中国阿尔泰花岗岩类源岩组分的氮同位素分辨

The source components of granitoids in the Chinese Altay (CA) include clastic sediments or meta-sediments eroded from the juvenile crustal rock of the Caledonian province (component 1) east of the CA, those from the Tuva-Mongol microcontinental block (component 2) further east of the CA, and the mantle-derived mafic rock (component 3). The nitrogen concentration and δ15N value are much higher in the sediments and meta-sediments than in the mafic rock and, hence, can be used to sensitively distinguish source component 1 or 2 from 3 for the CA granitoids. This project selects nine granitoid plutons from the CA to study. First, the N concentrations and δ15N of the granitoids and their mafic microgranular enclaves, with the alumina saturation index, accessory mineral assemblage and ISr, are used to discriminate source component 3 from 2 and, especially, 1 for each pluton, although the Nd, Hf isotopic compositions of components 1 and 3 overlap. Then, the Nd, Sr and zircon Hf isotopic compositions of the above rocks are compared with those of the three source components, whereas the U-Pb ages of the inherited zircon from the former compared with the zircon U-Pb age range of the latter. The mineral and geochemical criteria and isotopic parameters are correlated to determine the combination of source components for each pluton. After that, the Nd-Sr isotopic compositions of the granitoids and the involved source components are employed to model fraction of each of the components. The N concentration varies and 15N/14N fractionates significantly with devolatilization through the prograde metamorphism to the partial melting. The variations of the fraction of residual N (NH4+) and its δ15N will be modeled through the prograde-metamorphic zones, migmatite to the gneissic granitoid using both the batch devolatilization and Rayleigh distillation of N2 and NH3. Moreover, the Nd-Sr and zircon Hf isotopes and detrital- and inherited-zircon U-Pb ages will be compared among the metamorphic rock, migmatite and granite. The δ15N variations consistently modeled by the devolatilization, and similar Nd-Sr-Hf isotopic compositions and detrital- / inherited-zircon U-Pb ages will suggest generation of the gneissic granite through partial melting of metamorphic rocks or their protolith; otherwise, the gneissic granite would have stemmed from the deeper crustal rocks with possible involvement of mantle-derived material. A much higher or lower proportion of source component 3 than component 1 for most of the CA granitoids will support or deny the abrupt continental-crustal growth in the area at about 400 Ma; the latter case would imply a continuous, progressive continental-crustal growth in the western segment of the Central Asian Orogenic Belt. In addition, this project will stimulate the isotopic analysis and research of the fixed nitrogen from silicate rocks in China, and accumulate data of granitoids for the international comparison.

中国阿尔泰花岗岩类的源岩包括：来自东邻加里东省新生地壳（组分1）和更东部的图瓦-蒙古微陆块（组分2）的碎屑沉积物及其变质岩，幔源镁铁质岩石（组分3）。沉积物及其变质岩的N含量和δ15N显著高于幔源岩石。本项目拟研究阿尔泰的九个花岗岩体。将花岗岩以及镁铁质微粒包体的δ15N-N含量与地球储库对比，结合铝过饱和指数和ISr，高灵敏度地分辨各岩体的源岩组分1、2与3尤其组分1与3。将上述岩石的Nd-Sr同位素、锆石Hf同位素以及继承锆石的U-Pb年龄与三个源岩组分的同位素组成以及锆石U-Pb年龄范围对比，确定各岩体的源岩组分的组合，定量模拟相关源岩组分的比例。N含量和15N/14N分馏是去挥发分的灵敏指示剂。运用N2、NH3的批式去挥发分和瑞利蒸馏模型，模拟从各递增变质带→混合岩和片麻状花岗岩，残余N的分数和δ15N的变化，回答片麻状花岗岩是否由变质岩或其原岩熔融形成，有无深部物质参与及其比例。

受本项目资助，本单位稳定同位素实验室首次在国内建立了硅酸盐矿物和岩石中固定氮的同位素分析方法，提供了一个新的源区/岩示踪剂。在像中亚造山带这样以新生地壳物质为主要成分的地区，主要岩石的Nd-Sr-Hf同位素大幅重叠，N同位素的分辨率或灵敏度显著高于Nd-Sr-Hf同位素。N-O同位素、锆石U-Pb年龄-Hf同位素、Nd-Sr同位素多重判别表明，中国阿尔泰在早古生代是一个海底扩张中心，形成了一套基性到中性的火山岩夹碎屑岩。中国阿尔泰海底扩张中心的范围从南缘向北东伸展到山区，其发育时限也从早泥盆世前溯到早古生代，向北东阿尔泰山区被掩埋覆盖。揭示阿尔泰山区地壳柱由上覆岩系、下伏岩系和新近底侵的镁铁质岩石组成。阿尔泰花岗岩主要由下伏岩系熔融形成。埋藏的海底扩张中心不仅为地壳熔融提供热量，还为青格里英云闪长岩基提供了物质。对东准噶尔花岗岩类的源岩给出了有效的鉴别。碱性A型花岗岩来源于相对贫水的下洋壳，I2亚组花岗岩类来源于含水的上洋壳。部分熔融提取I型花岗岩浆使得上洋壳脱水，后者再度熔融产生A型花岗岩。东准噶尔北东部具有埃达克质特征的火山岩不是俯冲板片脱水熔融的产物，而是由俯冲组分交代的地幔楔部分熔融产生的岩浆发生分离结晶的产物。玄武质岩浆补给和结晶分离综合控制了岩浆的演化。索尔库都克Cu-Mo矿床的成矿和矽卡岩化发生于320 Ma左右。远源流体沿着断裂和裂隙上升，淋滤围岩中的Ca、Fe及Cu、Mo成矿物质，单向渗滤交代火山岩-火山碎屑岩，受流体渗滤方向和组分活动性差异的控制形成分带明显的层状、似层状矽卡岩，成矿物质发生卸载形成Mo、Cu矿体。. 项目执行期间，项目组成员发表7篇SCI论文，其中，5篇发表于国际SCI刊物，均注明得到国家自然科学基金资助和项目批准号。第一作者和通讯作者均为项目组成员。

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