SHRIMP U-Pb Zircon Geochronological, Geochemical And Nd-Hf-O Isotopic Evidences For Early Neoproterozoic Sibaoan Magmatic Arc Along The Southeastern Margin Of Yangtze Block

It is generally accepted that the South China was formed through amalgamation of the Yangtze and Cathaysia blocks during the Proterozoic Sibaoan orogenesis, whereas the timing and kinematics of the Sibao orogeny is still not well constrained. We report here SHRIMP U-Pb zircon geochronological, geochemical and Nd-Hf-0 isotopic 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 calc-alkaline granitoids (ACG). These rocks are characterized by high eNd(t) and eHf(t) values and lowδ¹⁸O values, i.e., εNd(t) = 6.9-8.6, εHf(t) = 8.2¹2.8 and δ¹⁸O = 5.7⁶.1‰ forthe Taohong Stock, and εNd(t) = 6.4⁷.8, εHf(t) = 9.5¹1.5 and δ¹⁸O = 5.9⁷.6‰ for the Xiqiu Stock,. Thus, their parental magmas were most likely derived from a depleted mantle source, and the granites were formed through fractional crystallization of basaltic magmas with little, if any, crustal contamination. On the Rb-Hf-Ta and Rb-Y+Nb discrimination diagrams, they plot exclusively into the volcanic arc granitoid (VAG) field, suggesting that they were 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 Sibao orogenesis took place at least during atime interval between -1.0 and -0.9 Ga in its eastern segment. It is noted that the Sibao orogeny in South China is generally contemporaneous with some early Neoproterozoic (-990-900 Ma) 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.