Barium Isotope Study On Granitic Magmatism And Rare Metal Enrichment

As an important component of the continental crust,granite is one of the most important features that distinguish the Earth from other planets in the solar system.Studying granitic magmatism is crucial for understanding the formation and stability of the continental crust and the evolution of life,as well as for securing national economic development.The granite as a whole is characterized by significant geochemical diversity.Although previous multidisciplinary studies have evaluated the influence of several processes,such as partial melting,crystal fractionation,and wall rock contamination,on the composition of granitic magmas,there are still great controversies about the crystal-melt separation mechanism of granitic magma,the role of hydrothermal fluids in the granitic magma evolution and the transport of ore-forming rare metal elements,and the genesis of granitic pegmatites.Ba is compatible and fluidmobile during the evolution of granitic magmas,and there are differences between its bonding environment in the mineral-melts-fluids.Hence,Ba isotopes may be fractionated in the above processes,and have great application potential.In this thesis,we present Ba isotope studies on three sets of representative granitoid samples,with the aim of providing new insights into the crystal-melt separation and magmatichydrothermal evolution of granitic magma and the internal evolution mechanism of granitic pegmatite veins.Crystal-melt separation in the crystal mush is currently regarded as one of the most important mechanisms responsible for the compositional variation of granitic magmas and is key for understanding the connection between plutonic and volcanic rocks.However,the existing geochemical indicators for crystal-melt separation in many granitic plutons are largely concealed by the remaining interstitial melt.There is also a lack of corresponding geochemical indicators for the specific mechanisms of crystalmelt separation in crystal mush.To address these issues,we measured the Ba isotope compositions of whole-rock and Ba-bearing mineral samples from the Huili granitic pluton within the Jiaobei Terrane of the North China Craton.Among them,the more differentiated albite granites have higher δ138/134Ba(0.50‰～0.95‰)than the Kfeldspar granites(-0.14‰～0.16‰).Trace element modeling suggests that the Kfeldspar-controlled crystal-melt separation in a crystal mush formed by the K-rich parental magma can explain the features of variation in Ba and Sr contents between the two types of granites.As the K-feldspar granites represent the residual crystal mush.the Ba isotope data of the coexisting Ba-bearing minerals can impose critical constraints on crystal-melt separation.The K-feldspar shows the highest Ba content(387μg/g～1465μg/g)and the lowest δ138/134Ba(-0.23‰～0.01‰)among all the investigated minerals,suggesting that its crystallization should drive the interstitial Na-and Si-rich melt towards an isotopically heavy composition.The apparent fractionation values of coexisting biotite and muscovite(Δ138/134BaBiotite-Muscovite)vary from-0.98‰ to 1.01‰,indicating that the coexisting minerals are not in Ba isotope equilibrium.Combined with microstructural features,the Ba isotope disequilibrium signatures of coexisting minerals reflect crystal repacking and accumulation during crystal-melt separation.Granitic magmatic-hydrothermal evolution is an important reason for the compositional variation of highly differentiated granites in the shallow continental crust,and is a key step in the formation of many rare metal(Sn,W,Nb,Ta,Mo,etc.)deposits.The transcrustal magmatic system model established in recent years suggests that the fluids exsolved from the deep magma reservoirs can participate in the shallow magmatic-hydrothermal evolution,and can also carry fluid-mobile ore-forming elements.However,it is difficult to identify the influences of deep magmatic fluids on shallow magma compositions and the contribution to the transport and enrichment of rare metal elements by using conventional geochemical indicators.Here,we report Ba isotope compositions for samples from the Jurassic Qitianling A-type granite batholith in the Nanling Range,South China.The apparent age gap and sharp contacts between the three stages of granite suggest an underlying long-lived crystal mush-dominated transcrustal magmatic system.The first-stage granites are formed by the melts separated from the deep crystal mush,and the latter-two stage highly differentiated granites formed by episodic remobilization of the shallow crystal mush are closely associated with several large tin-polymetallic deposits.The small variation of δ138/134Ba in the first-stage granites(-0.24‰～0.37‰)indicates that fractional crystallization in deep crystal mush does not cause significant Ba isotope fractionation.In contrary,the latter-two stage highly differentiated granites show more variable and overall lowerδ138/134Ba(-1.79‰～0.14‰),which cannot be explained by K-feldspar-controlled fractional crystallization in shallow crystal mush.The distinctively low Ba contents(<100μg/g)and magmatic-hydrothermal evolution features of the latter-two stage granites suggest that their low δ138/134Ba are due to modification by magmatic fluids.Further modelling demonstrates that exsolved fluids from deep crystal mush can explain the light Ba isotope compositions of the latter-two stage granites.Because these ascending deep magmatic fluids can efficiently scavenge Sn and other fluid-mobile elements then transport to shallow level,these fluids could provide critical materials for the Sn-polymetallic mineralization.LCT(Li-Cs-Ta)-type granitic pegmatite,as a kind of granite with special texture and highly differentiated features,is also the main host rocks for many rare metals(Li,Be,Rb,Cs,etc.).However,there is great controversy over the specific role of H2Odominated fluid components in the internal evolution and Li enrichment of granitic pegmatite veins.The Jiajika granitic pegmatites in western Sichuan mainly intrude into the metasedimentary rocks in the mantle of the Jiajika gneiss dome at the eastern edge of the Songpan-Garze Orogenic Belt.The granitic pegmatite veins are spatially grouped and zoned around the Majingzi two-mica granite,and are typical LCT-type granitic pegmatite.The Li-mineralized pegmatites are mainly distributed in the Ⅲ and Ⅳ zones,which as a whole constitute the largest hard rock-type Li deposit in Asia.In this study,Ba isotope compositions of borehole samples with different depths and lithologies were measured.The granitic pegmatite samples show large Ba isotope fractionation(-2.44‰～0.47‰).Combined with the metasedimentary rock data,this Ba isotope fractionation does not support the petrogenetic mechanism that granitic pegmatite formed by low degree partial melting of the metasedimentary source.Although the granite and aplite samples show variations in the degree of differentiation,their δ138/134Ba variation is small,indicating that the large Ba isotope fractionation between granitic pegmatite samples should be the result of internal evolution in the veins.According to the variation of mineral assemblage from the proximal to the distal end of the granitic pegmatite veins,the evolution of the initial water-unsaturated melts through rapid crystal growth cannot explain the low δ138/134Ba of most granitic pegmatite samples.At least for the Jiajika granitic pegmatite,initial supercritical fluids occurring phase separation at the low formation temperature of granitic pegmatite is a plausible process capable of producing such large Ba isotope fractionation.The H2O-rich silicate-poor melt formed by phase separation can effectively transport rare metal elements to the distal end of the vein to enrich and mineralization.In summary,the results of this thesis show that Ba isotopes are helpful for identifying highly differentiated granites and deciphering petrogenetic links between high silica rhyolites and granitic plutons,and can trace fluid-mediated material transport within the transcrustal magmatic system.Fluid components should play an important role in the internal evolution and Li enrichment of LCT-type granitic pegmatite veins,which may be of great significance for the exploration of giant rare metal deposits.