Asynchronizing paleo-Pacific slab rollback beneath SE China: Insights from the episodic Late Mesozoic volcanism
Asynchronizing paleo-Pacific slab rollback beneath SE China: Insights from the episodic Late Mesozoic volcanism
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中国东南部下方古太平洋板片的异步回滚：晚中生代幕式火山活动的见解
DOI：
10.1016/j.gr.2015.09.009
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发表时间：
2016-09
影响因子：
6.1
通讯作者：
Xia Yan
中科院分区：
文献类型：
--
作者：
Liu Lei;Xu Xisheng;Xia Yan
中国东南地区广泛分布的晚中生代火成岩被普遍认为是在古太平洋构造体制下形成的。以往的研究表明，火成岩的基本趋势是向洋俯冲，进一步暗示了一个北西向的古太平洋俯冲模式，板片逐渐回滚。新的LA-ICP-MS锆石U-Pb定年结果表明，福建晚中生代火山岩系（又称下火山岩系）早期形成于3幕：长林组160-148 Ma，南苑组145-130 Ma，小溪组130-127 Ma，其中第2幕为火山活动的高潮。因此，福建下部火山岩系的形成早于浙江下部火山岩系（140-118 Ma），呈与潜在俯冲带平行的NE向展布趋势。此外，对福建下火山岩系锆石的Hf同位素分析表明，其εHf（t）值介于− 19.2 ~+ 1.7之间。大多数研究的火山岩有更多的放射性Hf比变质基底，需要在其起源中的少年成分的参与。除长林组的锆石εHf（t）值随时间变化趋势相反外，其它地区的锆石εHf（t）值随时间的变化趋势与中国东南部其他地区的火成岩一致。锆石εHf（t）值的增加意味着同期底侵幔源岩浆的物质贡献增加，这可能是地壳逐渐减压的结果。与下部火山岩系向北东方向的演化趋势一样，岩浆成因中的年轻物质贡献也是沿沿着方向逐渐向同一方向演化的。因此，我们提出了一个150-120 Ma的古太平洋板块回滚模型，以解释中国东南部的幕式岩浆活动和地壳伸展。而长林组火山岩锆石εHf（t）值的降低则表明它们是在板块推进引起的地壳挤压作用下形成的。福建上部火山岩系形成于110-88 Ma，与浙江火山岩系形成时间相同，其锆石Hf同位素组成也比下部火山岩系亏损，表明当时整个东南沿海处于弧后构造环境。
Extensive Late Mesozoic igneous rocks in SE China have been widely considered to be generated under the paleo-Pacific tectonic regime. Previous studies suggested a fundamental oceanward younging trend for the igneous rocks, further implying a NW-ward paleo-Pacific subduction model with gradual slab rollback. New LA–ICP–MS zircon U–Pb dating results show that early stage of the Late Mesozoic volcanic sequences (also known as the lower volcanic series) in Fujian Province was formed in three episodes: 160–148 Ma for the Changlin Formation, 145–130 Ma for the Nanyuan Formation, and 130–127 Ma for the Xiaoxi Formation, among which the second episode made the volcanism climax. Thus the entire lower volcanic series in Fujian were formed earlier than those in Zhejiang (140–118 Ma), displaying a NE-ward younging trend parallel to the potential subduction belt. Besides, in situ Hf isotope analyses on dated zircons yield anεHf(t) range of − 19.2 to + 1.7 for the lower volcanic series in Fujian. The majority of the studied volcanic rocks have more radiogenic Hf than that of the metamorphic basement, requiring the involvement of juvenile components in their origin. Moreover, the zirconεHf(t) value increases with time in each single area, as well as the igneous rocks elsewhere in SE China, except for the Changlin Formation which shows very opposite isotopic varying trend. Increasing zirconεHf(t) values imply an increasing material contribution of contemporaneous underplated mantle-derived magmas, which was plausibly induced by gradual crustal decompression. As well as the NE-ward younging trend of the lower volcanic series, it is also identified that the juvenile material contribution in the igneous petrogenesis gradually took place in the same direction along the coastal area. Thus here we propose an asynchronizing paleo-Pacific slab rollback model during 150–120 Ma to account for the episodic magmatism and crustal extension in SE China. On the contrary, decreasing zirconεHf(t) values of the Changlin Formation volcanics indicate that they were formed under enhanced crustal compression probably induced by slab advance. On the other hand, the upper volcanic series in Fujian were formed during 110–88 Ma coevally to those in Zhejiang, with more depleted zircon Hf isotopic compositions than the lower volcanic series as well, indicating that the entire coastal SE China was under the back-arc tectonic setting during that time.
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3.9
作者：
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通讯作者：
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2.9
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