Geochronology And Geochemistry Of Alkali-rich Porphyries And Its Xenoliths In Liuhe And Xiaoqiaotou,Western Yunnan

Intrusions of Liuhe,and Xiaoqiaotou,located at weastern margin of Yangze terrane,are adjacent to the Jinshajiang-Ailaoshan fault belt.The petrology,minerology,geochronology and geochemistry characteristics of alkali-rich porphyries and xenolith are researched in this paper to constrain the age of emplacement and to build spatio-temporal framework and columnar section of lithosphere.It will give more evidence of interaction between crust and mantle at western Yunnan by revealing the correlation between xenolith and its host rock and by exploring the characteristic of magmatic source,petrogenesis,tectonic setting and geodynamics.(1)Lithologic characters of Alkali-rich porphyries in Liuhe and Xiaoqiaotou are pyroxene orthophyres and masanophyres respectively and both of them are stocks.Xenoliths hosted in pyroxene orthophyre consist of biotite-rich xenolith,biotite plagioclase gneiss,granitic xenolith,plagioclase amphibole schist and garnet amphibole biotite plagioclase gneiss.While the xenoliths hosted in masanophyre consist of two-pyroxene granulite and plagioclase amphibole schist.(2)Intrusions of Liuhe and Xiaoqiaotou emplaced between 37-35 Ma,which is consistent with the peak of alkali-rich magmatism.Lowest crystallization temperature of Liuhe porphyries is exceeding the Xiaoqiaotou porphyries according to the Ti-in zircon thermometers.(3)Liuhe pyroxene orthophyres belong to shoshonite series while Xiaoqiaotou masanophyres are high-K calc-alkaline series.Both porphyries are characterized as alkali-rich,high-K,low-Ti,enriched in LREE and LILE,depleted in HREE and HFSE,and negative Eu anomalies,indicating that they are derived from same magma.(4)The experimental results from the electron probe analysis of the inclusion and host rock minerals show that the least original depths of pyroxene orthophyres and masanophyres are 43 km and 33 km respectively,and their xenoliths mainly are captured from upper and middle crust.Taking phase equilibrium research into consideration,the magma source depth should be the bottom of lower crust.(5)Indicated by the EPMA analysis results,the Hf isotope analysis of the porphyrys and the bulk rock trace element analysis,the magma source region for the porphyrys is the enriched mantle which had undergone crustal contamination with metasomatism possibly of a superimposed one of the Mesoproterozoic and Permian Paleo Tethys ocean.(6)Beginning from around 70 to 60 Ma,affected by the Eurasia-India continental collision,the lithosphere in Sanjiang region thickened and shortened,which eventually resulted in the thinning of the lithospheric mantle,causing the uprising of the asthenosphere,which brought enough energy for the enriched mantle to melt.From about 40 Ma,translithospheric deep fracture generated by late collisional orogenesis created favourable conditions for asthenosphere upwelling and growth of magma chamber,which promotes the interaction with bottom of lower crust.The alkali-rcih magma reaches the subsurface through fault zone,which result in the spatial and temporal distribution of alkali-rich porphyrys in western Yunnan.