Geochemistry and Tectonic Significance of the Ophiolite and Associated Volcanics in the Mianlue Suture, Qinling Orogenic Belt, China

The Mianlue suture is a component of Qinling orogenic belt, marking the final position of collision between the North and South China Blocks. The suture contains a complicated ophiolitic complex with many slabs of different tectono-stratigraphic units. The ophiolite, mainly distributed in the Pipasi and the Zhuangke areas, are composed of ultrabasic rocks, like harzburgites and dunites, which exhibit LREE depletion with remarkable positive Eu anomaly, and diabase dyke swarms that show LREE enrichment but slightly negative Eu anomaly. Metamorphosed volcanic rocks can be divided into three groups in terms of their REE geochemistry and trace element ratios of T₁/V, Th/Ta, Th/Yb and Ta/Yb. The first group is the MORB-type basalt with LREE depletion, representing fragments of oceanic crust and implying an association of MORB-type ophiolite and a proto-Tethyan Qinling Ocean between Qinling and South China Block during middle Paleozoic-early Mesozoic times. The second group is the oceanic island basalt assemblage, and the third is the island arc volcanic rocks including tholeitic basalt and a large amount of calc-Alkaline intermediate-acidic volcanic rock associations. These facts indicate that the Mianlue ophiolite represents an ocean basin that experienced evolution from Devonian to Perman, and the Qinling terrane became an independent lithospheric microplate during the late Paleozoic-Early Mesozoic.