Nitrogen and oxygen isotopic profiles and redox structure across the Chinese Altay reveal the source rocks of granites, core of the Central Asian Orogenic Belt
Nitrogen and oxygen isotopic profiles and redox structure across the Chinese Altay reveal the source rocks of granites, core of the Central Asian Orogenic Belt
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DOI：
10.1016/j.lithos.2023.107392
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发表时间：
2023-10
期刊：
影响因子：
3.5
通讯作者：
Wei Liu;Lianjun Feng;Xiujin Liu;Tiejun Li;Hongwei Li;Xiaofei Pan;Cuiqin Si
中科院分区：
文献类型：
--
作者：
Wei Liu;Lianjun Feng;Xiujin Liu;Tiejun Li;Hongwei Li;Xiaofei Pan;Cuiqin Si
在中国阿尔泰（CA）的两个岩石系列被确定。上覆岩系的原岩为细砂岩和粉砂岩，由石英和粘土组成，但含有少量长石。下伏岩系为CA花岗岩、混合花岗岩和早泥盆世康布铁堡组夹持的黑云母-斜长片麻岩和斜长角闪岩。是最晚形成的西南边缘CA花岗岩包括奥长石微斜长石花岗岩和黑云母斜长石花岗岩，具有广泛的斜长石碎斑钾钠交代作用。这两个系列都具有高的正εNd-ε Hf值，但N单键O同位素可区分。本研究利用N-单键O同位素示踪CA花岗岩的源岩，探讨从变质、混合岩化到部分熔融的脱挥发作用，上覆岩系的N含量较高（20.2-484 ppm;平均值：87.3 ± 99.0 ppm; 1σ;n= 36），δ 15 N（−3-19‰;平均值：5.8 ± 4.3‰; n = 36）和δ 18 O（6.9-15.6‰;平均值：13.6 ± 3.05‰;n= 29）比产生N（9.9-71.1 ppm;平均值：32 ± 17.1 ppm;n= 22），δ 15 N（−7.3-8.1‰;平均值：1.1 ± 4.7‰; n = 22）和δ 18 O（4.5-12.1‰;平均值：8.8 ± 1.9‰; n = 22）。上覆岩系的原岩堆积于CA的富氧浅海，δ 15 N值较高;下覆岩系的原岩延伸至缺氧深海，经历了高温海水反应，δ 18 O和δ 15 N值较低。CA花岗岩的N、δ 15 N和δ 18 O含量比上覆系列低得多，但其值与下覆系列的值重叠，δ 18 O略有增加。从下伏系列到CA花岗岩，δ 15 N值随元素标准化N含量的降低而增大，表明下伏系列的混合岩化作用和部分熔融作用导致了15 N/14 N分馏。定量模拟结果表明，下伏系列在550 ~ 750 °C的NH3挥发分演化轨迹与下伏系列和CA花岗岩相吻合，而上覆系列的15 N/14 N分馏轨迹则与CA花岗岩相偏离。这些结果表明，CA花岗岩已从下伏系列。大多数数据的基础系列和CA花岗岩可以模拟瑞利蒸馏表明脱气NH3从本质上固态原岩/源岩从变质混合岩化逃逸。部分转化为N2或N2 O导致15 N-富集在两个系列的上覆系列，东部CA的前缘。下伏系列岩浆与交代地幔岩浆的混合，以及围岩流体的贡献，导致青阁里花岗闪长质和微闪长质包体具有高δ 15 N值。
Two rock series in the Chinese Altay (CA) were identified. The protoliths of the overlying series are fine sandstone and siltstone that consist of quartz and clay but contain little feldspar. The underlying series includes biotite–plagioclase gneiss and amphibolite entrained by the CA granite, migmatitic granite and the Early-Devonian Kangbutiebao Fm. as the uncovered, latest-formed southwestern margin. The CA granites include oligoclase microcline granite and biotite plagioclase granite that exhibit extensive K-Na-metasomatism of plagioclase porphyroclast. The two series both have high positive εNd-εHfvalues but are Nsingle bondO isotopically distinguishable. This study employed Nsingle bondO isotopes to trace source rocks of the CA granites and to explore devolatilization from metamorphism, migmatization to partial melting.The overlying series yielded much higher N content (20.2–484 ppm; average: 87.3 ± 99.0 ppm; 1σ;n= 36), δ15N (−3–19‰; average: 5.8 ± 4.3‰; n = 36) and δ18O (6.9–15.6‰; average: 13.6 ± 3.05‰;n= 29) than the underlying series that yielded N (9.9–71.1 ppm; average: 32 ± 17.1 ppm;n= 22), δ15N (−7.3–8.1‰; average: 1.1 ± 4.7‰; n = 22) and δ18O (4.5–12.1‰; average: 8.8 ± 1.9‰; n = 22). The protoliths of the overlying series were accumulated on oxic shallow sea in the CA and, hence, recorded high δ15N, whereas the protoliths of the underlying series extended to anoxic deep-ocean and experienced high-temperature seawater reaction, which resulted in low δ18O and δ15N values of the protoliths of the underlying series. The CA granites yielded much lower N content, δ15N and δ18O than the overlying series but have values overlapping those of the underlying series with a marginal increase in δ18O. The δ15N values tend to increase with decreasing element normalized-N contents from the underlying series to the CA granites, suggesting that migmatization and partial melting of the underlying series have caused15N/14N fractionation. Quantitative modeling demonstrates that NH3devolatilization of the underlying series at 550 to 750 °C can fit the underlying series and the CA granites, whereas15N/14N fractionation trajectories of the overlying series miss the CA granites. These results suggest that the CA granites have been derived from the underlying series. Most data of the underlying series and the CA granites can be modeled by Rayleigh distillation suggesting escape of degassed NH3from essentially solid-state protolith/source rock from metamorphism to migmatization. Partial conversion to N2or N2O caused15N-enrichment in the two series at the leading edge of the overlying series, the eastern CA. The magma derived from the underlying series mixed with magma stemming from the metasomatized mantle with contribution from country rock-derived fluid result in high δ15N of the Qinggeli granodiorite and microdioritic enclaves.