Study On Genesis For The Carbonatite In Bangong Lake Area, Tibet

Numerous carbonatite bodies, associated intimately with the ultrabasic rocks in ophiolite zone, are discovered through field survey. They occur as stock or dike with the width about tens to hundreds of meters. Some carbonatites contain sulphide-nickel ore while others don't contain the ore. Based on mineralography, geochemistry and Sr-Nd-Pb analysis, I research on the petrogenesis of carbonatites in Bangong Lake area. And the forming condition is discussed according to the C-O isotopes and zircon age.Two types are subdivided according to chemical compositions:calciocarbonatite and magnesiocarbonatite. The former consists mainly of calcite and secondarily of plagioclase, magnesite and quartz without sulphide-nickel while the latter is mainly comprised of magnesite and siderite and secondarily of calcite, chlorite and steatite including sulphide-nickel, with metasomatic texture developing locally.Geochemical characteristics of magnesiocarbonatites differ from either typicall mantle-sourced or crust-sourced carbonatites but are similar to those of associated ultrabasic rocks, accompanied by the similar Pb composition. So this indicates magnesiocarbonatites have the same material source with associated ultrabasic rocks. However, Sr-Nd-Pb isotope data shows that crust especially lower-crust accounts for a greater proportion in magnesiocarbonatites than in ultrabasic rocks. I conclude that magnesiocarbonatite is the product of ultrabasic rocks metasomatized by CO2-rich fluid, because Sr transports easily with fluid but Pb with melt. The magnesiocarbonatites plot near to marine limstone in theδ13Cv-PDB～δ18Ov-SMOW graph with the mean value of 1.25%o and 18.65%o respectively forδ13CV-PDB andδ18OV-SMOW, implying the CO2 fluid origins from surrounding limstone. The magnesiocarbonatite formed at Late Cretaceous Epoch (80.3±1.2 Ma) when it was the crustal uplift and extension stage and a suit of intermediate-acid dike formed. So that CO2-rich fluid mainly comes from limstone melted by deep-seated heat and then ascends through extensional structures to replace the ultrabasic rock and lastly forms the magnesiocarbonatite.The dock occurrence of calciocarbonatite proves its magmatic genesis. Abundance pattern of trace elements normalized by primitive mantle for calciocarbonatite coincides with that of the crust-sourced magmatic carbonatite in Daqingshan, Inner Mongolia in the north boundary of North China Block. However, evidence from Sr-Nd-Pb isotopes indicates calciocarbonatite derives from mantle, close to EMII inεNd-Isr diagram. In view of its TDM(697.4 Ma～881.9 Ma), I conclude the forming process of calciocarbonatite as follows:firstly carbonatite magma differentiated from mantle in late Proterozoic, then remelted in Mesozoic heated by the intrusion of basic-ultrabasic magma and lastly emplaced with ultrabasic magma.