西太平洋海山省Magellan海山链MA1-MA10海山的形成年代与成因研究

The seamount trails in the West Pacific Seamount Province(WPSP) record the magmatic activities for Pacific plate in Cretaceous, and thus are important to understand the absolute plate motion and deep geological process under the Pacific plate, more detailed analysis of seamounts is needed in the future. Magellan Seamount Trail(MST) is one of the typical seamount Trails in WPSP, this study focus on 10 seamounts(MA1-MA10) of MST, through the research on 40Ar/39Ar geochronology, geochemistry and Sr-Nd-Pb-Hf isotope, to get the age of the seamounts, and discuss the magma source and mgma evolution. Then combine with the results of previous stduy of 7 seamounts to establishment an integrated framework of geochronology for MST, and reveal the relationship of their magma source with the present-day hot spots in the South Pacific Isotopic and Thermal Anomaly(SOPITA), to give a better undertanding of the ptrogenesis of MST combing with the plate reconstruction and regional geological tectonic background. The result is helpful to understand the petrogenesis of small seamounts and the motion of Pacific plate in Cretaceous, and meanwhile offer further basic data to our cobalt ore area. Using better method which has little influence from alteration to do age dating and mantle source tracing, and fine analysis of a regional seamount trail by comprehensive factors are the innovation and features of this study.

西太平洋海山省的海山链记录了白垩世太平洋板块下的岩浆活动，对指示板块运动方向及了解下覆深部地质过程有重要作用，对海山链更细致的剖析是未来的发展方向。Magellan海山链是西太平洋海山省的典型海山链之一，本课题通过对Magellan海山链MA1-MA10海山的40Ar/39Ar年代学和岩石地球化学、Sr-Nd-Pb-Hf同位素的研究，获得海山的年龄，探讨岩浆来源与岩浆演化，结合已有的7座海山的研究结果，建立Magellan海山链完整的年代学框架，揭示岩浆喷发期次，分析海山源区与南太平洋同位素与热异常区热点的对应关系，结合板块重建和区域构造地质背景认识海山链的成因。研究结果对理解小型海山的成因及白垩世太平洋板块的运移有一定意义,也为我国富钴结壳矿区勘探提供进一步的基础地质资料。采用受蚀变影响小的方法进行定年和示踪地幔源区，以及综合多种因素对区域海山链进行精细剖析是本项目的创新与特色之处。

海山样品为海底深部地幔地球化学组成提供了重要证据，对海山的研究可以更好的揭示板内火山活动。西太平洋海山省（WPSP）仍有许多尚未研究的海山，它们的年龄和地球化学组成仍然未知。对它们的研究可以了解其运动轨迹及演化历史，有助于更好地理解下覆太平洋板块运行的地质过程。本研究提供了WPSP中4座海山的7个玄武岩样品的40Ar/39Ar年龄和微量元素以及Sr-Nd-Pb同位素数据。数据表明，这些岩石都属于洋岛碱性玄武岩（OIA型）。对其中橄榄石斑晶的分析表明，岩浆经历了强烈的橄榄石分馏，并且在其演化过程中经历了从橄榄石+斜长石到橄榄石+斜长石+单斜辉石的岩浆演化过程。样品的REE模式和蜘蛛图显示了OIB（洋岛玄武岩）的特征。87Sr/86Sr范围为0.70460±23~0.70624±15，206Pb/204Pb范围为18.241±16~18.599±15，143Nd/144Nd范围为0.512646±8~0.512826±7，暗示源区为EMI和EMⅡ。40Ar/39Ar年龄数据显示，这些海山形成于~97Ma和~106Ma，一部分海山的形成可能经历了与麦哲伦海山链的东侧海山相同的形成过程，但是其他海山可能具有不同的岩浆起源。