U-Pb zircon geochronology, geochemistry and Nd isotopic study of Neoproterozoic bimodal volcanic rocks in the Kangdian Rift of South China implications for the initial rifting of Rodinia
SHRIMP U–Pb zircon age, geochemical and Nd isotopic data are reported for the Neoproterozoic Suxiong volcanic rocks in the Kangdian Rift, western South China. These volcanic rocks are bimodal, consisting mainly of mildly alkaline basalts and trachydacites to rhyolites. SHRIMP U–Pb zircon age determination indicates that they were erupted at 803±12 Ma. Most basaltic rocks are characterized by high positiveεNd(T) values (+5 to +6),pronounced enrichment in Th, Ta, Nb, LREEs, Sr, P, Zr, Hf, Ti, smooth LREE-enriched patterns and generally‘humped’ trace element spidergrams. They resemble the alkali basalts of the Hawaiian oceanic island basalts (OIB) and the Ethiopian continental flood basalts (CFB). These features suggest that the basaltic rocks were most probably derived from an OIB-like mantle source without appreciable crustal/lithospheric contamination. Differentiated basalt and trachyandesite samples show relatively lowεNd(T) values (+1.7 to +2.4) and Nb-Ta depletion due to contamination by the mafic lithosphere and/or crustal materials. The rhyolite and dacite samples have small positiveεNd(T) values (+1.1 to +2.6), general enrichment in most incompatible trace elements (K, Rb, Th, Zr, Hf and REEs) but significant depletion in Nb, Ta, Sr, P, Eu and Ti. They share geochemical characters of A2-type granites, and are likely generated by shallow (P≤4 kbar) dehydration melting of hornblende-bearing granitoids. Geochemical and Nd isotopic characters and high-volcanicity of the Suxiong bimodal volcanic successions are consistent with their formation in a continental rift environment, such as the Ethiopian rift. The Kangdian Rift is considered as part of a wider continental rift system produced by a starting mantle plume beneath South China during the Neoproterozoic breakup of Rodinia.
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
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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