The late mesozoic granodiorites from the southwest basin in the South China Sea and its tectonic implication

Abstract

The southwest basin is a key to study the origin and development of the South China Sea (SCS). We do not know much about its boundaries, geological history, and the formation of its sea floor because it has a complex and highly regional structural background, notable sediment activity, and yet few floor rocks. Here a granodiorite sample was collected from the southern margin of the southwest basin of the South China Sea. The results indicated that the ⁴⁰Ar-³⁹Ar ages of biotites in the sample are 110.3±0.5 Ma, suggesting that they were products of magmatic intrusion during the Early Cretaceous period. The sample’s geochemistry showed it had high SiO₂, K₂O, and Al₂O₃ but low TiO₂ levels. Tectonic discriminant diagrams suggested that the sample might represent extrusion-related magmatism, either in an arc or forearc setting in the SCS area and that the sample mainly belonged to the syncollision type, whose formation was related to orogenies. The sample may be part of the main rock that made up the boundaries of rift system. The process of tension cracking was similar to the development of the Red Sea, in which the rifting and sagging occurred in the continental crust. The southwest basin may not be an original ocean, but a rift developed through finite extension on continental crust basement. The oceanic crust came into being when the width and depth of the rift valley reached a certain scale. The granodiorite sample we collected provides a means of determining the boundary of the southwest basin and the clues that may help researchers expand relevant models. It constitutes an important datum regarding the analysis of the formation and development of the SCS.

Keywords

Late Mesozoic / granodiorite / the southwest basin / the South China Sea / ⁴⁰Ar/³⁹Ar age / tectonic implication

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