Geochemistry of Nanhuan-Cambrian Sandstones in Southeastern Hunan, and Its Constraints on Neoproterozoic-Early Paleozoic Tectonic Setting of South China

Southeastern Hunan lies in the transition area of the Yangtze and Cathaysia block. Thick flysch formation formed during the Nanhuan—Cambrian in the area. The sandstones of the sediments on the whole are characterized by intermediate SiO2 contents, high K2O/Na2O and Al2O3/ (Na2O+CaO) ratios, clearly high Fe2O3*+MgO and CaO contents. The REE distribution patterns are uniform and similar to typical post-Archean shales and the upper continental crust, with LREE enrichment, falt HREE, and significant negative Eu-anomalies. Characteristics of major and trace elements show information of several different tectonic settings of passive continental margin, active continental margin and continental arc. According to the corroded protoliths, efflorescent conditions and carry-subside processes for sediment, the rocks formed in continental arc and active continental margin should possess geochemical characteristics different observably from those in passive continental margin, while the rocks formed in passive continental margin can embrace some geochemical information of continental arc and active continental margin. Therefore the Nanhuan—Cambrian sandstones from southeastern Hunan were formed in a tectonic setting of passive continental margin. The idea coincide with the objective fact that Paleo-Mesoproterozoic rocks formed in continental arc and active continental margin were major protoliths of the Nanhuan—Cambrian sandstones, and that the violently down-faulted sedimentary basin took on some activity. Some evidences such as at least 40～60 Ma absence of sediment between the Banxi Group and Lengjiaxi Group, Neoproterozoic magmatism related with collisional orogeny in Jiangnan orogens, the absence of island-arc volcanic material in the Nanhuan—Early Paleozoic sediments and the continuous Mesoproterozoic (even earlier) crystalline basement in southeastern Hunan indicate that the Neoproterozoic—Early Paleozoic sedimentary basin between the Yangtze and Cathaysia block was not an oceanic but intracontinental rift basin. The idea about the tectonic setting of the Neoproterozoic—Early Paleozoic sedimentary basin between the Yangtze and Cathaysia block based on the geochemistry of the sandstones from southeast Hunan correspond with that based on regional materials.