Structural Deformation Characteristics and Geological Significance of the Mesoproterozoic Augen Granite Gneiss in Hainan Island, China

The oldest basement rock discovered so far in Hainan Island is the augen granite gneiss of the Gezhencun Formation in the Middle Proterozoic era. By studying its petrology and geometry, the stress and temperature pressure conditions during rock formation can be restored and reconstructed, which is of great significance for reconstructing the early tectonic framework and geological environment of Hainan Island. Field geological, geochemical, and tectonic geological studies have shown that the upper part of the Gezhencun Formation is mainly composed of biotite plagioclase gneiss or mixed biotite plagioclase gneiss; The lower part is composed of mixed granodiorite and biotite plagioclase mixed gneiss. The main body of the original rock is calcium alkaline volcanic rock, which belongs to a set of moderately acidic volcanic rock formations. The composition of volcanic rocks is generally equivalent to that of andesite and quartz andesite (lava and tuff), with characteristics of modern island arc or continental margin arc volcanic rock series. Research suggests that the Gezhencun Formation may have formed in a continental margin arc fragmentation environment and was a product of the fragmentation from the Laurentian continent. 1. INTRODUCTION Gneiss is commonly found in the basement of ancient landmasses or high-grade metamorphic terranes in orogenic belts, formed under specific geological conditions and tectonic environments, providing key information about the composition and formation age of the basement material of landmasses, regional metamorphism, and tectonic attributes. Therefore, the study of gneiss in Hainan Island helps to understand the tectonic properties and evolutionary history of Hainan Island. Previous researchers have conducted detailed chronological studies on the gneiss in the Qiongxi area, all of which have obtained an age of about 1.4 billion years, belonging to the Middle Proterozoic era (Li et al., 2013; Yao et al., 2017; Zhang et al., 2018; Xu et al., 2020). Hainan Island is located at the junction of the Tethys tectonic domain and the Pacific Rim tectonic domain, with a unique tectonic location. The study of the eastward extension of the Tethys and its relationship with the Pacific Rim tectonic domain is of great geological significance (Figure 1). The geological strata (including volcanic rocks) in Hainan Province can be traced back to the Middle Proterozoic. After multi cycle sedimentation and stage tectonic movement, the strata mostly remained in the islands of the South China Sea Islands and surrounding basins as islands. Since the Middle Proterozoic, except for the absence of the Devonian and Jurassic strata, all other strata have been distributed. The Proterozoic is mainly exposed in Hainan Island, the Xisha Islands and the Nansha Islands. Hainan Province has a multi-stage and cyclical geological evolution characteristic. The Mesoproterozoic was an important geological period for the formation of crystalline basement in Hainan Island, followed by important geological evolution stages such as multi-stage intra plate cracking and orogeny from the Neoproterozoic to the Triassic, intra continental basin mountain evolution from the Jurassic to the Cretaceous, and Cenozoic cracking. This article conducts a detailed analysis of field geology, structural geology, and other factors in order to explore its formation mechanism and geodynamic background, and provide new constraints on key issues such as the eastward extension and evolution of the ancient Tethys Ocean.