Abstract:A relatively complete ophiolite combination in the Qingping area, Changning- Menglian suture zone is mainly composed of altered peridotite, serpentine, pillow basalt, amphibolite and other tectonic units. The LA- ICP- MS U- Pb zircon geochronology of amphibolite in the ophiolites mélange from the area yielded 206Pb/238U mean weighted ages of 433. 9±2. 9 Ma and 436. 7±2. 4 Ma indicating that the amphibolite formed during the Early Silurian. The in- situ zircon Hf isotopes have negative εHf(t) value of 5. 5~6. 7, with an average value of 6. 2. The SiO2 of the amphibolite ranges from 45. 65% to 50. 71%, with an average value of 48. 02%，and K2O/Na2O=0. 07~0. 23 indicating that they belong to the Na- rich and K- poor were sub- alkaline series of rocks.They showed a strongly fractionated REE pattern, (La/Yb)N=1. 87~4. 35, δEu=0. 80~1. 26, with no pronounced Eu anomlies. They are also enriched in LILEs（Rb, Ba and Sr), but strongly depleted in HFSEs (Nb, Ta, Ti and Zr) suggestive of subduction zone characteristics. Their geochemistry showed a typical E- MORB form of the amphibolite. The tectonic environment diagrams show that all the samples fall into the E- MORB region, indicating that the Changning- Menglian Ocean was in existence during the subduction in the Early Silurian. The Sr- Nd- Hf isotopic composition (i. e. (87Sr/86Sr)t=0. 704289~0. 704742, (143Nd/144Nd)t=0. 512107~0. 512172, εNd(t)=0. 54~1. 81 average, 0. 99) shows that magma source area of amphibolite is mainly from the enriched mantle (EMII). It may be affected by the subduction of separated fluids Changning- Menglian Ocean while not being contaminated by crustal materials, and has the characteristics of Dupal anomlies in Pb isotopes. It is suggested that there existed a Dupal anomalous mantle domain in the Changning- Menglian suture zone during the early Silurian in the Qingping areaShuangjiang Countywhich is consistent with the present Indian Ocean. The Changning- Menglian suture zone is located in the southern hemisphere in the early Silurian, suggesting that Early Silurian oceanic basin in Changning- Menglian formed a part of the Gondwana continent.
