Petrogenesis of the U-rich Permian Akkulen syenite intrusion, Tien Shan, Kyrgyzstan: insights into its magmatic evolution and geodynamic setting

Permian alkaline granitoids are widely distributed throughout the Tien Shan Orogen and adjacent region of the southern Central Asian Orogenic Belt. However, their petrogenesis and related tectonic setting remain equivocal. A detailed mineralogical, U-Pb zircon dating, in situ elemental and Sr-Nd-Hf isotopic studies of the uranium-rich Akkulen syenite intrusion in the Northern Tien Shan of Kyrgyzstan were undertaken to better understand its magmatic processes and geodynamic evolution. Four independent oxygen barometers have shown that the oxygen fugacity of the Akkulen syenitic magma evolved from initial low fO2 (2 (>NNO +1) during magma ascent and cooling; Apatite geochemical characteristics also indicate that the magma is relatively high F (2.9–5.1 wt.%), with low Cl (0.053–0.13 wt.%) and anhydrous (<4 wt %) contents. According to whole-rock geochemical data, the syenite samples have abnormally high U (38.5–57.1 ppm) and Th (96.2–137 ppm) concentrations, we suggest that high F content and moderate oxygen fugacity are beneficial for uranium enrichment. Zircon grains from a syenite sample yielded a weighted 206Pb/238U age of 283.5 ± 2.6 Ma. The intrusion has an A-type granite affinity with high alkali (Na2O + K2O) contents and Fe/(Fe+Mg) ratios, high LREE/HREE (~5), and low Sr, Ba, and Eu contents. They have negative εNd(t) (−5.2 to −3.6) values with Mesoproterozoic two-stage model ages (T DM2 = 1.35–1.47 Ga), and variable εHf(t) values (−4.6 to +2.4) with Mesoproterozoic two-stage model ages (T DM2 = 1. 2–1.59 Ga), which suggest these rock derivations from the relatively old crustal with minor contribution from juvenile crustal melts. In comparison with regional available dataset, we propose that the alkaline Permian granitoids in Kyrgyz Northern Tien Shan may have been originated from large-scale partial melting of Mesoproterozoic metamorphic basement with minor juvenile crustal material in a post-extensional setting. In contrast, the Early Permian granitoid magmatism in the Tarim Craton may have been caused by the partial melting of Neoproterozoic basement rocks associated with Permian mantle plume activities.

二叠纪碱性花岗岩类（Permian alkaline granitoids）广泛分布于天山造山带（Tien Shan Orogen）及中亚造山带（Central Asian Orogenic Belt）南部邻区，但其岩石成因与相关构造背景至今仍存争议。为深入阐明其岩浆作用过程与地球动力学演化，研究团队对吉尔吉斯北天山（Northern Tien Shan of Kyrgyzstan）的富铀阿库伦正长岩岩体（uranium-rich Akkulen syenite intrusion）开展了详细的矿物学、锆石U-Pb测年（U-Pb zircon dating）、原位元素及Sr-Nd-Hf同位素研究。四种独立的氧逸度计（oxygen barometers）测算结果显示，阿库伦正长岩岩浆的氧逸度（oxygen fugacity）在岩浆上升与冷却过程中由初始的低fO2（2 (>NNO +1)，即相对于镍-镍氧缓冲剂（NNO）的+1倍）演化；磷灰石地球化学特征（apatite geochemical characteristics）亦表明，该岩浆氟含量相对较高（2.9–5.1 wt.%），氯含量较低（0.053–0.13 wt.%）且不含水（<4 wt %）。基于全岩地球化学数据（whole-rock geochemical data），正长岩样品的铀（U）含量异常偏高（38.5–57.1 ppm），钍（Th）含量介于96.2–137 ppm之间；研究认为，高氟含量与适中的氧逸度有利于铀富集（uranium enrichment）。对一件正长岩样品的锆石颗粒（zircon grains）进行测年，获得的加权平均206Pb/238U年龄（weighted 206Pb/238U age）为283.5 ± 2.6 Ma。该岩体具有A型花岗岩（A-type granite）亲和性：碱质（Na2O+K2O）与Fe/(Fe+Mg)比值偏高，轻稀土/重稀土（LREE/HREE）比值约为5，锶（Sr）、钡（Ba）及铕（Eu）含量偏低。其εNd(t)值为负值（−5.2至−3.6），对应的中元古代二阶段模式年龄（Mesoproterozoic two-stage model ages）T_DM2为1.35–1.47 Ga；εHf(t)值变化范围为−4.6至+2.4，对应的中元古代二阶段模式年龄T_DM2为1.2–1.59 Ga，指示该岩石源自相对古老的地壳，并有少量新生地壳熔体（juvenile crustal melts）的贡献。对比区域已有数据集（regional available dataset），研究提出吉尔吉斯北天山的二叠纪碱性花岗岩类可能形成于伸展后构造环境（post-extensional setting），源自中元古代变质基底的大规模部分熔融，并有少量新生地壳物质的加入。与之相对，塔里木克拉通（Tarim Craton）内的早二叠世花岗岩类岩浆作用，可能与二叠纪地幔柱活动（mantle plume activities）引发的新元古代基底岩石（Neoproterozoic basement rocks）部分熔融有关。