南海洋脊抽吸作用对其周边新生代玄武质岩浆活动的影响

Influence of Ridge Suction on Cenozoic Basaltic Magmatism in the Surroundings of the South China Sea

洋中脊与地幔柱相互作用诱发的洋脊抽吸作用,被认为是产生富集型洋中脊玄武岩以及某些洋岛和海山的重要地质过程。但是,对于边缘海扩张是否同样会产生洋脊抽吸作用并控制其周边的玄武质岩浆活动,目前仍不清楚。南海海盆是世界上最大的边缘海海盆之一,它的扩张时间为~32~16 Ma。由于其周边分布有大量的软流圈来源的玄武岩,因而是讨论这一问题的理想研究区。文章对相对远离南海扩张中心的浙江省新生代玄武岩进行了K-Ar定年工作,发现浙江余姚的新生代玄武岩主要形成于渐新世至早中新世(32.2~21.7 Ma),属于南海扩张期的玄武岩。前人研究发现,南海周边的新生代玄武岩主要是形成于南海盆地扩张结束之后。结合前人数据和此次研究的年龄结果发现,越远离南海扩张中心,中国东南部的南海扩张期玄武质岩浆活动越密集。洋脊抽吸作用可以很好的解释这一时空分布规律,因为上涌的地幔流会汇聚在南海扩张中心的区域。而在浙江地区发现了南海扩张期玄武岩,则表明该区域深部的地幔流并没有受到南海洋脊抽吸作用的显著影响。

Ridge suction triggered by ridge-plume interaction has been documented to generate enriched mid-ocean ridge basalts, some oceanic islands and seamounts. However, it remains unclear whether a similar process may apply in settings of an opening marginal sea and basaltic magmatism in its surrounding areas. The South China Sea （SCS） basin,which has been actively spreading between ？32 and 16 Ma,is one of the largest marginal basins that spatially associated with voluminous asthenosphere-derived basalt. It is thus an ideal case for studying this unresolved question. Here, we present new age data for Cenozoic basalts from Zhejiang province, which are basalts that erupted relatively distally from the SCS spreading center. Basalts in Yuyao County of Zhejiang province have predominantly erupted from the Oligocene to early Miocene （ 32. 2 ？21. 7 M a ） ,and thus formed during active spreading （ syn-spreading） of SCS basin. This is striking, as most erupted intraplate basalts in the surroundings of SCS basin have formed following the cessation of the SCS basin. Using published age data and our new results, we can establish that syn-spreading basaltic magmatism in southeastern China is gradually increased in provinces that are relatively distant to the spreading center. The temporal and spatial distribution can be explained by ridge-suction, where upwelling mantle flows were focused towards the SCS spreading center. The presence of syn- spreading basalts in Zhejiang province as well as in the neighboring Jiangsu and Anhui provinces indicates that the mantle flows beneath this area was not significantly affected by the suction of the SCS spreading.