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    Geochronological and geochemical features of the Cathaysia block (South China) New evidence for the Neoproterozoic breakup of Rodinia [查看] Liang-ShuShu、MichelFaure、Jin-HaiYu、Bor-MingJahn
    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. 1.9–1.8 Ga, and the tectonic–magmatic event dated at 0.45–0.40 Ga 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 εHf(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 860–800 Ma, whereas the older stage represents an eruption of volcanic arc rocks at about 970 Ma. 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.
    SHRIMP U–Pb Zircon geochronology of the Altai No. 3 Pegmatite, NW China, and its implications for the origin and tectonic setting of the pegmatite [查看] TaoWang、YingTong、Bor-mingJahn、Tian-renZou、Yan-binWang、Da-weiHong、Bao-fuHan
    The “Altai No. 3 granitic pegmatite” in NW China is the largest pegmatite in the world that has been found in an orogenic belt; it forms one of the largest muscovite deposits in Asia and is most famous for its concentric-ring structure. Although it has been mined for more than 70 years, the age of formation and mineralization of the pegmatite has not been well determined, despite numerous previous geochronological studies. Here we present new results of SHRIMP U–Pb zircon dating. Three weighted mean 206Pb/238U ages of 220±9 Ma, 198±7 Ma and 213±6 Ma were obtained for pegmatite zones 1, 5, and 7, respectively. The analyses were made on clear,recrystallized domains and overgrowth rims of large, opaque high-U zircon grains. We interpret the age of 220±9 Ma for zone 1 to approximately represent the initial time of formation for No. 3 pegmatite. An additional date of 213±2 Ma was determined on zircon from a small subordinate vein formed in the late-stage phase of the No. 3 pegmatite. This age set the late age of emplacement of the pegmatite. The time span of 213 to 198 Ma may reflect late stage hydrothermal alteration. In addition, a weighted mean 206Pb/238U age of 277±6 Ma was obtained from small, clear, transparent low-U zircon grains which are more probably xenocrysts. Our new age data indicate that the pegmatite has no genetic relationship with the wall granitic rocks (409 Ma), but is likely related to the early Mesozoic (220 Ma) granitic magmatism that was just recognized. The No. 3 pegmatite was formed in an early Mesozoic anorogenic extensional regime. A stable tectonic setting was undoubtedly required for the formation of the large pegmatite.
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