Geochemistry of Ophiolites from the Mian-Lue Suture Zone: Implications for the Tectonic Evolution of the Qinling Orogen, Central China

Abstract

The Mian-Lue suture zone along the southern margin of the Qinling orogenic belt contains abundant ophiolite fragments that record collision between the North and South China terranes. The ophiolites are well exposed in the areas of Lueyang, Pipasi, and Derni. In Lueyang, ultramafic rocks comprise serpentinized harzburgite and dunite exhibiting LREE depletion and marked positive Eu anomalies, gabbroic rocks enriched in Sr and Ba and positive Eu anomalies, diabase dikes enriched in LREE with slightly negative Eu anomalies, and tholeiite metabasalts with MORB-type characteristics of LREE depletion, low Nb (3.9-8.4 ppm), TiO2 (0.92-1.86 wt%,), Ti/V (18.14-23.76), Th/Ta (0.04-1.86), Th/Y (0.01-0.02), and Ta/Yb (0.01-0.13). Ophiolites from Pipasi comprise tholeiitic basalt with average low (La/Yb)N(0.65-0.97), Ti/V (26.98), Th/Ta (1.06), Th/Y(0.01), and Ta/Yb(0.05). Ophiolites from Derni are represented by basalt, gabbro, and metaperidotites. The basalts are LREE depleted and lack prominent Eu anomalies, indicating a depleted asthenosphere source. Isotopic ages of 350-245 Ma imply the existence of a Paleozoic ocean basin from Devonian to Permian time in central China. The MORB-type ophiolites in the Mian-Lue suture zone are regarded as the fragments of Paleo-Tethys oceanic lithosphere that separated the Qinling and South China terranes during middle Paleozoic to early Mesozoic.