Tectonic setting and mantle source characteristics of Jiugequan basalts, North Qilian: Contraints from trace elements and Nd-isotopes.

Basalts from Jiugequan ophiolite, North Qilian mountains, are of MORB-type, and are composed of N-MORBs in the lower section and E-MORBs in the upper section. Most of the basalts show negative Nb anomaly. From the bottom to t he top, Th, Nb, LREE and Zr contents and (La/Yb)(N), (La/Sm)(N), Ce/Zr, Zr/ Y, Th/La, Th/Yb ratios of the basalts are gradually increased, companioned with continuous descreasing of Y, Yb, Lu and Sc contents, Zr/Nb and La/Nb r atios, as well as epsilon (Nd)(t) In addition, the samples show significant correlations between incompatible element ratios and epsilon (Nd)(t), that is, incompatible elements and isotopes are coupled. We interpret these cor relatios as mixing trends of N-MORB source and OIB source resulting from me lting of a heterogeneous mantle domain. Trace elemental and Nd-isotopic cha racteristics indicate that the Jiugequan ophiolite was from a seamount in a back-are basin, and the geochemical variations records the motive geologic al processes that the seamount being gradually developed and moved away fro m the spreading ridge. It is proposed that many ophiolites were actually fr om seamounts.