The evolution of the Central Yangtze Block during early Neoarchean time: Evidence from geochronology and geochemistry

Although some Archean zircons (xenocrysts) have been reported in various places in the South China, Archean outcrops within the Yangtze Block have been spotted only in the Kongling terrain. Recently, the findings of a Neoarchean K-granite pluton emplaced within the Yangpo Group have opened a new window to study the formation and evolution of the Yangtze Block. We conducted a combined study of zircon U-Pb and whole-rock major and trace elements for seven quartz schist and two granite samples from the Yangpo Group in the Huji region, South China. It is another Archean outcrop in the Yangtze Block except for the Kongling terrain. Results of LA-ICP-MS dating for the detrital zircons from the quartz schist of the Yangpo Group indicate two age groups originating from two significant magmatic events at ∼2.8 Ga and ∼3.05 Ga, as well as the ∼2.9 Ga and ∼3.2-3.3 Ga magmatism identified in a previous study. The quartz schists of the Yangpo Group show high SiO2 content and K2O/Na2O ratios, and low TiO2, Fe2O3, and MgO contents. Two granitic samples from the Yaozishan pluton display relatively high SiO2 and high alkali contents, low MgO, TiO2, and P2O5 content, and high A/KNC ratios (1.21-1.31), exhibiting an aluminous feature. At the same time, the two samples show positive anomalies of HFSE (Zr, Hf, Y) and LILE (Rb, U, Th, La), and high Rb/Sr and 104 × Ga/Al ratios. These geochemical observations are typical of A-type granites. The LA-ICP-MS dating results of the Jinshan and Yaozishan Granitic plutons further confirm the within-plate rifting and A-type granite emplacing event at ∼2.65 Ga. Compared with the Jinshan A-type granite, the Yaozishan A-type granite has a distinct feature of abnormally high 104 Ga/Al ratios (6.19-6.23) and lower zirconium saturation temperature (772-778 °C), which indicates that the Yaozishan A-type granite emplacing at ∼2673 Ma be associated with an early episode of the plate rifting event in the middle Neoarchean. These results suggest that the Yangpo Group be developed from ∼2.8 Ga to ∼2.7 Ga, as filling sequence of a passive continental margin or intracratonic basin. Thus, our study reveals for the first time that the preliminary cratonization of the Central Yangtze Block ended ∼2.8 Ga followed by the filling of the craton basin, plate collision at ∼2.7 Ga, and post-collision rifting at ∼2.65 Ga.