A GEOCHEMICAL DETERMINATION OF THE SUBDUCTION OCEAN CRUST IN EAST QINLING MOUNTAINS

The relic of subduction palaeooceanic crust is recognized in Song Shugou area of Shangnan in the eastern Qinling with the geochemical methods proposed by the authors. The existence of ophiolite suite and volcanic rock zone is the reliable evidence for the determination of subduction palaeooceanic crust. Ultrabasic rocks, which are pure olivinfels and orthorhomic pyroxene in Song Shugou ophiolite suite, are geochemically characterized by rare earth elements of loss mantle rocks. The basic (metamorphic) lava mixed in the ophiolite suite is practically a oceanic tholeiites and they have some characteristics such as less SiO₂ (52%), high TiO₂ (0.80%), high ratios of TiO₂ /SiO₂, high Q value, low K value, low REE value, low ratios of ΣCe/ΣY and poor in light rare earth elements, these and tholeiites are resemble to oceanic basalts. The volcanic rocks of Danfeng group adjacent to the ophiolite suite are calc-alkali island arc-typed rocks with low ratio of TiO₂/SiO₂, low Q value, and high values of K, ΣREE, and ΣCe/ΣY, enriched with light rare earthelements, which are similar to the volcanic rocks of island arc type. Based on the data mentioned above and the results of correlation analysis of common elements, the authors suggested that the volcanic rocks of Danfeng group were formed in island arc environment, while the volcanic rocks in Song Shugou area were formed in oceanic environment.