Detrital Zircon Provenance of Pliocene Yinggehai Formation in the Ledong Gas Field of the Yinggehai‐Song Hong Basin

Over the past decades, a number of hydrocarbon reservoirs have been discovered in the deepwater area of the Yinggehai-Song Hong Basin, South China Sea. The NW-oriented Yinggehai-Song Hong Basin, located on the continental shelf at water depths of 50-200 m in the northwestern South China Sea, is one of the largest Cenozoic pull-part basins in the world. The previous studies illustrated that three major source terrains were involved in the basin, including Red River (southern Yangtze Block), central Vietnam (Eastern Indochina Blcok) and Hainan Island (Cathaysia Block) (Xie, 2009). The provenance of Pliocene Yinggehai Formation in the southeast of the basin is still controversial due to there is no reliable data to identify the source regions. Recently, the studies of modern river sediments around the basin have shown that detrital zircon populations derived from the Red River are fundamentally different to those derived from central Vietnam and Hainan Island (Hoang et al., 2009; Yan et al., 2011; Wang et al., 2014). Therefore, the method of detrital zircon U-Pb dating could be used to constrain the provenance of the Yinggehai Formation. In this study, sandstone sample (LD30-1) was collected from the drill core in the Ledong gas field in order to determine the provenance of the Yinggehai Formation in this area. U-Pb geochronologic analysis of detrital zircon grains extracted from the sample using a laser ablation inductively coupled mass spectrometer yields ages ranging from Archean to Cenozoic (Fig.1D). The sample have two major age peaks at ca. 234 Ma and 430 Ma, with subordinate peaks at ca. 98 Ma, 158 Ma, 2526 Ma and a “broad” age group between 750 Ma and 1060 Ma. The Late Jurassic-Cretaceous zircon grains with age peaks at ca. 98 Ma and ca. 158 Ma are consistent with the ages of Yanshanian granites which mainly present in the Hainan Island (Wang et al., 1991; Yan et al., 2011) and could be regarded as the symbol of the Hainan (Wang et al., 2014). The Late Early Cretaceous adakitic intrusive rocks and volcanic rocks (around 100 Ma) have been discovered in the southeast of the Baisha Fault (Jia et al., 2009; Wang et al., 2011). Abundant ages of around 234 Ma are consistent with the ages of the “HercynianIndosinian Movements”. The ages well developed in the surrounding source areas and probability related to Indochina and South China collision and subsequent lateral extrusion of Indochina (Şengör et al., 1988; Lu et al., 1999; Lepvrier et al., 2008). The early Paleozoic grains related to Caledonian Orogen in South China with peak at ca. 430 Ma are widely distributed in Yangtze and Indochina Block but rare and poorly understand in Hainan Island. These zircons in this sample are younger than those in the Indochina (Fig. 1), thus, we suggest that they were derived from southern Yangtze through the Red River. The Neoproterozoic grains (750-1060 Ma) corresponding to the tectonic event termed “Jinningian Movement” are rare or absent in other source areas and can be interpreted as a signature of the Yangtze Craton. Neoproterozoic magmatism, including granites, volcanic rocks and ophiolites, occurred sporadically around the Yangtze Block (Lu, 1999; Zhou et al., 2006; Xiao et al., 2007), which probably produced by a mantle plume that marked the pre-breakup of Rodinia and subsequent migration of South China. In addition, a number of ages around 2526 Ma has been suggested to the basement sedimentary sequences of the Yangtze Craton. During the Pliocene, the majority of sediment was supplied from the northwest delivered by the “Red River delta”. The source region WANG Ce, LIANG Xinquan, FU Jiangang, ZHOU Yun, JIANG Ying and DONG Chaoge, 2015. Detrital Zircon Provenance of Pliocene Yinggehai Formation in the Ledong Gas Field of the Yinggehai-Song Hong Basin. Acta Geologica Sinica (English Edition), 87(supp.): 279-280.