The Study On Zougouyouchacuo And Nadingcuo Cenozoic Volcanic Rocks Of South Qiangtang In Petrology And Geochemistry

Since the collision between India continent and Eurasia continent , the multiphase volcanic activity are formed in northern Tibet . According to the isotopic dating, the volcanic activity of Qiangtang began from the sodium-rich alkali basalt , the age is 60⁴4Ma, since 42Ma, the high Mg^# and K calc-alkaline volcanic activity has a plenty of appearance in north Qiangtang area whose peak age is 42³7 Ma. the high Mg^# and K calc-alkaline volcanic rock of south Qiangtang was formed in 35-31Ma, and it is mainly distributed in Zougouyouchacuo and Nadingcuo,yulinshan-gemucuo alkali potassic-ultrapotassic volcanic rocks erupted at 31-24Ma .Geochemical characteristics of the high Mg^# and K calc-alkaline volcanic rock of south Qiangtang is the intense fractionation of HREE, high Sr and low Y of adakites, and there are always some divergences on the genesis of it , the relationships between the nature of origin region and the generation process of magma and lithospheric thinning-after- thickening become important scientific issues in research on formation and evolution of Tibetan Plateau.This paper takes the genesis study of the ougouyouchacuo and Nadingcuo high Mg^# and K calc-alkaline volcanic rock of south Qiangtang as study emphasis, and explores the relationships between the magmatic origin and lithospheric thinning-after- thickening.The main lithological associations of the Nadingcuo volcanic rock are basalt- latite and latite , while the one of Zougouyouchacuo volcanic rock is mainly trachyte. The volcanic rock is mainly lava which is produced in the form of lava sheet and volcanic unaka and covers the pre-Jurassic strata.The chemical constituents of the Nadingcuo basalt- latite is of the characteristics of alkaline series , Na₂O/K₂O=2.53-2.06, while latite is of the blend feature of alkali-series and subalkali-series , Na₂O/K₂O=1.3-0.94 , the Zougouyouchacuo trachyte enters subalkali-series composition region, Na2O/K2O=1.06-0.85. In SiO₂-K2O diagram, volcanic rocks enter high K calc-alkaline-series composition region, partial latite enter shoshonite-series composition region, which displays the magmatic property from basalt- latite to trachyte evolves from the sodium to the potassic ,and is of the blend feature of kohalaite-series and high K calc-alkaline-series.The results show that Zougouyouchacuo and Nadingcuo Cenozoic volcanic rocks is the product resulted from the evolution of a cognate magma. In the harker diagram, major oxidates presents regular transformation with the increasing of SiO₂ from the Nadingcuo basalt- latite to the Zougouyouchacuo trachyte, which displays the characteristics from the evolution of a cognate magma. Strong compatible elements, as Cr,Ni, vary in a large range, Cr varies between 156²Ã—10^-6 from basalt- latite to latite, and indicates that magma composition change is under the control of fractional crystallization. But incompatible elementsΣLREE and Nb decrease with the increasing of SiO₂, which indicates entry of crust materials during the magma evolution. TheεHf of rocks decreases with the increasing of SiO₂, which further proves entry of crust materials during the magma evolution.The Mg# of Zougouyouchacuo and Nadingcuo Cenozoic volcanic rocks varies between 57.7-43, and are of characteristics of high Mg^# generally.They aslo have a strong enrichment in light rare earth element, relative HREE depletion and strong fractionation of light and heavy rare earth egative, andδEu is from non-anomaly to weak positive anomaly, also high Sr/Y, the above geochemical characteristics of latite and trachyte is consistent with adakites,and shows that garnet residues exists in magma source. But the∑LREE of the rocks decreasing with the increasing of SiO₂ and the existence of basic rocks show that their genetic interpretation is not the melting of crust eclogite.In the Th/La-Zr/Nb diagram, the basalt- latite enters low-Th/La and low-Zr/Nb mantle composition region constituted by depleted and OIB type mantle, while latite and trachyte vary towards active continental margin high K calc-alkaline composition region in turns, and differe from island arc volcanic rocks composition region. In the Ba/Nb-La/Nb diagram, the trachyte locates in the edge of volcanic arc composition region, while the basalt- latite has a transition towards MORB and OIB mantle composition region, these characteristics indicate that the primitive basaltic magma is from asthenospheric mantle and lithosphere mantle formed by paleosubdution which has a blend source. theεHf from basalt- latite to trachyte varies between 5.5-2.2 which indicates source region is of the properties of weak depleted mantle and is consistent with low-Th/La and low-Zr/Nb.Based on the comprehensive comparison with the geochemical characteristics of the Cenozoic volcanic rocks of north Qiangtang, the paper puts forward that Zougouyouchacuo and Nadingcuo Cenozoic volcanic rocks of south Qiangtang derives from the blend souce of asthenosphere upwelling mantle and lithosphere mantle, the alkaline basaltic magma of enriched incompatible elements of garnet lherzolite low melting is primitive magma, the primitive magma during the bottom invasion by means of AFC forms latite and trachyte. asthenosphere upwelling has possibly a relationship with the breaking-off of subduction slab, and indicates that the breaking-off of Tethys ocean subduction slab may happen before 35Ma.