Carbonatitic pockets in intra-ocean arc volcanics (Qilian orogen): Petrogenesis and implications for carbon recycling in subduction zones

Abstract

Carbonatites in island arcs are crucial for understanding the processes of magmatism and carbon recycling in subduction zones. Here we report carbonatitic pockets in Ordovician intra-oceanic island-arc volcanic rocks and in Ocean island basalts (OIB) of Cambrian ophiolites in the South Qilian Accretionary Belt, NW China. All these carbonatitic pockets are composed of relatively pure calcites, some of which have accessory minerals of apatite, sphene, hematite, magnetite and allanite. The 87Sr/86Sr(i) ratios of calcite range from 0.703833 to 0.706732, concentrated at about 0.705, which is close to the composition of Enriched Mantle 1 (EMI) affected by fluids from the subduction zone. The C-O isotopic compositions show trends that evolve away from primary carbonatite (δ13CPDB = −9.0 ~ −1.0 ‰, 18OSMOW = 10.1– 16.5 ‰), probably caused by calcite fractional crystallization. High Sr, Th, U, Pb and low Nb, Zr, Hf, Ti contents and 87Sr/86Sr(i) ratios indicate that these carbonatitic pockets derived from carbonatite magma, but may have been altered by subduction-zone fluids. Carbonatitic pockets in ankaramites represent primary mantle-derived carbonate melts/liquids with low REE (rare earth elements) abundances (<10 ppm) and δ13CPDB. Elevated REE contents and LREE/HREE (light / heavy rare earth elements) ratios suggest that carbonatitic pockets in high-Al andesite may have experienced fractionation crystallization. The EMI-type carbonatitic pockets occurrence in the two different rock assemblages may reflect an interaction between an oceanic arc and an oceanic plateau, deriving from the recycling of Proterozoic marine carbonates through the lower mantle.