海洋地质与环境重点实验室知识库

  冲绳海槽是西太平洋唯一一个在陆壳基底扩张而成、目前尚处于扩张早期的弧后盆地，区域内岩浆、热液活动发育，是研究多圈层相互作用、壳幔物质循环的理想场所。

  为了解该弧后盆地的壳幔相互作用，本文以冲绳海槽南部玄武质安山岩、英安岩和流纹岩为研究对象，利用岩石和单矿物的元素和同位素地球化学研究方法，揭示了不同类型火山岩的成因。其中，第四与那国热液区（Yonaguni Knoll IV）中流纹岩具有高SiO₂（>70.01 wt.%）、K₂O + Na₂O（6.71~7.16 wt.%）含量和分异指数（DI > 93），属中钾钙碱性系列，其富集大离子亲石元素和轻稀土元素、强烈亏损高场强元素，具有海槽火山岩中最为富集的同位素组成特征：εNd（-3.2~3.5）、⁸⁷Sr/⁸⁶Sr（0.705552~0.705713）、δ¹⁸O值（+8.1‰~ +8.6‰）和Pb同位素组成（²⁰⁶Pb/²⁰⁴Pb = 18.568~18.589，²⁰⁷Pb/²⁰⁴Pb = 15.641~15.656和²⁰⁸Pb/²⁰⁴Pb = 38.848~38.904）。对该流纹岩的全岩地球化学特征研究表明其由幔源玄武质岩浆与壳源长英质岩浆混合后在浅层岩浆房中经历高程度的演化分异形成；角闪石成因矿物学研究进一步佐证该流纹岩为壳-幔混合成因。此外，高氧化条件和缺乏出溶高盐性流体导致金属元素和岩浆Cl保留在该流纹质熔体中和/或分配进入其结晶矿物相中。因此，大量该类流纹质岩浆侵入上部地壳，有可能构成Yonaguni Knoll IV热液系统中Cl、金属元素和其他物质的一个潜在来源。

  横穿弧后火山带（cross back-arc volcanic trail；CBVT）区域英安岩中磷灰石为氟磷灰石，F含量为1.58~2.54 wt.%，Cl含量为0.57~0.88 wt.%，Cl含量低于周边岛弧英安岩中磷灰石Cl含量，磷灰石具有富集的原位微区⁸⁷Sr/⁸⁶Sr同位素组成特征（0.70645 ~ 0.70687），进一步研究表明俯冲板块脱水过程释放的含Cl流体进入上覆地幔楔，引起地幔楔部分熔融，形成具有高Cl含量的初始铁镁质岩浆。该铁镁质岩浆上升经由地壳过程中，与低Cl含量和高⁸⁷Sr/⁸⁶Sr比值的壳源长英质岩浆发生混合，稀释了初始铁镁质岩浆Cl的含量并升高了⁸⁷Sr/⁸⁶Sr比值，形成该英安岩的母岩浆；进一步，磷灰石的U-Th/He年龄约为0.2 Ma，表明该英安质岩浆喷发年龄约为0.2 Ma，其喷发机制为玄武质岩浆注入浅层英安质岩浆房，带来的高温驱使英安质岩浆向上运移并最终喷发。

  冲绳海槽南部的玄武质安山岩富集大离子亲石元素、亏损高场强元素。C5玄武质安山岩的Sr-Nd-Pb-Hf同位素组成最为亏损，具有“印度洋型”地幔特征，而C4和TVG9-1玄武质安山岩的放射性成因的Sr-Nd-Pb-Hf同位素更为富集，朝向EM2端元，主要受少量（1%）、高Nd/Hf比值的俯冲沉积物影响。俯冲的菲律宾海板块较老的年龄、较快的俯冲速率，以及所具有的冷俯冲带的特征是冲绳海槽火山岩岩浆源区缺乏俯冲沉积物熔体贡献的原因。

因此，俯冲板块与上覆地幔楔的相互作用形成的初始岩浆及其上升过程中再度与减薄地壳的相互作用是冲绳海槽南部玄武质安山岩、流纹岩和英安岩形成的主要机制。进而，冲绳海槽的岩浆作用是壳（俯冲洋壳+减薄陆壳）、幔（地幔楔）多圈层相互作用的产物，其岩浆演化过程更为复杂。

  The Okinawa Trough (OT) is the only back-arc basin in the western Pacific which was formed in the continental crust basement and is still in the early stage of expansion. It has active volcanism and hydrothermal activities. Thus, the OT is an ideal place to study the interaction of multi-circle layers and the circulation of crust-mantle materials.

