Petrological Study Of Ordovician Wanhe Ophiolitic Melange In Shuangjiang,Yunnan Province

Ophiolitic melange in Wanhe, located between Tongchang Street ophiolitic melange zone and Lincang granite zone, is close to the zone of Lincang granite in the Changning-Menglian belt. The 1:250000 regional geological survey projects in Lincang and Gunlong discovered and named the ophiolitic melange in Wanhe, which is to the east of Shuangjiang County and extends to about 25 kilometers in south-north orientation. The former experts assumed that ophiolitic melange in Wanhe might be the branches of that in Tongchang Street by reason of the migration of tectonic movements, with its forming age and forming mechanism similar to that in Tongchang Street. Recently, it has been discovered in the regional geology and mineral resources survey conducted by Institute of Geological Survey in Yunnan Province that ophiolitic melange in this region is continuously exposed along the South Shuangjiang County to the Yun County, with its spatial distribution of length over 100 kilometers and its forms as xenoliths. Investigations to the ophiolitic melange zone in Wanhe expose that there exist relatively complete mid ocean ridge basalt(MORB), gabbro dykesm, hot-water ocean basin sedimentation, ocean silicon- gray- mud sedimentation, arc volcanic rocks, continental shelf turbidite sedimentation and other combinations.This thesis focuses on the investigations on the ophiolitic melange in Wanhe by adopting field geological investigation, petrography, geochemistry and chronology. The main results and discoveries are as follows:1. Wanhe Ophiolitic melange is flourished with plagioclase amphibolite, amphibolite schist, metamorphic gabbro cumulates and garnet gabbro.While the edge of the zone is two-mica gneiss plagioclase, leucocratic rocks include plagiogranite, tonalite, with all the rocks continuously exposed. It is worth noting that melange is mixed with retrograde metamorphism eclogite, which is found in 4 spots along Mengku to Dananmei.2. According to the analysis of rare earth sample in Wanhe, the ophiolitic melange in this region is combined with N-MORB, OIB and E-MORB, with E-MORB as its main geochemical microelement and N-MORB as its auxiliary element. In addition, the amount of Al2O3 occupies large proportion while the figure of TiO2,Ni, Cr varies dramatically, classifying the ophiolitic melange as magmatic rocks formed in oceanic tectonic environment.3. The average age of zircon U-Pb attained from Wanhe ophiolitic melange is 470.8Ma, indicating that ophiolitic melange in this region is formed in late Ordovician. The forming age of Wanhe ophiolitic melange differentiates with that in Tongchang Street, with the former representing the ocean basin in early Paleozoic Era and the latter representing the ocean basin in late Paleozoic Era, classifying them as different plate suture in two eras.4. It is inferred that the south Lancang River exists the proto-Tethys basin, whose ridge stretches from south to north along Shuangjiang County to Yun County. Actually, the ridge has subducted to the east in late Ordovician, forming into arc magmatic rocks; meanwhile, the east ridge develops eminently with collision-type granites, indicating that the east ridge was passive continental margin basin.