Geochemistry of Meso-and Neoproterozoic intra-oceanic arc volcanic rocks in the eastern segment of the western Kunlun orogenic belt

Abstract: Study shows that the Meso-and Neoproterozoic volcanic rocks distributed south of the line of Cele-Mingfeng-Yutian on the northern margin of the eastern segment of the West Kunlun Mountains, Xinjiang, consist dominantly of a suite of low-grade (basaltic ) andesite with very small amount of rhyolite. Andesite contains 52.36% -58.30% (mean54.55%) SiO2, <0.3% (mean0.22%) TiO2 and 1.70% -3.75% Na2O + K2O, with Na2O> K2O and Na2O / K2O = 2.44-4.61, M / F = 0.62-0.78 and MgO / (MgO + TFe) = 0.36-0.45. The alkali-silica composition shows that the rock is mainly calc-alkaline. REE study indicates that the ΣREE content of andesite is low, exhibiting a narrow range between 15.52 and 17.92 ppm, close to that of oceanic tholeiite. The (La / Yb) N of andesite is 0.69-1.33 and the (Ce / Yb) N is 0.75-1.17, the fractionation between HREE and HREE being not pronounced. Except for the δEu of sample 1028-13 that is 0.6, the δEu values ??of other samples are in the range of 0.91-1.17, basically no Eu anomaly being present. On the chondrite-normalized REE distribution patterns all the andesite forms a colony. The REE distribution patterns are close to that of oceanic tholeiite. Study of trace elements in andesite indicates the following: Th / Yb = 0.04-0.10, Th / Ta = 5.92-11.22, Zr / Hf = 20.46-29.40 and Th / Y = 0.04-0.08. The N-MORB distribution patterns of trace elements show that the rock is enriched in such large-ion lithophile elements as Sr, Ba, Rb, K and Th and depleted in Cr, as is similar to tholeiitic volcanic arc basalt. An integrated petrologic and geochemical analysis of the volcanic rocks, combined with the tectonic setting, suggests that this suite of volcanic rock occurred in an intra-oceanic arc environment.