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

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.