新疆伊宁地块晚泥盆世火山岩的确认及其地质意义

最新发现的泥盆纪火山岩位于西天山伊犁地块北缘,主要岩石组成为流纹岩,英安岩夹少量安山岩。锆石LA-ICP-MS U-Pb定年结果显示,尼勒克北流纹岩类的形成时代一致,分别为369.7±4.9Ma,363.4±2.5Ma,均为晚泥盆世。岩石SiO₂含量较高且变化范围较宽,Al₂O₃含量较高,富碱,贫镁和钙,高FeO^T/MgO+FeO^T比值,低TiO₂,里特曼指数δ较小(1.09~2.88),A/CNK值变化较大(0.8~1.46),属于钙碱性过铝-准铝质低钛流纹岩类。稀土元素总量(∑REE)变化较大(80.66×10^-6~241.8×10^-6),稀土配分模式为轻稀土富集右倾((La/Yb)_N=5.96~7.82),铕负异常明显(Eu/Eu^*=0.47~0.86),相对富集Rb、Th、K等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,较高的Rb/Sr、Nb/Ta和Th/Ta比值(分别为0.18~5.15,11.18~13.96,3.54~9.5),较低的Ti/Zr、Ti/Y及Ta/Yb比值(分别为4.95~25.79,43.7~295.5,0.45~1.29),这些地球化学特征表明其可能为上地壳部分熔融的产物,源区残留物组成可能为斜长石和角闪石,同时显示其具有大陆边缘弧亲缘性,这与西天山在早石炭世处于岛弧环境相匹配,即在晚泥盆世-早石炭世期间,准噶尔洋向伊犁-中天山地块之下发生俯冲,形成了伊犁地块北缘的大陆边缘弧岩浆带。尼勒克北晚泥盆世流纹岩类成岩时代的确定及构造环境分析为进一步认识伊犁地块北缘地区的构造演化提供了新的证据。

Confirmation of Devonian volcanic rocks from Yining block, Xinjiang and its geological significations

Newly discovered Devonian volcanic rocks occur on north margin of the Yili plate,western Tianshan,including rhyolite,dacite and a small amount of andesite,High-precision zircon LA-ICP-MS U-Pb dating results show that weighted mean 206Pb/238U ages of 369.7±4.9Ma and 363.4±2.5Ma for the rhyolite,corresponding to the Late Devonian,Petrogeochemistry analyses suggest the north Nileke rhyolite belong to calc-alkaline peraluminous-metaluminous rhyolitic rocks with low Ti,which is characterized by high and wide silica,high aluminum and alkaline,low magnesium and calcium,high FeOT/MgO+FeOT ratio,low Rittman index(δ=1.09～2.88) and wide A/CNK(0.8～1.46),The rocks also have relatively high total Rare Earth Element(REE) content(∑REE=80.66×10-6～241.8×10-6) with LREE-rich patterns((La/Yb)N=5.96～7.82) of chondrite-normalized and significant negative Eu anomalies(Eu/Eu*=0.47～0.86),The rocks enriched in large ion lithophile elements such as Rb,Th and K,strongly depleted high strength field elements such as Nb,Ta and Ti,with relatively high Rb/Sr,Nb/Ta and Th/Ta ratios(0.18～5.15,11.18～13.96,3.54～9.5,respectively) and low Ti/Zr,Ti/Y and Ta/Yb ratios(4.95～25.79,43.7～295.5,0.45～1.29,respectively),The geochemical characteristics indicate that the rocks may be partial melting of the upper crust,with the source composed of plagioclase and hornblende,also show their affinity with the continental margin arc,this is consistent with the Early Carboniferous island arc setting in the western Tianshan,That is to say,the Junggar Ocean southward subducted to the Yili-Central Tianshan plate in the Late Devonian-Early Carboniferous Period,forming island arc magmatic belt in northern margin of the Yili plate,The age and tectonic setting of the north Nileke Late Devonian rhyolite provided new evidence for further understanding for tectonic evolution of northern margin in the Yili plate.