International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

PETROGENESIS OF CONVERGENT-MARGIN CALC-ALKALINE ROCKS AND THE SIGNIFICANCE OF THE LOW OXYGEN ISOTOPE RATIOS: THE RODNA-BÂRGĂU NEOGENE SUBVOLCANIC AREA (EASTERN CARPATHIANS)

Pages: 77 - 90

Authors: DELIA CRISTINA PAPP, IONEL URECHE, IOAN SEGHEDI, HILARY DOWNES, LUIGI DALLAI

Abstract: Neogene calc-alkaline magmatites (from basaltic andesites to rhyolites including mafic cognate enclaves) of the Rodna-Bârgău 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 garnet bearing rocks, or by the sporadic occurrence of mafic cognate enclaves. The δ18O values of amphiboles vary from 4.2 to 5.4 ‰ (SMOW). The δ18O value measured on garnet is 4.3 ‰. The range of 87Sr/86Sr ratios is from 0.70588 to 0.70887. The decrease of the δ18O values as 87Sr/86Sr ratios and SiO2 increase is interpreted as a progressive contamination of a mantle derived magma with a contaminant depleted in δ18O and enriched in 87Sr/86Sr (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 δ18O values measured on clinopyroxenes vary from 4.6 to 5.7 ‰ and on amphiboles from 3.8 to 6.7 ‰. The range of 87Sr/86Sr ratios is from 0.70605 to 0.70950. The covariation of the δ18O values and 87Sr/86Sr ratios is scattered. The highest δ18O values correspond to the highest 87Sr/86Sr ratios and are consistent with assimilation of the local upper-crustal rocks. The lower δ18O values and the observed oxygen isotope disequilibrium between coexisting pyroxenes and amphiboles are explained by interaction with heated meteoric water.

Keywords: Eastern Carpathians, subvolcanic intrusions, calc-alkaline magmas, crustal interaction, enclaves, low δ18O

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