Petrogenesis of convergent-margin calc-alkaline rocks and the significance of the low oxygen isotope ratios: the Rodna-Bargau Neogene subvolcanic area (Eastern Carpathians)

Neogene calc-alkaline magmatites (from basaltic andesites to rhyolites including mafic cognate enclaves) of the Rodna-Bargau subvolcanic area (East Carpathian arc) are evaluated on the basis of new mineral compositional data, major and trace elements, as well as Sr and O isotope data. Two different series of rocks have been separated. The magmas of the medium-K series had a rapid ascent toward the surface, as proven by the presence of primary garner bearing rocks, or by the sporadic occurrence of mafic cognate enclaves. The 8180 values of amphiboles vary from 4.2 to 5.4 parts per thousand (SMOW). The delta(18)O value measured on garnet is 4.3 parts per thousand. The range of (87)Sr/(86)Sr ratios is from 0.70588 to 0.70887. The decrease of the delta(18)O values as (87)Sr/(86)Sr ratios and SiO(2) increase is interpreted as a progressive contamination of a mantle derived magma with a contaminant depleted in delta(18)O and enriched in (87)Sr/(86)Sr (i.e. hydrothermally altered lower crustal rocks). Within the high-K series the presence of intermediate magma chambers where assimilation-fractional crystallization processes took place is considered. The delta(18)O values measured on clinopyroxenes vary from 4.6 to 5.7 parts per thousand and on amphiboles from 3.8 to 6.7 parts per thousand. The range of (87)Sr/(86)Sr ratios is from 0.70605 to 0.70950. The covariation of the delta(18)O values and (87)Sr/(86)Sr ratios is scattered. The highest delta(18)O values correspond to the highest (87)Sr/(86)Sr ratios and are consistent with assimilation of the local upper-crustal rocks. The lower delta(18)O values and the observed oxygen isotope disequilibrium between coexisting pyroxenes and amphiboles are explained by interaction with heated meteoric water.