The Petrogenesis And Magmatic Process Of Bengge Syenite In Zhongdian Area, Yunnan Province

The Zhongdian Bengge is located in the southeast margin of the Tibet Plateau.It is within the "Sanjiang" Tethys structural domain and lies to the south of Yidun arc.The syenite outcropped in this area is mainly divided into ultrapotassic rocks and alkaline porphyry.Previous research remains controversial on its chronology.The precondition for applying zircon U-Pb dating and isotopic tracing is neglected,because the Bengge syenite is alkalic and its whole-rock Zr-saturated temperature is very low.Inaccurate ages would be obtained without identifying the origin of zircons which were used for U-Pb dating."Crystal population" such as xenocryst,antecryst and autocryst commonly exist in magmatic system.Analyzing their genesis is an important way to obtain the origin,process,composition and physical and chemical conditions of the host magma.The biotite aegirine syenite,biotite pyroxene syenite and biotite syenite are ultrapotassic rocks(MgO>3%,K2O>3%,K2O/Na2O>2),which are alkalic to peralkalic.The whole-rock Zr-saturation temperature of Bengge UPR is very low(86±13?634± 13? for biotite aegirine syenite,346± 14?-549± 13? for biotite pyroxene syenite and 281±14?-525±13? for biotite syenite),and they are far lower than the Ti-inzircon temperature of zircon(758?-1044?)that used for U-Pb dating.This indicates autocrystic zircon should be hardly present in the Bengge ultrapotassic rocks,and the zircon U-Pb ages(209±2 Ma-240±2 Ma)obtained in this study are all inherited ages.By contrast,biotite 40Ar/39Ar dating result(205.7±1.5 Ma)can better constrain the consolidation age of the Bengge ultrapotassic rocks.The values of ?HF(t)of these inherited zircons range from-5.0 to 0.42.By combining their U-Pb ages,they are thought to be derived from arc magmatic rocks.Some samples(biotite aegirine-augite syenite)of the ultrapotassic rocks contain idiomorphic aegirine-augite phenocrysts,and some of them contain pale-sieved diopside-salite cores.The distinctive structure and composition features of these diopside-salite cores are different from autocrystic aegirine-augite in the same sample,indicating they are xenocrysts.The F1-F2 diagram of clinopyroxene shows that the diopside-salite xenocrysts are mainly from arc volcanic rocks and oceanic crust,similar with the source of the inherited zircons.Aegirine-augite crystallization is favored by the fluids-rich and high oxygen fugacity condition of magma,which may be generated within intraplate environment.The ultrapotassic rocks of Bengge are enriched in large ion lithophile elements(LILE),relative depleted in high field strength elements(HFSE),which is typical feature of arc magmatic rocks.High Rb/Sr,low Ba/Rb ratios,together with high 87Sr/86Sr,low 143Nd/144Nd isotope ratios indicate the Bengge ultrapotassic rocks are derived from the phlogopite-bearing lithospheric mantle,and the source region is modified by subduction fluid/melt and contaminated by crustal material.The Bengge ultrapotassic magma did not undergo significant crystallization differentiation,assimilation,contamination of wall-rock,and magma mixing after its generation.The whole-rock Zr-saturation temperature of orthophyre ranges from 332±14? to 741 ± 16?,so authigenic zircon is likely formed.Zircon U-Pb dating reveals the Bengge orthophyre emplaced at about 35 Ma within post-collisional environment,consistent with the peak age of alkaline-rich porphyries along "Ailao shan-Red River" shear zone.The Sr-Nd isotopic characteristics of Bengge orthophyre suggest a hybrid source region of crust and mantle.The Bengge orthophyre is likely generated by mixing the partial melts of mantle and lower crustal,which were induced by intraplate extension.