   In order to understand crust-mantle interaction of back-arc basin, the basaltic andesite, dacite and rhyolite in the southern OT were studied. The petrogenesis of the different types of volcanic rocks was revealed by the methods of element and isotope geochemistry of whole rocks and single minerals. The rhyolite from the Yonaguni Knoll IV hydrothermal field have high SiO₂ (>70.01 wt.%), K₂O + Na₂O (6.71~7.16 wt.%) contents and differentiation index (DI) values (DI>93), and are medium-K calc-alkaline in composition. The rhyolites enrich in LILEs and LREEs and strong depletion in HFSEs, with the most enriched isotopic compositions, including εNd (from -3.2 to -3.5), ⁸⁷Sr/⁸⁶Sr ratios (0.705552 to 0.705713), δ¹⁸O values (+8.1‰ to +8.6‰) and Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.568~18.589, ²⁰⁷Pb/²⁰⁴Pb = 15.641~15.656, and ²⁰⁸Pb/²⁰⁴Pb = 38.848~38.904). Based on these characteristics, we propose that the rhyolites were most likely generated by the mixing of a mantle-derived basaltic magma and crustal felsic magma, followed by extensive fractional crystallization in shallow magma chamber. The genetic mineralogy of amphibole further proves that the crust-mantle mixed origin. The lack of exsolved hydrosaline fluid and the high oxidation conditions resulted in the metal elements and magmatic Cl remaining dissolved in the rhyolitic melt and/or partitioning into crystalline minerals upon eruption. Thus, the large volumes of rhyolitic magma intruded in the upper crust of Yonaguni Knoll IV hydrothermal field are a potential fertile source of these elements via leaching by heated sea waters.

  Apatite grains in the dacite from the CBVT are fluorapatites with F contents of1.58~2.54 wt.% and Cl contents of 0.57~0.88 wt.%. The Cl contents of the apatites in the dacite were slightly lower than those in surrounding subduction related systems. Apatites have high in situ analyzed ⁸⁷Sr/⁸⁶Sr ratios (range from 0.70635 to 0.70697). Based on these characteristics, we propose that the dehydration of subducted slab released fluids into the overlying mantle wedge and triggered mantle melting, which produced primitive mafic magma with high Cl contents. Then, the mafic magma ascended into the crust, which has low Cl concentrations and high ⁸⁷Sr/⁸⁶Sr ratios. The mixing of a mantle-derived basaltic magma and a crustal felsic magma in a shallow magma chamber diluted the Cl content and elevated the ⁸⁷Sr/⁸⁶Sr ratio, thus producing the parental magma of the dacite. When the dacitic magma chamber was replenished by mafic magma at approximately 0.2 Ma, the high temperature from the injection would have caused the felsic melt to move upward and finally erupt.

  Basaltic andesites in the southern OT are enrichment of LILEs, depletion of HFSEs. The Sr-Nd-Pb-Hf isotope composition of the C5 basaltic andesite is the most depleted, with the characteristics of the “Indian MORB” mantle, while the Sr-Nd-Pb-Hf isotopes of the C4 and TVG9-1 basaltic andesite are more enriched, towards the EM2 endmember, mainly affected by a small amount (1%) of subducted sediments with high Nd/Hf ratio. The cold subducted Philippine Sea plate is characterized by older age and faster subduction rate, which is the reason of lack of contribution of the subduction sediment melt to the mantle source.

  Therefore, the initial magma formed by the interaction between the subduction plate and the overlying mantle wedge and its interaction with the thinned crust during the ascending process are the main mechanisms for the petrogenesis of basaltic andesite, rhyolite and dacite in the southern OT. Furthermore, the magmatism of the OT is a product of multi-layer interactions between the crust (subducted oceanic crust and thinned continental crust) and mantle (mantle wedge), and its magma evolution process is more complicated.