Geochemical characteristics and tectonic setting of the laohushan basalts, north Qilian mountains

摘要

The Ordovician Laohushan ophiolite, located in the eastern part of the North Qilian Mountains, is mainly composed of meta-peridotites, gabbros and basalt alternating with sediments. The sediments are mainly turbidites, in- cluding sandstones, siltstones, cherts etc. Major elements show that the basalts are subalkaline tholeiites and may be analogous to ocean-floor basalts. Except a few N-MORBs, most of the basalts of E-MORBs as indicated by incompatible element ratios such as(La/Ce)_N, La/Sm, Ce/Zr, Zr/Y and Zr/Nb. Negative Nb anomaly is common but negative Zr, Hf and Ti anomalies are quite rare.