Geochemical characteristics, and tectonic setting of the Laohushan basalts, North Qilian Mountains

The Ordovician Laohushan ophiolite, located in the eastern part of the Nort h Qilian Mountains, is mainly composed of meta-peridotites, gabbros and bas alts alternating with sediments. The sediments are mainly turbidites, inclu ding sandstones, siltstones, cherts etc. Major elements show that the basal ts are subalkaline tholeiites and may be analogous to ocean-floor basalts. Except a few N-MORBs, most of the basalts are E-MORBs as indicated by incom patible element ratios such as (La/Ce)(N), La/Sm, Ce/Zr, Zr/Y and Zr/Nb. Ne gative Nb anomaly is common but negative Zr, Hf and Ti anomalies are quite rare. Based on the geochemical characteristics, it is suggested that the La ohushan basalts were formed in a back-are basin. epsilon (Nd) (t) of the ba salts ranges between +3.0 and +8.9 and (Sr-87/Sr-86)(i) ranges between 0.70 30 and 0.7060, indicating a depleted mantle source which was mixed with mor e or less enriched mantle components. Furthermore, the petrography of the s andstones and geochemistry of the cherts suggest that the sediments were de posited near a continental margin.