Study Of The Petrogenesis Of Tarim Permian Flood Basalts,NW China

Two groups of basalts have been recognized in the Tarim Permian Continental Flood Basalts Province?TCFB?,Northwest China.Two series of representative samples for the two groups of basalts from Keping and Yingmai were collected for detailed petrological and geochemical analyses including Sr-Nd-Zn isotopes.The volume of erupted basalt is also roughly calculated in this study,and the result shows about21,600km³,which is far less than the defined LIP.All of the basalts are porphyritic in texture with Keping basalts predominated by plagioclase phenocrysts and Yingmai basalts by clinopyroxene phenocrysts.Magmatic carbonate solid solutions grown within globules and filled in the trellises of plagioclase and clinopyroxene phenocrysts are identified in the Keping basalts.The Keping basalts are characterized by low Mg#?28–37?and low Nb/Zr ratios?0.09–0.11?while those in Yingmai have higher Mg#?46–71?and higher Nb/Zr ratios?0.15–0.18?.Both types of basalt display negative anomalies in Zr-Hf,higher Zr/Hf ratios than BSE and higher La/Nb and Nb/Ta ratios than OIB,which indicates carbonated mantle sources.Both Keping(?⁶⁶Zn between 0.29–0.32‰,±0.05‰,2SD)and Yingmai basalts(?⁶⁶Zn between 0.29–0.32‰,±0.05‰,2SD)display higher Zn isotopes than primitive mantle,which suggests that sedimentary carbonates may involved in their mantle sources.The Ti-rich characteristics of two type of basalts are inconsistent with a carbonated mantle source,and imply that Ti-rich components are required for both types of basalts.We suggest that the amphibolite veins may exist in the Keping mantle source while recycled garnet pyroxenite may involve in the Yingma mantle source.The slightly negative anomalies in Nb-Ta and radiogenic ⁸⁷Sr/⁸⁶Sr_i?0.70720–0.70779?,?Nd_?t??-3.2to-1.9?signature of Keping basalts indicate that they may derive from subcontinental lithospheric mantle?SCLM?.The disappearance of negative anomalies in Nb-Ta and the moderate ⁸⁷Sr/⁸⁶Sr_i?0.7050–0.7052?and?_Nd?t??-2.5 to-1.3?isotope characteristics of Yingmai basalts imply that they are likely derived from asthenospheric mantle.An ancient subduction model is favored and the distinct geochemical characteristics of two groups of basalts are attributed to different ways of interaction among asthenospheric mantle,subducted slab and SCLM.