Petrogenesis And Geological Significance Of The Within-plate Volcanic Rocks From Microcontinent In Sanjiang Area, Southestern Yunnan Province

Sanjiang Orogenic Belt is consisted of many suture zone and microcontients. Thewithin-plate volcanic rocks formed in these microcontinents has great significance forgeological reaserch. It can be used to detective the deep structure of lithosphere as‘electronic probes’ and to distinguish the differences between microcontinents as a‘rule’.The Late Carboniferous low-Ti tholiitic basalts in Baoshan microcontinents, withvarying grade of Crustal contamination, is enriched in LREE, LILE and depleted inNb, Ta. The Sr, Pb isotopic content is relatively high compared with the Nd isotopiccontent. The structural setting shows a combination of WPB and MORB, whichsuggests the typical products of magmatism under continental-rift setting. Thegeochemical characteristics shows the signs of “Gondwana type” of Baoshanmicrocontinent is totally different with the Emeishan basalts, but is affinity with theDeccan and Panjal basalts in India.The Late Permian high-Ti basalts in Zhongza microcontinents is mainlydistributed in Bogexi and Gangdagai area. The typical OIB-type trace elementscharacter of these basalts, enriched in LREE, LILE, HFSE and Sr isotope, indicatedthat the mantle source maybe consisted of the mixture of asthenosphere andlithospheric mantle, which is exactly same with the Emeishan high-Ti basalts. Thewithin-plate structural setting is proved in these rocks, indicated that the lithospherethinning of Zhongza microcontinent during Permian. The primary magma wasproduced through low degree partial melting of the ‘Yangtze type’ lithospheric mantle,and then formed the OIB-type basalts of Zhongza microontient.The Cenozoic potassic alkali basalt in Simao microcontinents is mainlydistributed in Puer and Tonguan area. The trace elements character of these basalts,enriched in Ti, Al, k, LREE, LILE and HFSE, contradict with depleted isotopicevidence (high-Sr and low-Nd). Both the forming age and geochemical characteristicspointed out that the alkali basalts in Simao microcontinents is belong to the late periodCenozoic magmatism in Sanjiang area. The mantle source, enrich in K, LREE, LILE and HFSE, is a lithospheric mantle modified by metasomatic fluids extracting fromthe asthenosphere partial melting process. Lithospheric mantle with. The lithosphericthinning may associated with the lateral movement of the Tibet asthenosphere flowalong the Red River-Ailaoshan fault zoneTwo groups of arc-type (Nb, Ta, Ti negative anomaly) volcanic rocks discoveredin Changdu microcontinent. They may represents the products of Jinshajiangsubduction during different stage. The zircon U-Pb age of Permian basaltandesite(269.4~270.7Ma) offer a well constraint on the end time of Jinshajiang subduction.The Late Triassic trachyandesite is ultra-potassic rocks formed under thepost-collisional setting. The mantle source of these two groups rocks are characterizedby EMII-type end member which is modified by metasomatic fluids/melts formedduring the subduction of Paleo-Tethys...