Geochemistry And Petrogenesis Of Volcanic Rocks In Changning-Menglian Suture Zone

Changning-Menglian suture zone is considered as one of the most important ophiolite melange belts in Sanjiang orogeny, which extends northward to Bitu belt in eastern Tibet, connecting southward to Nan-Uttaradit belt in Thailand, and finnally reaches the Bentong-Raub suture in Malaysia. It represents the residue of main branch of Paleo-Tethyan ocean located in northeastern margin of Gondwana continent in late Paleozoic. Field geology, petrography, trace elements and isotopic geochemistry were adopted in volcanic rocks sampled from Wumulong-Tongchangjie, Ganlongtang, Nongba, Minle and Xiaodingxi areas to discuss petrological geochemistry and petrogenesis. This thesis also summarizes and recovers the tectonic evolution of Paleo-Tethyan oceanic basin, and the main conclusions can be drawn as follows:(1) The ultramafic rocks in Wumulong-Tongchangjie areas show high magnesium and low calcium. As a whole, it represents the characteristic of residual mantle rocks after the partial melting with high proportion. The tholeiites in Tongchangjie are enriched MORB and originated in a small oceanic basin, while the alkaline basalts were generated in oceanic island, and some enriched components were added in sources with high oxygen fugacity. The simulation shows alkaline basalts were formed by mixing enriched garnet lherzolite and depleted spine lherzolite with certain ratio which gave rise to partial melting at the proportion of 1%-3%.(2) The tholeiites in Nongba areas show the features of Enriched MORB, which derive from a weakly oxidative source mixed with MORB and HIMU. What's more, the source is not contaminated by crust basically. The simulation reveals the tholeiites in Nongba areas were derived from partial melting at the ratio of 0.2%～0.25%, and they were formed at the pressure of 15-20 Kbar in mantle source. In addition, The alkaline basalts generated in ocean island, which were the products of partial melting of the oceanic intraplate hotspot. The simulation represents it was generated from partial melting of mantle peridotites at the proportion of 0.1% in higher pressure than 25 Kbar.(3) The tholeiites in Ganlongtang areas were derived from depleted mantle source with low oxygen fugacity, which were obviously contaminated by crustal materials. Batch melting shows the tholeiites in Ganlongtang areas were generated from partial melting of mantle source with residual harzburgite above 10% proportion in pressure of 15-20 Kbar.(4) Meso basaltic volcanic rocks in Minle and Xiaodingxi areas were proved to form in the settings of post-collisional arc, which were derived from partial melting of residual slab of oceanic lithosphere rich with volatile matters. The former derived from low or moderate partial melting of reductive mantle source with amphibole rutile and garnet risidues. Fractional melting simulation reveals volcanic rocks in Minle areas generated from 1.573～3.377% proportion partial melting of mantle peridotite. But the volcanic rocks in Xiaodingxi areas have the characteristics of multicycle eruption with high intensity, and the magma keeps balance with orthorhombic pyroxene and garnet residual mineral phases in sources. The simulation shows the volcanic rocks in Xiaodingxi area were derived from partial melting of 1.2516～1.8951% with low degree.(5) There existed a wide tethyan ocean in Changning-Menglian area and even all over the southeast Asia, which underwent a whole Wilson cycle including oceanic breakup, generation of oceanic crust, subduction and collision between the Sibumasu and Simao terrane. Hence fingerprint of oceanic mantle in every stages was recorded by the volcanic records, which is also a response for the breakup in northeatern border of Gondwana. And it plays a key role in researching on the plate tectonic framework and the transformational mechanism between ocean and continent in Sanjiang area in late paleozoic.