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Geochronological and geochemical features of the Cathaysia block (South China): New evidence for the Neoproterozoic breakup of Rodinia

机译:华夏地块(中国南方)的地质年代和地球化学特征:罗迪尼亚新元古代分裂的新证据

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The Cathaysia block is an important element for the reconstruction of the Proterozoic tectonic evolution of South China within the Rodinia supercontinent. The Pre-Devonian Cathaysia comprises two litho-tectonic units: a low-grade metamorphic unit and a basement unit; the former was a late Neoproterozoic-Ordovician sandy and muddy sedimentary sequence, the latter consists essentially of metamorphosed Neoproterozoic marine facies sedimentary and basaltic rocks, and a subordinate amount of Paleoproterozoic granites and amphibolites. This block has undergone several tectono-magmatic events. The first event occurred in the late Paleoproterozoic, at ca. 1.9-1.8Ga, and the tectonic-magmatic event dated at 0.45-0.40Ga was resulted from the early Paleozoic orogeny that made the Pre-Devonian rocks to undergo a regional lower greenschist to amphibolite facies metamorphism. The Neoproterozoic geodynamic event is poorly understood. In this paper, new U-Pb zircon age, whole-rock chemical and zircon Hf isotopic data for mafic and felsic igneous rocks are used to constrain the tectonic evolution of Cathaysia. Zircon SHRIMP U-Pb analyses on four mafic samples yielded rather similar Neoprotorozoic ages of 836±7Ma (gabbro), 841±12Ma (gabbro), 847±8Ma (gabbro) and 857±7Ma (basalt). Combined with the published isotopic age data, most of the mafic samples dated at 800-860Ma show geochemical characteristics of continental rift basalt. By contrast, rhyolitic samples with an age of 970Ma have a volcanic arc affinity. All mafic samples have LREE-enriched REE patterns, and non-ophiolitic trace element characteristics. However, the zircon Hf isotopic data of mafic samples show positive epsilon e{open}_(Hf)(t) values (+4.1 to +10.5), suggesting that they were originated from a long-term depleted mantle source. All the available ages indicate that the Cathaysia block has registered two stages of Neoproterozoic magmatism. The younger stage corresponds to a continental rifting phase with emplacement of mafic rocks during the period of 860-800Ma, whereas the older stage represents an eruption of volcanic arc rocks at about 970Ma. These two magmatic stages correspond to two distinct tectonic settings within the framework of the geodynamic evolution of Cathaysia. Such a similar Neoproterozoic stratigraphy and magmatism between the Cathaysia, Yangtze and Australian blocks provide a significant line of evidence for placing the Cathaysia block within the Rodinia supercontinent.
机译:华夏地块是在Rodinia超大陆内重建华南元古代构造演化的重要元素。前泥盆纪华夏由两个岩石构造单元组成:低品位变质单元和基底单元;前者是新元古代-奥陶纪晚期的泥泞泥质沉积序列,后者主要由变质的新元古代海相沉积和玄武质岩石以及少量的古元古代花岗岩和两性辉石组成。该地块经历了几次构造-岩浆事件。第一次事件发生在古元古代晚期,大约在1.9-1.8Ga,构造-岩浆事件的发生时间为0.45-0.40Ga,是由于早古生​​代造山运动使前德文统岩石经历了区域性的低绿片岩到角闪岩相的变质作用。对新元古代的地球动力学事件了解甚少。本文利用镁铁质和长英质火成岩的新的U-Pb锆石年龄,全岩化学和锆石Hf同位素数据来约束华夏的构造演化。 Zircon SHRIMP U-Pb分析在四个铁镁质样品上产生的新元古代年龄分别为836±7Ma(gabbro),841±12Ma(gabbro),847±8Ma(gabbro)和857±7Ma(玄武岩)。结合已发布的同位素年龄数据,日期为800-860Ma的大多数镁铁质样品均显示了陆相裂谷玄武岩的地球化学特征。相比之下,年龄为970Ma的流纹岩样品具有火山弧亲和力。所有镁铁质样品均具有富含LREE的REE模式,并且具有非胶质微量元素特征。但是,铁镁质样品的锆石Hf同位素数据显示出正的eε_open(_Hf)(t)值(+4.1至+10.5),表明它们源自长期耗尽的地幔源。所有可利用的年龄都表明,华夏地块已经记录了新元古代岩浆作用的两个阶段。较年轻的阶段对应于陆相裂谷阶段,在860-800Ma时期内含镁铁质岩石,而较早的阶段则代表了约970Ma的火山弧岩喷发。这两个岩浆阶段对应于华夏地球动力学演化框架内的两个不同的构造环境。华夏,扬子和澳大利亚地块之间的这种类似新元古代地层和岩浆作用为将华夏地块置于罗迪尼亚超大陆内提供了重要的证据。

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