Geochemistry of Lenglongling Ophiolite, North Qilian, and Its Tectonic Implications

This paper reports a new ophiolite belt found in the Lenlongling area of north Qilian. From bottom to top, the rock belt is composed of mantle peridotites, gabbros, basalts and silicon rocks. Lithogeochemical characteristics indicate that the basalt can be divided into two groups according to its content of Ti. The high-Ti basalt show LREE enrichment patterns in chondrite-normalized REE diagram, and also display enrichment of incompatible elements such as K, Rb, Ba, Th, Nb, Ta in an uplift(hump) tendency when normalized to N-MORB. With typical geochemical characteristics of OIB, the rock may be formed in an intra-plate magma environment. In contrast, the low-Ti basalt has geochemical characteristics of island arc volcanic rock, showing deficiency of LREE similar to N-MORB, enrichment of the LILE and depletion of HFSE including Nb, Ta etc., which represent the back-arc basin setting. The rock assemblage and lithogeochemistry suggest that the Lenglongling ophiolite belt is formed in the back-arc basin and is the residual of the north Qilian ocean in Early Ordovician.