Element analyses and isotopic composition of basalts from the western Pacific

New Sr- Nd- and Pb-isotopic and trace element data are presented on basalts from the Sulu and Celebes Basins, and the submerged Cagayan Ridge Arc (Western Pacific), recently sampled during Ocean Drilling Program Leg 124. Drilling has shown that the Sulu Basin developed about 18 Ma ago as a backarc basin, associated with the now submerged Cagayan Ridge Arc, whereas the Celebes Basin was generated about 43 Ma ago, contemporaneous with a general plate reorganisation in the Western Pacifc, subsequently developing as an open ocean receiving pelagic sediments until the middle Miocene. In both basins, a late middle Miocene collision phase and the onset of volcanic activity on adjacent arcs in the late Miocene are recorded. Covariations between 87Sr/86Sr and 143Nd/144Nd show that the seafoor basalts from both the Sulu and Celebes Basins are isotopically similar to depleted Indian mid-ocean ridge basalts (MORB), and distinct from East Pacifc Rise MORB, defining a single negative correlation. The Cagayan Arc volcanics are different, in that they have distinctly lower epsilon-Ne(T) for a given epsilon-Sr(T), compared to Sulu and Celebes basalts. In the 207Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, the Celebes, Sulu and Cagayan rocks all plot distinctly above the Northern Hemisphere Reference Line, with high Delta 7/4 Pb (5.3-9.3) and Delta 8/4 Pb (46.3-68.1) values. They define a single trend of radiogenic lead enrichment from Celebes through Sulu to Cagayan Ridge, within the Indian Ocean MORB data field. The data suggest that the overall chemical and isotopic features of the Sulu, Cagayan and Celebes rocks may be explained by partial melting of a depleted asthenospheric N-MORB-type („normal“) mantle source with isotopic characteristics similar to those of the Indian Ocean MORB source. This asthenospheric source was slightly heterogeneous, giving rise to the Sr-Nd isotopic differences between the Celebes and Sulu basalts, and the Cagayan Ridge volcanics. In addition, a probably slab-derived component enriched in LILE and LREE is required to generate the elemental characteristics and low Ne(T) of the Cagayan Ridge island arc tholeiitic and calcalkaline lavas, and to contribute to a small extent in the backarc basalts of the Sulu Sea. The results of this study confirm and extend the widespread Indian Ocean MORB signature in the Western Pacifc region. This signature could have been inherited by the Indian Ocean mantle itself during the rupture of Gondwanaland, when fragments of this mantle could have migrated towards the present position of the Celebes, Sulu and Cagayan sources.本文报道了西太平洋海域苏禄海盆、西里伯斯海盆（Celebes Basin）以及已被淹没的卡加延脊弧（submerged Cagayan Ridge Arc）的玄武岩样品的Sr、Nd、Pb同位素及微量元素数据，这些样品由大洋钻探计划（Ocean Drilling Program, ODP）第124航次近期采集获得。钻探结果表明，苏禄海盆约于18 Ma前形成，为与现今已被淹没的卡加延脊弧相关的弧后盆地（backarc basin）；而西里伯斯海盆形成于约43 Ma前，与西太平洋区域的普遍板块重组事件同期，随后发育为开放大洋环境，持续接受远洋沉积物（pelagic sediments）沉积直至中中新世（middle Miocene）。两个海盆均记录了中中新世晚期的碰撞阶段，以及晚中新世（late Miocene）相邻弧系火山活动的起始事件。87Sr/86Sr与143Nd/144Nd的协变关系显示，苏禄海盆与西里伯斯海盆的海底玄武岩在同位素组成上与亏损型印度洋洋中脊玄武岩（MORB）相似，而区别于东太平洋海隆（East Pacific Rise）MORB，二者呈现显著的负相关趋势。与苏禄海盆和西里伯斯海盆的玄武岩相比，卡加延弧火山岩具有独特特征：在给定εSr(T) (epsilon-Sr(T))的条件下，其εNd(T) (epsilon-Ne(T))值显著更低。在207Pb/204Pb、208Pb/204Pb对206Pb/204Pb的同位素图解中，西里伯斯、苏禄及卡加延地区的岩石样品均显著分布于北半球参考线（Northern Hemisphere Reference Line）之上，具有较高的Δ7/4Pb（5.3~9.3）与Δ8/4Pb（46.3~68.1）值。在印度洋MORB的数据范围内，三者呈现出从西里伯斯经苏禄到卡加延脊的放射性铅富集单一演化趋势。本次研究的数据表明，苏禄、卡加延及西里伯斯地区岩石的整体化学与同位素特征，可通过亏损型软流圈N-MORB型（“正常”）地幔源区的部分熔融来解释，该源区的同位素组成与印度洋MORB源区相似。该软流圈源区存在轻微的不均一性，这导致了西里伯斯与苏禄海盆玄武岩、以及卡加延脊火山岩之间的Sr-Nd同位素差异。此外，还需要一种可能源自俯冲板片、富集大离子亲石元素（Large Ion Lithophile Elements, LILE）与轻稀土元素（Light Rare Earth Elements, LREE）的组分，用以解释卡加延脊岛弧拉斑玄武岩（island arc tholeiitic）与钙碱性熔岩（calcalkaline lavas）的元素特征及低εNd(T)值，同时该组分也对苏禄海弧后盆地玄武岩存在微弱贡献。本研究结果证实并拓展了西太平洋区域广泛分布的印度洋MORB同位素特征。该特征可能是在冈瓦纳大陆（Gondwanaland）裂解时期由印度洋地幔本身继承而来，当时该类地幔碎片迁移至现今西里伯斯、苏禄及卡加延源区的位置。