| 扬子陆块北缘西大别地区新元古界定远组双峰式火山岩地球化学特征、成因及其地质意义 |
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| 引用本文： | 朱江,邱啸飞,周豹,张海军,吴越,邓新.扬子陆块北缘西大别地区新元古界定远组双峰式火山岩地球化学特征、成因及其地质意义[J].地球科学,2021,46(4):1311-1327. |
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| 作者姓名： | 朱江 邱啸飞 周豹 张海军 吴越 邓新 |
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| 作者单位： | 1.长江大学资源与环境学院, 湖北武汉 430100 |
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| 基金项目： | 油气资源与勘探技术教育部重点实验室基金;中国地质调查局项目;湖北省自然科学基金 |
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| 摘 要： | 西大别地区定远组双峰式火山建造主要由流纹岩、流纹质凝灰岩和玄武岩组成，受区域变质作用影响较小，保留了良好的新元古代火山岩构造，成为理解扬子地块北缘大别山地区前寒武纪地质演化的极佳研究对象.为深化理解其成因和动力学背景，对该火山岩系开展了详细的岩相学、岩石地球化学和Sr-Nd-Hf同位素研究.流纹岩具较高的SiO2（70.06%~75.46%）、总碱（7.13%~7.89%）和Al2O3（12.96%~14.84%）含量，属于过铝质（A/CNK=1.05~1.27）、钙碱性系列，具S型花岗岩的亲缘性.岩石富集轻稀土（（La/Yb）N=5.30~19.81），负Eu异常不明显.流纹岩锶同位素初始比值ISr为0.703 5~0.707 7，钕同位素εNd（t）值为-10.6~-6.5，两阶段Nd模式年龄TDM2=1.69~2.00 Ga.其锆石εHf（t）值介于-19.2~-7.2，两阶段Hf模式年龄TDM2=1.77~2.59 Ga.玄武岩发育典型的气孔和杏仁构造，SiO2含量为48.26%~51.71%，Mg#值为0.32~0.59，轻稀土元素弱富集（（La/Yb）N=3.21~10.23）.锶同位素初始比值ISr为0.706 1~0.708 1，钕同位素εNd（t）值为-6.6~+3.4，单阶段Nd模式年龄TDM=1.44~2.02 Ga.岩石地球化学和Sr-Nd-Hf同位素特征显示，定远组流纹岩起源扬子古老陆壳物质的部分熔融；玄武岩原始岩浆可能起源于富集地幔物质的部分熔融.该玄武质岩浆演化过程以分离结晶为主，可能存在少量陆壳物质混染.在陆缘拉张背景下岩石圈地幔部分熔融产生热的基性母岩浆，其底侵或上侵使得地壳岩石部分熔融，产生酸性岩浆，基性岩浆和酸性岩浆交替喷出形成了定远组双峰式火山岩系.扬子地块北缘大别地区新元古代中期（740 Ma左右）岩石圈地幔表现为富集特征，并处于岩石圈持续伸展的动力学背景.
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| 关 键 词： | 火山岩 地球化学 岩石成因 Sr-Nd-Hf同位素 岩石圈地幔 大别山 |
| 收稿时间： | 2020-05-26 |
Neoproterozoic Bimodal Volcanic Rocks from Dingyuan Formation in Western Dabie Area,Northern Margin of Yangtze Block,China:Geochemistry,Petrogenesis and Geological Implications |
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| Zhu Jiang,Qiu Xiaofei,Zhou Bao,Zhang Haijun,Wu Yue,Deng Xin.Neoproterozoic Bimodal Volcanic Rocks from Dingyuan Formation in Western Dabie Area,Northern Margin of Yangtze Block,China:Geochemistry,Petrogenesis and Geological Implications[J].Earth Science-Journal of China University of Geosciences,2021,46(4):1311-1327. |
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| Authors: | Zhu Jiang Qiu Xiaofei Zhou Bao Zhang Haijun Wu Yue Deng Xin |
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| Abstract: | Neoproterozoic bimodal volcanic rocks from the Dingyuan Formation within the western Dabie terrain consist of rhyolite, rhyolitic tuff and basalt. Elemental, whole-rock Sr-Nd and zircon Hf isotopic analyses were carried out in this study for these volcanic rocks, in order to better understand their petrogenesis and geodynamic processes along the northern margin of Yangtze block. The rhyolite is characterized by high concentrations of SiO2 and Al2O3, belongs to the calc-alkaline series, and is peraluminous. It is enriched in light rare earth elements (LREEs) and shows unapparent Eu anomaly. The basalt has SiO2 of 48.26% to 51.71%, Mg# values of 0.32 to 0.59, and is weakly enriched in light rare earth elements. The rhyolite has initial 87Sr/86Sr ratios ranging from 0.703 5 to 0.707 7, and negative εNd(t) values of -10.6 to -6.5, with calculated two-stage Nd model ages of 1.69 to 2.00 Ga, while the basalt has (87Sr/86Sr)i of 0.706 1 to 0.708 1, εNd(t) of -6.6 to +3.4, and the Nd model ages (TDM) values of 1.44 to 2.02 Ga. In situ zircon Hf isotopic analyses show that the εHf(t) values of zircons from the rhyolite vary from -19.2 to -7.2, with calculated two-stage Hf model ages of 1.77 to 2.59 Ga. The geochemical and Sr-Nd-Hf isotopic signatures suggest that the rhyolite has a close affinity to S-type granite, and is derived from partial melting of the ancient crust of the Yangtze block. The basalt is most likely derived from an enriched lithospheric mantle, with fractional crystallization and assimilation of crustal materials during the evolution of the magma. It is proposed that the Neoproterozoic bimodal volcanic suites from the Dingyuan and the Qijiaoshan formations are erupted coevally, but have different sources and magmatic processes. The bimodal volcanic rocks in this study provide evidences for an enriched lithospheric mantle beneath the Dabie terrain at ca.740 Ma, and a rifting extensional setting for the north margin of the Yangtze block. |
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