Formation Mechanism Of Tourmaline In The Himalayan Leucogranites:Constraints From The Natural And Experimental Studies

The Himalayan leucogranites derive from crustal anatexis in the process of Indian-Eurasian plate collision,which can record the formation and evolution of the Himalayan orogenic belt.Tourmaline leucogranite is one of the main types of Himalayan leucogranites.Owing to tourmaline can crystallize in a different stage of magma evolution,studying the structure,composition and crystallization condition of tourmaline can help us constrain the generation of the evolution process of Himalayan leucogranites.In this study,the study of natural sample and tourmaline crystallization experiment have been conducted for understanding the formation mechanism of tourmaline in Himalayan leucogranites.Two different types of tourmaline are found in Gurla Mandhata tourmaline leucogranite,the structure and major element composition show these tourmalines crystallize in a different stage of magma evolution.The compositions of core and mantle of the II-type tourmaline are similar with the tourmalines which present as inclusion in other minerals,both show high Mg and low Fe contents,suggest that the core and mantle of the II-type tourmaline formed at the early magma evolution stage.?-type tourmaline always includes other minerals,similar to the rim of II-type tourmaline on major element composition,have relatively high Fe and low Mg contents,these features show that?-type tourmaline and the rim of II-type tourmaline are crystallized from the highly fractional magma.Tourmaline in the leucogranite has a bimodal B-isotope distribution,the core and mantle of II-type tourmaline share similar boron isotopic composition with?¹¹B value of-7‰^-8‰,the B-isotope composition of the rim of II-type tourmaline and?-type tourmaline are uniform,with the range of?¹¹B value from-12‰to-15‰.Calculation of boron isotope fractionation between melt,fluid and tourmaline suggest that the boron isotope compositional variations are a result of volatile degassing of magma,which results in the strong decrease of?¹¹B value in evolved magma.The difference in the same type of tourmaline suggests the tiny fractionation of B-isotope composition with the crystallization of tourmaline.In combination with previous study,the B-isotope composition for tourmalines from Himalayan leucogranite is inconsistent,the variations could potentially result from multiple sources of boron,each with different isotopic composition,or from a single source of boron undergoing an isotopic fractionation process.Several granitic glasses with different boron content were used to conduct tourmaline crystallization experiment at 650-800?,300MPa,aH2O=0.5¹ and fO2^NNO and NNO+2.3 for determining the tourmaline crystallization condition in granitic melt.The experimental results show that boron content is the most important factor for tourmaline saturation in granitic melt,the boron content required for tourmaline crystallization changes with the difference of other conditions,and strongly depends on temperature.The tourmaline saturation boundary at aH₂O=1 and 0.7,logfO₂=NNO and NNO+2.3 has been fitted in the diagram of B₂O₃ concentration in melt versus temperature,and the boundaries can be used to speculate the boron content in granitic magma during tourmaline crystallization.Moreover,Tourmaline prefers to crystallize at low water activity and high oxygen fugacity magma.The relationship between aluminum saturation index?ASI?and tourmaline crystallization was not found in our experiments,the experimental results show tourmaline is stable in metaluminous melt?ASI<1.1?if the melt contains sufficient boron.Mineral assemblage of experimental runs demonstrates that the liquidus phase of boron-bearing granitic melt is controlled by the boron concentration and water activity of the melt.In addition to tourmaline,another boron-rich phase,dumortierite also crystallized in some experimental runs,and it will be a liquidus phase in the melt with high boron content and low water activity.Tourmaline in some experiments shows typical hourglass sector zoning,display that hourglass sector zoning can also exist in magmatic tourmaline.Because the compositional differences among the sectors are pronounced in major elements,the presence of sector zoning must be established and taken into account when making inferences from tourmaline chemistry.Fluid-rock interaction experimental between a two-mica leucogranite and boron-rich fluids were carried out at 600-700?and 200MPa.The experimental results show that tourmaline granite can be produced by the reaction between boron-rich fluid and two-mica granite.At 700?,adding boron-rich fluid made partial melting of two-mica granite and tourmaline crystallized from the boron-rich partial melt.No melts were produced in the experiments at 600?,the Fe,Mg and Al released from biotite dissolution combined with boron in the fluid to form tourmaline.The tourmaline produced by the experiment generally has core-rim structure,indicating that the composition of melt or fluid was changing with the tourmaline crystallization.Based on the experimental result,the boron-rich fluid produced by degassing of boron-bearing granitic magma chamber reacts with the two-mica granite in the margin of the magma chamber to form tourmaline granite vein or dyke.Based on the results of natural tourmaline research and tourmaline crystallization experiments,the genesis of tourmaline in Himalayan leucogranite is diverse,but mainly crystallizes in the late stage of magma evolution.Combined with previous studies,we summarize the genetic relationship of different types of leucogranites two main modes:the composition of initial melt determines the type of leucogranite,and fractional crystallization determines the type of leucogranite.