矿床地球化学国家重点实验室知识库

The Neoproterozoic mafic rocks of the Jiangnan Orogen in South China provide a critical geological record for unraveling regional tectonic history and testing different tectonic models. However, the geochronology, geochemistry and petrogenesis of >835 Ma mafic igneous rocks in the Central and Southwestern Jiangnan Orogen have not been well reported so far, in contrast to the extensive study of such rocks in the Eastern Jiangnan Orogen. An integrated study of geochronology and geochemistry has been carried out for the basaltic rocks from NE Hunan and NW Jiangxi Provinces of the Central Jiangnan Orogen. SHRIMP, SIMS and LA-ICP-MS zircon U–Pb dating of three mafic samples yielded zircon U–Pb ages of 860 ± 20 Ma, 838 ± 12 Ma and 847 ± 18 Ma, respectively. This provides evidence for the development of early Neoproterozoic (∼850 Ma) mafic lava along the Central Jiangnan Orogen. SiO₂ ranges from 49.43 wt% to 52.07 wt% and MgO from 7.36 wt% to 9.04 wt% with mg-number of 68–57. Their chondrite-normalized REE patterns exhibit a left-sloping pattern with more depleted LREEs relative to HREEs. On the N-MORB-normalized multi-element patterns, they are characterized by enriched highly incompatible elements, depleted moderately incompatible elements relative to those in N-MORB, insignificant Nb–Ta depletion and remarkably positive Sr and weakly negative Ti anomalies, resembling those of modern Eastern Lau back-arc basin basalts. In addition, these rocks have positive ɛ_Nd(t) values of 1.30–9.36 with initial ⁸⁷Sr/⁸⁶Sr ratios in the range of 0.70796–0.71126. The geochemical characteristics above indicate an origination of the MORB-like source modified by the flux of the slab-derived fluid and input of the recycled sediment-derived melt. This finding, together with other geological observations, suggests that the mafic rocks in the Central Jiangnan Orogen might be formed in a back-arc basin in the eastern SCB. In conjunction with other data, it is inferred that the closure of the back-arc basin along the Central Jiangnan Orogen is contemporaneous with that in the Fuchuan area.