustally contaminated. Assimilation andfractional crystallization (AFC) calculations predict the consumptionof 5–35% local upper-crustal metasediments or sedimentsfrom the palaeo-accretionary wedge. Variations in the isotopiccomposition of the contaminants and parental magmas caused variationsin the mixing trajectories in different parts of the arc Themost primitive isotopic compositions are found in low-K dacitesof the northern Cdlimani volcanic centre and are interpretedas largely mantle derived. A second possible mantle reservoirof lower 149 Nd/144 Nd and lower 206 Pb/204 Pb is identifiedfrom back-arc basic calc-alkaline rocks in the south of thearc Both magmatic reservoirs have elevated isotopic characteristics,owing either to source bulk mixing (between depleted or enrichedasthenosphere and <1% average subducted local sediment) orlower-crustal contamination. KEY WORDS: Carpathians; assimilation; calc-alkaline; Sr-Nd-Pb-0 isotopes; laser flurination |
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