The Guanshi Guyot with oceanic plagiogranite in the South China Sea: a potential preferred area for drilling deep crust and Moho discontinuity?

This paper closely focuses on the flagship project "deep earth exploration and the dream of Moho discontinuity in the 2050 Science Framework: Exploring Earth by Scientific Ocean Drilling" released by IODP. The discussion focuses on the possibility that the Guanshi Guyot of the South China Sea where oceanic plagiogranite was found, is a potential preferred area for deep crust and Moho discontinuity drilling, by studying the distribution pattern, associated intrusive rocks and genetic model of oceanic plagiogranites obtained around the global modern oceanic crust. The results show that oceanic plagiogranites were discovered under different tectonic backgrounds, including the ultraslow spreading background of southwest Indian Ocean, the slow spreading background of the Atlantic Ocean, the northwest Pacific Ocean, the middle Indian Ocean and the Indian Ocean, as well as the rapid-spreading background of the eastern Pacific Ocean, the marginal sea seafloor-spreading background such as the South China Sea, the intra-oceanic subduction background forming intra-oceanic arcs such as the Izu-Bonin-Mariana arc (I B M), the Kyushu-Palau ridge, and the Amami submarine plateau. Most oceanic plagiogranites are scattered in gabbro rocks as veins, which means that the oceanic plagiogranites were probably initially formed in the deep oceanic crust, but later were exhumed to the seabed surface or subsurface due to faults and other geological process, making them easily discoverable and providing a window for studying the magma process and structure of the oceanic crust. The Guanshi Guyot is located near the fossil spreading ridge of the eastern sub-basin in the South China Sea, and a rock sample of MORB-type oceanic plagiogranite was obtained by dredging. Geochemical data of the rock samples indicates that it may have been formed by partial melting of gabbro. Therefore, the oceanic plagiogranite of the Guanshi Guyot probably came from the exhumation of deep oceanic crustal rock and is expected to be an important tectonic window and a potential preferred area for deep crustal and Moho drilling in the South China Sea. In order to verify this speculation, further work is suggested as follows: (1) As the previous oceanic plagiogranite was obtained by dredging, it has a large positioning error. Therefore, it is necessary to use an accurate positioning survey means, such as TV grab, shallow drilling or Remote Operation Vehicle (R O V) to obtain new rocks on the Guanshi Guyot. (2) At present, only oceanic plagiogranites were found on the Guanshi Guyot. It is necessary to investigate whether other deep oceanic crustal rocks such as gabbros are developed on the Guanshi Guyot. (3) It is necessary to conduct supplementary survey on gravity, magnetic, deep reflection seismic, Ocean Bottom Seismometer (OBS), magnetotelluric to reveal the crust and upper mantle structure characteristics, the faults distribution and the depth of the Moho discontinuity from a geophysical perspective. © 2022 Geological Society of China. All rights reserved.