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南海为什么有这么多岩浆？| NSR南海钻探专题

Magmatism plays a key role in the process of continental margin breakup and ocean formation. Even in extremely magma-poor Iberia and Newfoundland margin, studies of field outcrops have shown that syn-rift magmatism had participated in rifting from very early stage and contributed directly to the rifting process. The final transition from exhumed continental mantle to the ocean formation is also triggerred by the accumulation and eruption of magma [1]. Therefore, Atlantic-type passive continental margins are classified into two end-members: Magma-poor (non-volcanic) and magma-rich (volcanic). The differences between them lie on whether a large amount of intrusive and extrusive magmatism from mantle plume/hotspot is involved in the syn-rift and breakup stages. A magma-rich margin [2] should include the following characteristics: (1) A high velocity lower crust (HVLC) caused by syn-rift mafic magma underplating; (2) Continental crust intruded by abundant sills and dikes; (3) A large volume of Sea-ward Dipping Reflectors (SDRs) caused by flood basalt eruption or tuffs. All other margins are classified as magma-poor margin. The South China Sea (SCS) is a marginal sea surrounded by subduction zones on three sides (Fig. 1). Onshore outcrop and offshore drilling suggested that the SCS started rifting in late Cretaceous following the Mesozoic subduction and developed into ocean after early Oligocene. Although classified as a magma-poor margin, the northern continental margin of the SCS shows many features different from the classic examples of Iberia and Newfoundland, including the widely distributed (over 250 km) detachment faults, widespread HVLC and magmatic intrusion below extended crust, large amount of landward dipping fault systems, and especially the fast transition from continental margin to seafloor spreading [3]. Thus, the northern central SCS continental margin challenged the typical magma-poor and magma-rich models by having combined features of both. To test what kind of thinning process might have controlled the transition from continent to ocean in the SCS, International Ocean Discovery Program (IODP) Expeditions 367, 368, and 368X were carried out in 2017 and 2018. Seven sites were drilled from the Outer Margin High (OMH) to continent-ocean transition (COT) and to the early ocean basin [4] (Figs. 1 and 2). Neither serpentinized mantle nor flood basalt/tuff were encountered; instead, over 120 m fresh mid-ocean ridge basalt (MORB) with sediment interlayers were recovered at Site U1500 and ~180 m altered basalt at Site U1502. According to the magnetic anomaly interpretation, paleontological dating in sediment and ship-board major element analysis, the earliest MORB is about 30-34 Ma [4]. How is the transition from continent to ocean fulfilled? Discriminating the magmatic process of the SCS continental margin will greatly help to understand its thinning and breakup processes.

NSR海洋科学