Abstract:

Ophiolite mélange from the WumulongTongchangjie area, ChangningMenglian tectonic zone mainly consists of ultrabasic and basaltic rocks. The ultrabasic rocks are mainly composed of harzburgite and dunite. They exhibit distinctive geochemical characteristics of extremely low CaO (<01%), low rare earth element content (<5×10-6), (La/Yb)N=096114 and (Ce/Yb)N=104137, showing the depletion of light rare earth elements. The basaltic rocks are alkaline and have high TiO2 (257%333%) and K2O (229%552%) content, as well as high (La/Yb)N=1542～1981 and (Ce/Yb)N=12481476, which show strong enrichment in light rare earth elements. The Ti/V, Th/Ta, Th/Yb, Ti/Yb and Ta/Yb of the alkaline basalt are much similar to that of the typical oceanic island basalt (OIB); while are obviously different from that of the basalts originated from depleted mantle and/or generated by oceanic crust subduction. Therefore, the alkaline basalts in the studied area belong to typical oceanic island basalt (OIB) which was generated by magmatism within oceanic plate. It implies that the ChangningMenglian ancient ocean basin had undergone a whole evolutionary process from initial limited ocean basin to a mature ancient ocean basin.

Key words: geochemistry, oceanic island basalt, ChangningMenglian suture zone