Geochemistry and zircon SHRIMP U-Pb ages of the Guanzizhen ophiolite complex,the Western Qinling orogen,China.

The Guanzizhen ophiolite complex occurred along the Tianshui-Wushan Fault in the Western Qinling orogen,is composed of meta-basalts,meta-intermediate-basic plutonites and minor serpentinite.The meta-basalts are characterized by slight light rare earth element(LREE)depletion or flat REE distribution((La/Yb)_N=0.63～1.24)with no Nb,Ta or Ti negative anomalies in spider- diagram,indicating they were formed at the middle ocean ridge.The meta-intermediate-basic plutonites can be subdivided into two types.TypeⅠ,characterized by LREE enrichment((La/Yb)_N2.2)and significant Nb negative anomalies in spider-diagram ((La/Nb)_N2.4),is similar to island arc volcanic rocks.TypeⅡ,characterized by flat to LREE depletion REE distribution ((La/Yb)_N=0.44～1.38)with no Nb negative anomalies in spider-diagram((La/Nb)_N1.5),is similar to those meta-basalts mentioned above,indicating they are belong to the Guanzizhen ophiolite.The SHRIMP U-Pb ages of 534 9Ma and 5178Ma for gabbro and diorite(this diorite is belong tooceanic plagiogranite)from the ophiolite,respectively,suggest that the Guanzizhen ophiolite is an early-middle Cambrian paleo-oceanic crust fragment.Since the Tianshui-Wushan Fault in the Western Qinling could be the westward extension of the Shang-Dan Fault in the eastern Qinling and both were considered to be the major suture between the North China Block and the South China Block,these ages provided an important constraint on evolution of the Qinling paleo-ocean and the Shang-Dan tectonic zone.