卡洛琳海山链成因及验证地幔柱成因假说的大洋钻探设想

Origin of the Caroline Seamount Chain and an ocean drilling proposal to test the mantle plume hypothesis

地球上除了板块边界形成的岩浆活动之外,在板块内部也广泛存在大规模岩浆活动。其中,板内岩浆活动在大洋中形成了广泛的洋底高原和链状分布的海山。20世纪70年代,国际上提出“地幔柱”假说来解释洋底高原和有时间序列的海山链成因。“地幔柱”假说认为,深部地幔物质上涌过程中包裹周围地幔物质,并在浅部上地幔发生高程度部分熔融,首先形成大规模洋底高原,接着在运动的板块之上形成有年龄序列的洋岛/海山链。然而,长期以来,并没有找到明确有成因联系的洋底高原-海山链系统,使得该假说一直没有得到证实。卡洛琳火山系统由西部的卡洛琳高原和东部卡洛琳洋岛/海山链组成。回顾了卡洛琳海山链火山岩已有研究成果,结果显示,卡洛琳海山链很可能是由一个年轻(约30 Ma)地幔柱形成。尽管如此,在卡洛琳洋底高原和东部海山链之间还有大量平顶海山尚未进行过火山岩采样研究,目前对该海山链系统是否由一个地幔柱“热点”连续形成还缺少明确证据。由于该区海山通常被沉积物或礁灰岩覆盖,常规采样困难。本文提出对该平顶海山区进行5个站位的钻探采样,并开展岩石学、年代学和地球化学研究,并与卡洛琳洋底高原和海山链进行详细对比,将有望检验“地幔柱”假说是否成立。

In addition to magmatism along the tectonic plate boundaries,magmatism also occurs in intraplate setting.Intra-oceanic plate volcanism has formed a number of oceanic plateaus and seamount chains.In the 1970s,the mantle plume hypothesis was proposed to explain the genesis of oceanic plateau and seamount chain with time propagation.The mantle plume hypothesis considers that the deep mantle-sourced material upwells and entrains surrounding mantle materials and melts in the shallow mantle to form an oceanic plateau,and then an seamount chain with time on the moving plate.However,a clear evidence for genetic relationship between an oceanic plateau and a seamount chain is lacking,and,thus,the mantle plume hypothesis has not been proved.The Caroline volcanic chain consists of the Caroline plateau to west and the Caroline seamount chain to east.This study has reviewed the proceedings on the origin of the Caroline volcanic rocks in recent years,which indicates that the Caroline seamount chain could have been formed by a young deep mantle-sourced mantle plume.Because there is lack of studies on the volcanic rocks from the broad transition region between the Caroline plateau and seamount chain,robust evidence for the whole Caroline Seamount Chain formed by a deep mantle plume is lacking.This transition region is widely covered by pelagic sediments and reef limestone,and sampling of basement volcanic rocks is difficult with routine methods.This study proposes five drilling sites in this transition region,and studies of petrology,geochemistry and chronology of the drilled volcanic rocks will help to test the mantle plume hypothesis.