Geochemistry and Sr–Nd isotope composition of Carboniferous volcanic rocks of the Jueluotage Orogenic Belt: implications for the tectonic evolution of Eastern Tianshan, China

The major and trace element, and Sr–Nd isotopic compositions of the Carboniferous Qi’eshan, Wutongwozi, and Yamansu volcanic rocks from the northern and southern parts of the Jueluotage Orogenic Belt in East Tianshan, China, were analysed to understand their genesis and geodynamic implications. The early Carboniferous Qi’eshan basalts are characterized by high Al2O3, with La/Sm (1.38–1.79) and Ba/La (27.06–58.76) values higher than those of typical normal mid-ocean ridge basalt. They are relatively enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), and depleted in high field strength elements. Overall, their initial Nd–Sr isotopic compositions are εNd(t) = (5.6–7.0) and Isr = 0.70397–0.70429, implying the magma originated from a mantle wedge source that was metasomatized by subduction-related fluids. In contrast, the late Carboniferous Wutongwozi basalts have lower Ba/La (4.86–12.82), La/Nb (0.87–2.45), and LILE concentrations. They have the isotopic characteristics of depleted asthenosphere, relatively high and heterogeneous εNd(t) (9.3–9.4), and high Isr (0.70471–0.70533). Thus, the late Carboniferous Wutongwozi basalts may have been derived from the partial melting of mantle sources during asthenospheric upwelling. The early Carboniferous Yamansu acid volcanic rocks are characterized by high Mg# (46–48) and Lu/Y (~0.15), and low K2O/Na2O (0.01–0.20), similar to M-type granites. However, their εNd(t) (5.0–5.5) and Isr (0.70642–0.70768) values are lower than those of depleted mantle, indicating they were contaminated by lower crustal material. The magma source originated from a mantle-derived magma that was contaminated by middle Tianshan massif in a continental margin arc setting. Based on the results and previous field-based studies, we conclude that the Carboniferous volcanics in the Jueluotage Orogenic Belt formed in a complex trench–arc–basin setting in the Kuguertage–Aqikuduke Suture Zone.

为探讨中国东天山觉罗塔格造山带（Jueluotage Orogenic Belt）南北两段石炭纪七角山（Qi’eshan）、梧桐窝子（Wutongwozi）及雅满苏（Yamansu）火山岩的成因与地球动力学意义，本文对其主量元素、微量元素及Sr-Nd同位素组成（Sr–Nd isotopic compositions）开展了分析测试。早石炭世七角山玄武岩以高Al₂O₃为显著特征，其La/Sm比值（1.38~1.79）与Ba/La比值（27.06~58.76）均高于典型正常洋中脊玄武岩；该类玄武岩相对富集大离子亲石元素（LILE）与轻稀土元素（LREE），且亏损高场强元素，其初始Sr-Nd同位素组成为εNd(t)=(5.6~7.0)、Isr=0.70397~0.70429，指示岩浆源自受俯冲流体交代的地幔楔源区。与之形成鲜明对比的是，晚石炭世梧桐窝子玄武岩的Ba/La比值（4.86~12.82）、La/Nb比值（0.87~2.45）及大离子亲石元素含量均相对偏低，具备亏损软流圈的同位素特征：εNd(t)值较高且不均一（9.3~9.4），Isr值偏高（0.70471~0.70533），据此推测该类玄武岩可能源于软流圈上涌过程中地幔源区的部分熔融作用。早石炭世雅满苏酸性火山岩以高镁指数（Mg#，46~48）、Lu/Y比值（~0.15）及低K₂O/Na₂O比值（0.01~0.20）为特征，与M型花岗岩（M-type granites）相似；但其εNd(t)值（5.0~5.5）与Isr值（0.70642~0.70768）均低于亏损地幔，表明其受到了下地壳物质的混染，该类火山岩的岩浆源区源自大陆边缘弧背景下受中天山地块混染的幔源岩浆。综合本次测试结果与前期野外研究成果，本文认为觉罗塔格造山带内的石炭纪火山岩形成于库古尔塔格-阿其克库都克缝合带（Kuguertage–Aqikuduke Suture Zone）内的复杂沟-弧-盆体系中。