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Amalgamation between the Yangtze and Cathaysia Blocks in South ChinaConstraints from SHRIMP U-Pb zircon ages, geochemistry and Nd-Hf isotopes of the Shuangxiwu volcanic rocks [查看] Xian-HuaLi、Wu-XianLi、Zheng-XiangLi、Ching-HuaLo、JianWange、Mei-FangYe、Yue-HengYang

South China was formed through the amalgamation of the Yangtze Block with the Cathaysia Block, but the timing of this amalgamation is controversial, ranging from Mesoproterozoic to Mesozoic. We report here SHRIMP U–Pb zircon ages, geochemistry and Nd–Hf isotopes of the Shuangxiwu Group volcanic rocks from the southeastern Yangtze Block. These rocks were strongly deformed, metamorphosed to greenschist-facies, intruded by 849±7Ma dolerites, and unconformably overlain by Neoproterozoic rift successions of no older than ca. 820 Ma. The Beiwu and Zhangcun volcanic rocks from the middle and uppermost Shuangxiwu Group were dated at 926±15Ma and 891±12 Ma, respectively. All the studied rocks are characterized by highly positive Nd(T) (5.4–8.7) and Hf(T) (11.0–15.3) values. The Pingshui basaltic and andesitic rocks from the lower Shuangxiwu Group, whichwere previously dated at ca. 970 Ma, are high in Al2O3 (15–20%) but low in MgO (<8%), and are characterized by enrichments in Th and LREE but depletions in Nb, Ta, Zr, Hf and Ti, broadly similar to high-Al basaltic rocks in many volcanic arcs. The Beiwu andesitic to rhyolitic rocks have higherMgOthan the experimental melts of basaltic rocks, and their Al2O3 content decreases with increasing SiO2, similar to the regional coeval tonalites and granodiorites,suggesting their formation by crystal fractionation of basaltic parent magma. The Zhangcun volcanic rocks are high in SiO2 (mostly >69%), low in MgO (0.35–1.2%), and have nearly constant Al2O3 contents of 14–15%and relatively uniform trace element concentrations. Theywere generatedby remelting of juvenile mafic to intermediate arc rocks. Overall, the Shuangxiwu Group volcanic rocks and associated intrusive tonalites and granodiorites constitute a typical calc-alkaline magmatic assemblage of a 970–890Maactive continental margin. These results and the 849±7Ma zircon U–Pb age for the undeformed doleritic dikes intruding the Shuangxiwu Group suggest that the tectonic regime of the study region transformed from plate convergence to intracontinental rifting in the time period between ca. 890Ma and ca. 850 Ma.Previously reported 1.04–0.94Ga metamorphic and deformation ages from the nearby Tianli Schists and evidence for the final closure of the back-arc basin at ca. 880Ma (ophilitic obduction at Xiwan), further suggest that the amalgamation between the Yangtze and Cathaysia Blocks, likely through “soft docking”at the eastern segment of the Sibao orogen, was completed at ca. 880Ma or soon after.

SHRIMP zircon U-Pb geochronological and whole-rock geochemical evidence for an early Neoproterozoic Sibaoan magmatic arc along the southeastern margin of the Yangtze Block [查看] Mei-FangYe、Xian-HuaLi、Wu-XianLi、YingLiu、Zheng-XiangLi

It has been generally accepted that the South China Block was formed through amalgamation of the Yangtze and Cathaysia Blocks during the Proterozoic Sibaoan orogenesis, but the timing and kinematics of the Sibao orogeny are still not well constrained. We report here SHRIMP U–Pb zircon geochronological and geochemical data for the Taohong and Xiqiu tonalite–granodiorite stocks from northeastern Zhejiang, southeastern margin of the Yangtze Block. Our data demonstrate that these rocks, dated at 913±15 Ma and 905±14 Ma, are typical amphibole-rich calcalkaline granitoids formed in an active continental margin. Combined with previously reported isotopic dates for the ∼1.0 Ga ophiolites and ∼0.97 Ga adakitic rocks from northeastern Jiangxi, the timing of the Sibao orogenesis is thus believed to be between ∼1.0 and ∼0.9 Ga in its eastern segment. It is noted that the Sibao orogeny in South China is in general contemporaneous with some other early Neoproterozoic (1.0–0.9 Ga) orogenic belts such as the Eastern Ghats Belt of India and the Rayner Province in East Antarctica, indicating that the assembly of Rodinia was not finally completed until ∼0.9 Ga.

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