地质科技通报 (Jan 2021)
Geochemistry and tectonic implication of the sedimentary rocks in Lengjiaxi Group in northeastern Hunan
Abstract
Lengjiaxi Group is the earliest material record of Jiangnan orogenic belt in Hunan.There exist debates about the sedimentary tectonic setting of Lengjiaxi Group and the related location of Qinzhou-Hangzhou juncture.Based on field investigation, the authors collected fresh samples from the sandstones in lower-middle Lengjiaxi Group from Jinjing, northeastern Hunan, analysed their major and trace element compositions, studied the geochemistry of the sandstones in groups by age, and then probed into the basin type and tectonic framework.The sandstones on the whole are characterized by considerable variations of petrochemical components, lower SiO2 contents, high Al2O3 contents, high Al2O3/SiO2 and K2O/Na2O ratios.The REE distribution patterns are similar to typical post-Archean shales and the upper continental crust, with LREE enrichment, flat HREE, and significant negative Eu-anomalies.Characteristics of major elements suggest that the sedimentary environment was back-arc basin, and that the Yijiaqiao Formation and Panjiachong Formation in early stage possess higher maturity with sediment source from stable Yangtze Block to the north, while the Leishenmiao Formation and Huanghudong Formation in middle stage possess lower maturity with more sediment source from active continental arc to the south.Trace element-diagrams for discrimination of structural environment of all the formations show informations of continental arc, which actually reflects back-arc basin setting according to trace element inheritance from source rocks.Relative numeric size of trace elements parameters suggested that the sedimentary environment in the early stage was active continental margin, and in middle stage was continental arc, which is accordant with the information of major elements.According to the geochemistry evidences, the authors put forward the tectonic framework and evolution in Lengjiaxi Period as follows: in the early stage, upwelling of back-arc asthenosphere mantle caused lithospheric to extend for northwest-ward high-angle subduction of Huanan ocean block, and broad back-arc basin was thus formed, when Jinjing area was located in northern basin and there occurred deposits with source mainly from Yangtze Block to the north.In the middle stage, Huanan ocean block subducted with low-angle and pushed the continental arc northwest-ward, which caused the back-arc basin to contract, when for tectonic migration, there occurred deposits in Jinjing area with source mainly from continental arc to the south.Based on the studies of geochemistry of Lengjiaxi Group in this paper and regional geological features, it is considered that the continental arc to the south of back-arc basin was along An'ren-Shuangpai.
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