Geochemical Characteristics Of Wushaoling Ophiolites In North Qilian And Their Tectonic Significance

Ophiolite is a specific mafic - ultramafic association, not the name of a rock. Completely developed ophiolites from bottom to the upwards occur sequences: ultramafic complex - gabbro complex - mafic sheet complex - mafic volcanic complex. Wushaoling ophiolites include metamorphic ultramafic complex (serpentinite), crystalline ultramafic - mafic rocks (eclogite peridotite, pyroxenite, gabbro), mafic volcanic complex (basalt), which are typical ophiolite assemblages. The overlying rocks are basic- intermediate volcanic rocks and sediments, which are associated rocks.By detailed research on Wushaoling mafic - ultramafic complex in mineralogy, petrology, geochemistry, and combined with the regional tectonic setting of the North Qilian, we get the following conclusions:1,Wushaoling mafic - ultramafic rocks are SSZ Ophiolites. Geochemical characteristics of major elements, trace elements and REE show that, wushaoling basalts came from depleted mantle, owning both IAB and N-MORB geochemical characteristics. Combined with rock assemblages and regional geological features, Wushaoling mafic - ultramafic rocks should be ophiolites, formed in the back-arc basin environment.2,The basalts and gabbros of Wushaoling mafic - ultramafic rocks are homologous. Wushaoling basalts and gabbros have obvious crystallization differentiation trend, that is, the basalts are products of original magma after gabbros differentiated out. The gabbros' and basalts' REE patterns are similar to those layered-gabbros' and basalts' in ophiolite. The above showed that basalts are products of evolved magma's crystallization, gabbros represent crystalline phase, the two are homologous.3,Wushaoling ophiolites are components of Jiugequan - Laohushan ophiolite belt. At the geotectonic position, Wushaoling ophiolites should belong to the same opbiolite belt with Laohushan ophiolites, formed in the back-arc basin setting, and the spreading ridge was near to the island-arc. This ophiolite belt represents back-arc basin environment formed by the subduction of North Qilian ocean in Ordovician, are lithosphere pieces left on the continental margin by back-arc basin's closure.