Geological Characteristics, Geochronology And Tectonic Signiifcance Of The Langxian Ophiolitic Melange In The Eastern Yarlung Zangbo Suture Zone, Southeastern Tibet

In this thesis, the author investigates the field occurrence of the Langxian OphioliticMelange (LOM) exposed in the eastern Yarlung Zangbo suture zone and reports thepetrological and geochemical data, and zircon U-Pb ages of gabbros and basalts from themelange. These new data are used to reconstruct the history of tectonic evolution of theTethyan Ocean represented by the Langxian Ophiolitic Melange.The units of the LOM mainly include metaperidotite, cumulate complex (metadiabase),and basic metavolcanic rocks. These units occur as blocks or slices within an incompleteophiolitic sequence located around Lasuo, Bailu-Langxian, and Lilong villages. Theultramafic rocks in the LOM are dominated by the occurrence of dunite and clinopyroxenitethat strongly experienced metamorphism. These rocks have low Na₂O+K₂O (Na₂O > K₂O)and low P₂O₅with varying MgO and TiO₂contents. The ultramafic rocks from Bailu andLangxian are depleted in light rare earth elements (LREEs) similar to those of the peridotitefrom Alps, while the rocks from Lilong village are enriched in LREEs comparable to thoseof rocks from a refractory mantle that has been metasomatized by fluids or melts. In general,the metadiabases and metabasalts from the LOM are mostly calc-alkaline, depleted inlarge-ion lithophile elements (LILEs), similar in composition to MORB.One metadiabase sampe with N-MORB affinity has been dated to be 191.4±3.7 Maby zircon SHRIMP U-Pb dating method, providing the oldest record of ophioliticcomponents in the LOM so far. This means that the spreading of the Neo-Tethyan Oceanmay have been active during the Early Jurassic. The other metadiabase sampe withE-MORB affinity has been dated to be 147.2±3.4 Ma, coeval with the other twometabasaltic samples that have zircon SHRIMP U-Pb ages of 145.7±2.5 (E-MORB-type)and 147.8±3.3 Ma (OIB-type), respectively. These magmatic records with an age of ca.145 148 Ma obtained within the LOM indicate that ridge hotspot interaction may havebeen present within the Neo-Tethyan Ocean at that time.