Lithium Isotope,melt-fluid Inclusion And Niobium-tantalum Geochemical Studies Of Post-collisional Igneous Rocks From The Dabie Orogenic Belt

The Dabie orogenic belt located in eastern China,marks the final collision between the North and South China blocks during the Mesozoic.It is not only one of the largest and best-exposed high-pressure/ultrahigh-pressure metamorphic terranes on Earth,but also one of the predominant regions composed of large amounts of post-collisional granitic rocks and some mafic rock.Studying post-collisional mafic—ultramafic igneous rocks in the Dabie orogenic belt is of great significance for understanding the recycling of subducted continental crustal materials during the continental collision and the evolution of post-collisional mafic magma.This dissertation has carried out detailed petrology and geochemistry studies on the post-collisional mafic igneous rocks in the Dabie orogenic belt,and mainly obtained the following points:We systematic determined whole rock major and trace element,and Li isotopic compositions of Daoshichong,Zhujiapu,Jiaoziyan and Shacun post-collisional mafic-ultramafic rocks from Dabie orogenic belt.The analyzed samples have high MgO(up to 20.67 wt%)and low SiO2(41.06-56.20 wt%)contents,and arc-like trace element distribution patterns,suggesting derivation by partial melting of fertile mantle sources.The samples have ?7Li values of-2.9 to+7.2‰(average=+2.7‰)and Li concentrations of 2.50 to 28.1?g/g(average=9.20?g/g),both of which are beyond the relatively limited ranges of mantle values(?7Li=+2.9 to+4.3‰,average=+3.5±1.0‰,and Li=1.20±0.10?g/g).Excluding possible effects of wallrock assimilation and contamination,magmatic crystallization,and Li isotopic fractionation during subduction-exhumation processes,the observed Li isotopic heterogeneity is most likely related to the recycling of crustal materials.Based on a comparison with Li isotopic compositions of ultrahigh-pressure(UHP)metamorphic rocks from the Dabie-Sulu orogenic belt and terrestrial sediments,we suggest that the recycled continental crustal materials likely comprised gneisses,sediments,eclogites,and/or minor carbonate rocks(i.e.,marbles).Our study demonstrates that Li isotopes may provide a novel means of tracing deeply subducted continental materials.We conducted detailed microscope observations and laser Raman spectroscopy analyses of melt-fluid inclusions in Zhujiapu postcollisional mafic rocks from Dabie orogenic belt.Three types inclusions were identified in host minerals(olivine,orthopyroxene,clinopyroxene and amphibole)containing silicates(with the similar species as the host minerals)±carbonates±methane(CH4)±H2O±sulfates,carbonates+talc+CH4±sulfates,and silicates+carbonates+talc±CH4±sulfates,which respectively represent the captured melt,fluid,and melt-fluid coexisting components during the formation of host minerals.Cabonates,sulfates and CH4 are occurred in all three types of inclusions,suggesting the presence of CO2,CH4 and S components in lithospheric mantle source of the orogenic belt.The transformation of the melt-fluid inclusions by host minerals is negligible,we suggested carbonates and sulfates are mainly crystallized and precipitated from inclusion components.CH4 may be formed by the Fischer-Tropsch reaction between the CO2 fluid derived from deeply subducted continental crust and the mantle wedge peridotite or from the Earth surface.Combined with the previous studies,we proposed that the mantle source of Zhujiapu mafic rocks was together metasomatized by felsic melts and COH fluid,and that this kind of agent may be represent the supercritical fluid in silicate-CHO fluid system.Therefore,the melt-fluid inclusions of post-collisional mafic rocks from Dabie orogenic belt broadens our understanding of volatiles recycling in continental subduction zone.The daughter minerals of carbonate and sulfate in inclusions are mainly crystallized and precipitated from their own components.The CH4 component in inclusions may originate from the Earth surface and/or form by carbonic fluid derived from deep subduction of continental crust through Fischer-Tropsch reaction with the mantle wedge peridotite.Therefore,the melt-fluid inclusions in Zhujiapu post-collisional mafic rocks from Dabie orogenic belt record the carbon and sulfur cycles of the continental subduction zone.Combined with the petrological genesis,we suggest that there are felsic melt and COH fluid in the continental subduction zone,which may represent the supercritical fluid of the silicate melt+COH fluid system.Post-collisional mafic magmatic plutons from Daoshichong in the Dabie orogenic belt,central China,were studied for bulk geochemistry and in-suit major-trace elemental features in amphibole and biotite,which provide an attractive insight into the fractionation behavior of trace elements,in particular Niobium(Nb)-Tantalum(Ta)during mineral crystallization from mantle-derived magma.The investigated samples can be divided into three groups based on field and microscopic observations,which represent different stages of the magmatic evolution.From group I through group ? to group ?,MgO,Fe2O3,CaO,Cr and Ni contents continuously decrease,while contents of SiO2,Al2O3,Na2O,Ba and Sr as well as Nb/Ta ratios(19.5—26.2)constantly increase.These observations not only indicate that the post-collisional mafic rocks in Daoshichong pluton from Dabie orogenic belt is an excellent magmatic evolution series,but also demonstrate that Nb and Ta were significantly fractionated during this process.In-suit analysis data of amphibole display that concentrations of Nb-Ta and Nb/Ta ratios increase from 4.87 to 22.5 ppm,from 0.20 to 0.77 ppm and from 16.8 to 41.1,respectively,with decreasing Mg#(52.3—76.3)and increasing TiO2 contents(1.25—2.39 wt.%).Similar to amphibole,biotite shows Nb-Ta concentrations and Nb/Ta ratios increase from 8.50 to 25.1 ppm,0.32 to 0.65 ppm,21.1 to 49.6 with decreasing Mg#(52.0—71.0)and increasing TiO2 contents(1.02—5.07 wt.%),indicating that Nb and Ta could be remarkably fractionated through amphibole and biotite crystallization in magmatic process.In addition,Nb/Ta ratios of coexisting melt equilibrium with amphibole and biotite,and the partition coefficients between biotite and silicate melt(D Bt/melt Nb/Ta)were estimated by semi-quantitative modelling.The modelling results revealed that the Na/Ta ratios of coexisting melt could strikingly decrease from 31.6 to 15.6 accompanied with amphibole and biotite crystallization,and that the D Bt/melt Nb/Ta increase from 0.89 to 2.40 with decreasing Mg#.This study confirm from the natural samples that the crystallization of amphibole and biotite would cause significant Nb-Ta fractionation and reduce Nb/Ta ratios of the melt.We suggested that amphibole and biotite are two main crystallized-phases controlling Nb-Ta fractionation in the period of early continental crust formation,and that about 0.16 to 0.24 percent by weight of amphibole-biotite assemblages hidden in the middle-lower crust could balance the deficit of the silicate Earth.