Tectonic switching during Carboniferous-Early Triassic within the southern Alxa
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摘要:
阿拉善地区位于中亚造山带南缘中段，该地区出露大量古生代的蛇绿混杂岩、增生杂岩和岩浆岩，记录了古亚洲洋闭合过程对大陆地壳的改造。本文系统总结了阿拉善南部地区蛇绿混杂岩及石炭纪—早三叠世岩浆岩的研究进展，收集整理了该地区岩浆岩的年代学、全岩地球化学和同位素数据。结果发现，阿拉善南部地区分布有早二叠世俯冲相关蛇绿混杂岩，中、晚二叠世高镁安山岩和埃达克岩，表明该地区的古大洋在晚二叠世尚未闭合。阿拉善南部地区岩浆岩的时空分布反映出该地区岩浆岩带向北西方向迁移的特征，揭示了阿拉善地块晚古生代向北增生的过程。阿拉善南部地区从早石炭世的前进型俯冲带转换为晚石炭世—二叠纪的后撤型俯冲带。
Abstract:Numerous Paleozoic ophiolitic mélange, accretionary complex and plutons are exposed in the Alxa region, located in the central segment of the Central Asian Orogenic Belt, which may record the reworking process of continental crust during the closure of the Paleo−Asian Ocean. In this study, we collected and compiled new published data of zircon U−Pb age, whole−rock major and trace element and zircon Hf isotope for ophiolitic mélange and plutons in the southern Alxa. Based on these new data, we propose that Permian SSZ−type ophiolitic mélange and Middle−Late Permian high−Mg diorites and adakites support the hypothesis that a Permian oceanic slab subduction in the southern Alxa. The zircon U−Pb ages of Carboniferous−Permian magmatism show a younging trend toward the northwest in the southern Alxa region, indicating a northward accretion of the Alxa Block. Temporal−spatial variations of zircon Hf isotope for plutons suggest tectonic switching from advancing to retreating subduction during Carboniferous–Early Triassic within the southern Alxa.
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Keywords:
- Alxa /
- Central Asian Orogenic Belt /
- ophiolitic mélange /
- retreating subduction
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图 1 中亚造山带构造组成图（据Şengör et al., 1991；Xiao et al., 2015修改）
Figure 1. Tectonic map of the main components of the Central Asian Orogenic Belt
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图 3 恩格尔乌苏蛇绿混杂岩地质图（据Zheng et al., 2014）
Figure 3. The geological map of the Enger Us ophiolitic mélange
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图 4 阿拉善地区蛇绿混杂岩代表性岩石的稀土元素配分图（a）和微量元素蛛网图（b）（稀土元素和微量元素数据据Zheng et al., 2014, 2018）
Figure 4. The rare earth element patterns (a) and trace element spidergram (b) for representative samples of ophiolitic mélanges in the southern Alxa
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图 5 阿拉善南部地区侵入岩SiO2−(Na2O+K2O) (a，底图据Wilson, 1989)和SiO2−K2O图解(b，底图据Peccerillo et al., 1976)（地球化学数据据Zheng et al., 2022）
Figure 5. SiO2 versus (Na2O + K2O) diagram (a) and SiO2 versus K2O discrimination diagram (b) for plutons in the southern Alxa
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图 6 阿拉善南部地区地质图及侵入体年代学数据（年代学数据据Zheng et al., 2019b）
Figure 6. The geological map of southern Alxa, mainly showing zircon U−Pb age data of plutons
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图 7 阿拉善南部晚古生代岩体特征图
a—阿拉善南部晚古生代岩体年代学概率图；b—阿拉善南部晚古生代岩体及碎屑岩锆石U-Pb年龄-εHf(t)值图解；c—阿拉善南部晚古生代中酸性岩体锆石U−Pb年龄-锆石饱和温度图解；d—阿拉善南部晚古生代花岗岩锆石U-Pb年龄-(La/Yb)N值图解（据Zheng et al., 2022修改）
Figure 7. Characteristics diagrams of plutons in the southern Alxa
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图 8 阿拉善南部地区石炭纪—早三叠世构造演化示意图（据Zheng et al., 2022）
SZTZ—宗乃山-沙拉扎山构造带；NLTZ—诺尔公-狼山构造带
Figure 8. Tectonic evolution of the southern Alxa during Early Carboniferous—Early Trisstic
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