发育大洋斜长花岗岩的南海管事平顶海山：深部地壳和莫霍面钻探的优选区？
2022,96(8):2647-2656
摘要：本文紧密围绕IODP“面向2050年大洋钻探科学框架”中的“地球深部探测”旗舰计划和“莫霍面”梦想，研究并总结全球现代洋壳发现的大洋斜长花岗岩的分布规律、岩石组合特征和成因模式，探讨发育大洋斜长花岗岩的南海管事平顶海山是否为深部地壳和莫霍面钻探潜在优选区。统计结果表明大洋斜长花岗岩在多种不同构造背景形成的洋壳上均有发现，包括西南印度洋超慢速扩张构造环境，大西洋、西北太平洋、西印度洋和中印度洋慢速扩张构造环境，东太平洋快速扩张构造环境，南海等边缘海构造环境，伊豆- 小笠原- 马里亚纳(IBM)岛弧、九州- 帕劳海脊、Amami海底高原等洋内俯冲构造环境。多数大洋斜长花岗岩呈脉状零散分布于辉长岩中，意味大洋斜长花岗岩可能形成于洋壳深部，在后期断裂等地质作用下被剥蚀而更容易被发现，成为了解洋壳深部岩浆过程和洋壳结构的一个窗口。管事平顶海山位于南海东部次海盆古扩张脊附近，拖网获得MORB型大洋斜长花岗岩，前人基于地球化学数据认为其可能为辉长岩部分熔融形成。本文推测管事平顶海山大洋斜长花岗岩很可能为洋壳深部物质剥露海底，是南海的一个重要构造窗口，有望成为南海深部地壳和莫霍面钻探的潜在优选区，但需要开展进一步调查研究以验证推测：① 海山大洋斜长花岗岩为拖网所得，位置误差较大，需开展可精确定位的电视抓斗、浅钻或有缆遥控水下机器人(ROV)调查；② 海山目前只发现大洋斜长花岗岩，需调查海山是否发育辉长岩等深部地壳岩石组合；③ 需开展重磁、深反射地震、海底地震仪(OBS)、大地电磁等调查研究，建立管事平顶海山洋壳和上地幔结构模型，查明断裂带分布，揭示莫霍面深度。
关键词：南海；管事平顶海山；大洋斜长花岗岩；深部地壳；莫霍面
The Guanshi Guyot with oceanic plagiogranite in the South China Sea: a potential preferred area for drilling deep crust and Moho discontinuity?
Abstract：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 (IBM), 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 sub- surface 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 (ROV) 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.
Key words：South China Sea; Guanshi Guyot; oceanic plagiogranite; deep crust; Moho discontinuity
收稿日期：2022-05-30
基金项目：本文为南方海洋科学与工程广东省实验室项目（编号GML2019ZD0201）、国家自然科学基金重点基金项目（编号U20A201651）和中国地质调查局地质调查项目（编号DD20221719，DD20221712）联合资助的成果。
