Petrogenesis of Carboniferous volcanic rocks from the Gangou area, Chinese North Tianshan: Constraints on the evolution of the North Tianshan Ocean
Petrogenesis of Carboniferous volcanic rocks from the Gangou area, Chinese North Tianshan: Constraints on the evolution of the North Tianshan Ocean
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DOI：
10.1002/gj.3680
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发表时间：
2020-03
影响因子：
1.8
通讯作者：
Zheng-Yu Yang;X. Gong;Ji‐Feng Xu;Xijun Liu;Zhiguo Zhang;Hai-Long Zhou
中科院分区：
文献类型：
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作者：
Zheng-Yu Yang;X. Gong;Ji‐Feng Xu;Xijun Liu;Zhiguo Zhang;Hai-Long Zhou
中国天山造山拼贴中的主要北天山洋(又称准噶尔洋)的持续时间和演化仍未得到很好的约束。本文报道了甘沟地区(新疆，中国西北部)北天山增生杂岩中记录的石炭纪火山岩的年代学和地球化学新资料。所研究的岩石包括玄武质安山岩和安山岩，其中一块安山岩的锆石U-Pb年龄为332±4 Ma。这反映了火山岩的喷发与寄主早石炭世雅曼苏组的形成几乎同步。这些岩石的成分大多为拉斑玄武岩，少数样品转变为钙碱性。所有样品都显示富LREE和LILE，但亏损HFSE，这是在超俯冲带火山岩中观察到的普遍存在的俯冲特征。岩石中正的εNd(T)(+5.2~+6.7)和低的初始~(87)Sr/~(86)Sr(0.7038~0.7040)表明它们来自亏损的橄榄岩源区。然而，与典型的高镁安山岩相比，这些岩石中的镁含量(和镁#)总体上较低，这表明它们不太可能是由交代的地幔楔状橄榄岩简单熔融形成的。相反，由板岩熔体/流体-橄榄岩相互作用形成的镁铁质-超镁铁质交代岩(如辉石岩)的部分熔融可能解释了这些岩石。所研究的岩石与中国西天山广泛记录的早石炭世火山岩和典型大陆弧(如冈底斯弧)中的火山岩有许多地球化学相似之处。因此，结合北天山蛇绿岩的最新研究结果，我们认为火山岩倾向于支持寒武纪至石炭纪期间北天山大洋继续向南俯冲。
The time duration and evolution of the major North Tianshan Ocean (also the Junggar Ocean) in the Chinese Tianshan orogenic collage remain poorly constrained. Here we report new geochronological and geochemical data of Carboniferous volcanic rocks documented from the North Tianshan accretionary complex in the Gangou area (Xinjiang, NW China). The studied rocks include basaltic andesites and andesites, with one andesite yielding a zircon U–Pb age of 332 ± 4 Ma. This reflects nearly synchronous eruption of the volcanics with the formation of host Early Carboniferous Yamansu Formation. These rocks are mostly tholeiitic in compositions, with a few samples transitioning to calcalkaline. All the samples show enriched LREE and LILE but depleted HFSE, a ubiquitous subduction signature observed in supra‐subduction zone volcanics. The positive εNd(t) (+5.2 to +6.7) and low initial 87Sr/86Sr (0.7038 to 0.7040) in these rocks suggest derivation from a MORB‐depleted peridotite source. However, the overall low MgO contents (and Mg#) in these rocks compared with typical high‐Mg andesites indicate that they are less likely formed via simple melting of metasomatized mantle wedge peridotites. Instead, partial melting of mafic‐ultramafic metasomes (e.g., pyroxenites) formed by slab melts/fluids‐peridotite interactions may explain these rocks. The studied rocks share many geochemical similarities with those Early Carboniferous volcanic rocks widely documented from the Chinese Western Tianshan and those volcanics from typical continental arcs (e.g., the Gangdese arc). Therefore, we suggest the volcanic rocks, combining with recent results from the North Tianshan ophiolites, tend to support a continued southward subduction of the major North Tianshan Ocean from the Cambrian to the Carboniferous periods.