Trace Elements and Sr, Nd Isotope Geochemistry of the Lajimiao Norite‐Gabbro from the North Qinling Belt1

Abstract The Lajimiao norite‐gabbro complex, as a part of the ophiolites on the southern side of the North Qinling belt, consists of gabbro and norite‐gabbro. They were derived from different magma series: the gabbro was derived from tholeiitic magma series with higher TiO2, REE abundance and Fe3+ / Fe2+ ratio; norite‐gabbro was derived from calc‐alkali magma series with lower TiO2, Fe3+ / Fe2+ ratio and REE abundance and much lower HREE abundance, which suggests that the source of the norite‐gabbro magma was deeper and controlled by eclogite facies. Geochemical characteristics of both plutonic rocks are similar to those of island‐arc basalts, such as relatively high contents of Ba, Pb and Sr and relatively low contents of Nb, Zr and Ni. The Sr, Nd isotopic characteristics of the Lajimiao norite‐gabbro complex are similar to those of ophiolites. Its 8Nd values are constant, about +2; whereas 8Sr values have wide variation from — 6.4 to +31.2 and positively correlate with Na2O, H2O+ and CO2 contents and the Fe3+ / Fe2+ ratio. The ɛNd—Nd / Th, ɛNd—La/Nb and ɛNd—Ba/Nb diagrams clearly show that there were significant components of terrigenous sediments in the mantle source of the Lajimiao norite‐gabbro complex. It suggests that large amount of sediments had been carried into the mantle by the subducted ancient Qinling sea plate during the Palaeozoic.