Genesis Of The Luoji Ophioliticmélange From Shangri-La,Yunnan Province

This thesis aims to understand the ultramafic—mafic rocks, Permian basaltsand cherts from Shangri—La County at Yunnan Province. Based on the fieldinvistagtion and detailed stratigraphic sequence comparison, the concept “LuojiOphiolitic Mélange”is established. As compared to the Ganzi—Litang Ophiolite andHongshan—Shudu Ophiolitic Mélange, we discuss the spatial distribution andpotential reconstruction of the Ganzi—Litang Suture Zone.The initial sequence of the ophiolitic Mélange reconstructed to be mafic andultramafic (cumulates)�'basic dyke swarms�'Permian lavas�'layered radiolariancherts, basalts and carbonates. Present sequence contains external rocks, such asTriassic carbonates due to the tectonic activities. Generally it is compositionallysimilar to the Ganzi—Litang Karl Ophiolite. Presently, the Luoji Ophiolitic Mélangedistributes from the northeast to east—west and disappears at its west end. Asobserved in the field, the Luoji Ophiolitic Mélange is sectional in order but as awhole not.Mineral compositions and geochemistry of the clinopyroxenes show that theparent magma of the the Luoji Ophiolitic Mélange should be alkaline—sub alkalinetholeiitic basaltic magma. Because contanmination from crust and others magmareservoirs, it finally turned to be E—MORB (N—MORB+OIB). Themafic—ultramafic rocks are the parts of the oceanic crust and its sequences areconsistent to the normal ophiolite. Cumulates and early pillow basalts from LuojiFormation are E—MORB, basic dyke swarms and late stage massive basalts fromLuoji Formation are OIB+IAB. The magma sourses and evolutions variedsignificantly.The Luoji Ophiolitic Mélange belongs to the non—subduction MORand P type ophiolite (E—MORB+OIB)., which was controlled by the expansionaloceanic basin and mantle plumes. The Luoji Ophiolitic Mélange is the relics of alimited ocean basin under a high temperature but low pressure at the environment ofmid—ocean ridge and oceanic island. Zircon U—Pb geochronology suggestes thatcumulates formed at late Permian—early Trassic era (262.3±7.2Ma~253.3±7.5Ma),which can be compared to the the Ganzi—Litang Karl Ophiolite. The LuojiOphiolitic Mélange is more mature than the Ganzi—Litang Karl Ophiolite from the view of rock sequences. The cherts represent the gradual growth of oceanic basin butwe did not discover the relics related to subduction. This implies that either theoceanic basin was immature or the relics of the oceanic basin had been covered byYangtz craton or Zhongzan Block.Northwest to the Luoji Ophiolitic Mélange, the rocks assemblages ofHongshan—Shudu Ophiolitic Mélange are similar to the Wa’neng Ophiolite.Researches of the clinopyroxenes of dyke swarms from Hongshan—ShuduOphiolitic Mélange show that the parent magma was alkaline and formed under ahigher pressure. Comprehensively, Hongshan—Shudu Ophiolitic Mélange and LuojiOphiolitic Mélange represent two different stages of the Ganzi—Litang Ophiolite.The Hongshan—Shudu Ophiolitic Mélange seems to be MOR—type and related tothe subduction. The difference between the two ophiolitic Mélanges is caused by thelate stage tectonic activites.Based on the discussions above, the evolution of the Ganzi—Litang Ocean canbe summerised as followed. Late Permian—earlt trassic Era, due to the passiverifting of the Yangtz, the Ganzi—Litang opend at the first time and the LuojiOphoilite formed at the first Arc—Basin Sytem. The Luoji Ophoilite representedthree stages of the GanziLitang Ocean, from the continental rift�'initial oceanbasin�'ocean basin. Early—middle Trassic Era, the volcanical sediments from Luojiarea demonstrated that the gradual growth of the ocean basin. Early late Treassic Era,the Ganzi—Litang Ocean crutst subducted from southeast to northwest and theHongshan—Shudu Ophoilite formed. Late time at late Trassic Era, the LuojiOphoilite was disintegrated, transformed and mingled to be the present LuojiOphiolitic Mélange.