扬子陆块西南缘河口群沉积时限和构造属性的再厘定:兼论中元古代洋板块地层

Re-definition of depositional age and tectonic attribute for the Hekou Group from southwestern Yangtze block:A discussion on the Mesoproterozoic ocean plate stratigraphy

河口群、东川群和原通安组是扬子陆块中元古代早中期重要的地层单元,但其沉积时限和构造属性长期存在争论。本文在区域地质调查和同位素年代学研究的基础上,从“洋板块地质”的视角重新厘定了河口群、东川群和原通安组的沉积时限和构造属性。原通安组是由不同规模的岩块和基质共同构成的一套蛇绿混杂岩;东川群则较为完整地记录了从陆内裂谷到被动大陆边缘的演化过程,代表了盆地边缘的地层序列,东川群与原通安组(现定义为菜子园-通安蛇绿混杂岩)虽位于不同的断块,但应是同一盆地发展的产物;河口群与东川群、原通安组序列明显不同,但总体上也是粒度向上变细的海侵序列,并发育多套火山岩,在Columbia超大陆裂解的大背景下,河口群最有可能形成于夭折裂谷。对采自河口群长冲组的石榴子石云母片岩进行了LA-ICP-MS锆石U-Pb定年研究,绝大部分锆石具有核-边结构,核部最年轻一组207Pb/206Pb年龄的加权平均值为1468±28 Ma(MSWD=0.40,n=16),解释为原岩的最大沉积年龄;结合前人研究成果,将河口群和东川群的沉积时代分别限定在1.75~1.40 Ga和1.75~1.35 Ga。锆石边部的变质年龄集中在~830 Ma,结合海绵状结构特征,变质边最有可能是热液蚀变所致。锆石Hf同位素研究显示,扬子陆块西南缘在2.00~1.80 Ga期间,锆石εHf(t)值以负值为主,表明岩浆主要源于古老地壳的再造;而1.58~1.39 Ga的锆石εHf(t)多为正值,显示有大量地幔物质的加入。通过本次综合研究,扬子陆块西南缘古元古代晚期—中元古代早中期的演化经历了2.00~1.80 Ga碰撞造山、1.77~1.64 Ga陆内裂解、1.58~1.39 Ga洋盆形成和1.38 Ga之后的洋壳俯冲消减4个阶段,是Columbia超大陆聚合与裂解在扬子陆块的响应。

The Tong’an Formation,Hekou and Dongchuan groups,deposited during early to middle Mesoproterozoic,are important stratigraphic units on the Yangtze block.Their depositional age and tectonic attributes,however,remain in dispute.In this study,new regional geological evidences and geochronological data allow us to redefine the depositional age and tectonic attributes of the Tong’an Formation,Hekou and Dongchuan groups using the“Ocean plate stratigraphy”theory.The Tong’an Formation is an ophiolite complex that consists of different scale blocks and strong deformed matrix.Accordingly,the Tong’an Formation has been defined as the Caiziyuan-Tong’an ophiolite mélange.The stratigraphic sequence of the Dongchuan Group records a complete evolution from the intracontinental rift to the passive continental margin,and represents the basin marginal deposits.Although it outcrops on different fault blocks with the Caiziyuan-Tong’an ophiolite mélange,they were deposited in a same basin.The Hekou Group has obviously different stratigraphic sequence from the Dongchuan Group and the Caiziyuan-Tong’an ophiolite mélange,with a transgressive sequence of sediment grain size finer upwards and several volcanic layers.The Hekou Group could have been deposited in a failed rift in the context of breakup of the Columbia supercontinent.The garnet mica schist is from the Changchong Formation of the Hekou Group.Zircon grains from the schist exhibit core-rim structures.Zircon LA-ICP-MS U-Pb dating results from the zircon cores shows that the youngest age cluster yielded a weighted mean207Pb/206Pb age of 1468±28 Ma(MSWD=0.40,n=16),which is interpreted as the maximum depositional age of the garnet mica schist.Combined with existing geochronological data,the depositional age of the Hekou Group and Dongchuan Group were limited to 1.75~1.40 Ga and 1.75~1.35 Ga respectively.Meanwhile,the metamorphic age from zircon rims concentrated in ca.830 Ma.The spongeous structure on zircon CL images reveals that the metamorphic rims is most likely a product of hydrothermal alteration.Zircon grains of 2.0~1.8 Ga mostly give negativeεHf(t)values,indicating that the magmatism is dominated by the recycling of the ancient crust in the southwestern Yangtze Block;while zircon grains of 1.58~1.39 Ga are dominated by positiveεHf(t)values,suggesting that a large amount of mantle material has entered.Our comprehensive study identified four evolution stages of the southwestern Yangtze Block during late Paleoproterozoic and early to middle Mesoproterozoic:collisional orogenesis during 2.00~1.80 Ga,intracontinental rifting during 1.77~1.64 Ga,passive continental margin basin in 1.58~1.39 Ga and subduction and extinction of the oceanic crust since 1.38 Ga.