中国科学院海洋研究所机构知识库

西太平洋雅浦岛弧系统位于菲律宾海板块的东南边界处，岛弧形态和结构与西太平洋的其它火山岛弧有显著区别。雅浦岛弧是西太平洋一个广泛出露变质岩的岛弧。雅浦岛弧的变质岩与构造事件密切相关，可以有效指示岛弧早期阶段的岩石类型和构造演化历史。因此，雅浦岛弧变质岩的研究可以有效阐明西菲律宾海板块与太平洋板块边界的岛弧发育历史和构造演化过程。本学位论文以雅浦岛弧中段的Y3拖网站位和雅浦岛（Yap Islands）的变质岩为研究对象，通过矿物学、年代学和地球化学手段，研究了雅浦岛弧变质岩的成岩过程、形成年代和原岩组分，重点探讨了雅浦俯冲带的构造演化历史。取得的主要认识包括：

（1）雅浦岛弧Y3站位和雅浦岛变质岩主要为角闪岩相与绿片岩相。角闪岩的变质作用的研究结果表明，卡洛琳板块的俯冲侵蚀作用使得雅浦岛弧遭受了大面积侵蚀，导致埋藏岛弧深处的绿片岩相和角闪岩相变质岩出露于海底。

（2）雅浦岛弧Y3站位的角闪岩样品在矿物组合、主量元素、微量元素和同位素组成存在明显差异，恢复出3种原岩：弧前玄武岩、碱性洋岛玄武岩和岛弧拉斑玄武岩。这些原岩分别代表了不同的构造环境：初始的雅浦岛弧的弧前区域、卡洛琳洋底高原和初始雅浦岛弧。

（3）根据雅浦岛变质岩的地球化学分析结果，雅浦岛基性变质岩的原岩可分为两组：高镁的钙碱性玄武岩和低镁的拉斑玄武岩。两组样品的微量元素组成表现出洋岛玄武岩型的富集特征：富集 Nb、Ta、Zr、Hf、Ti等高场强元素；富集轻稀土元素，轻稀土与重稀土的分馏程度较高。雅浦岛基性变质岩的地球化学组成与卡洛琳洋底高原的拉斑玄武岩极为相似。因此，雅浦岛的构造背景可能代表拼贴在雅浦岛弧之上的卡洛琳洋底高原或海山，记录了洋底高原俯冲、碰撞和增生的地质过程。

（4）根据角闪石⁴⁰Ar/³⁹Ar测年和榍石SIMS原位U-Pb测年，雅浦岛弧角闪岩形成于~21 Ma，记录了卡洛琳高原与原雅浦岛弧碰撞事件的时间。金红石和榍石U-Pb年龄显示，雅浦岛绿片岩的变质作用的时间发生在大约22.2 ± 5.7 Ma。雅浦岛绿片岩的年龄结果与雅浦岛弧Y3站位的角闪岩的形成年代大致相同，共同指示了卡洛琳洋底高原与雅浦岛弧的碰撞时间。

（5）雅浦岛弧之所以广泛分布变质岩，是由于卡洛琳高原与初始雅浦岛弧的俯冲和碰撞作用导致的。雅浦岛弧的角闪岩的地球化学特征和年代学结果，从空间和时间上对卡洛琳洋底高原与初始雅浦岛弧的碰撞事件进行了精确地制约。雅浦俯冲体系至少经历了三个演化阶段：初始雅浦岛弧的俯冲起始阶段，产生了初始雅浦岛弧的弧前玄武岩和岛弧火山岩；初始雅浦岛弧与卡洛琳高原的碰撞阶段，包含岛弧、弧前、俯冲板片物质的增生楔发生区域变质作用，形成变质杂岩体，构成了现今雅浦岛弧的主要岩石类型；卡洛琳高原向雅浦海沟的超慢速俯冲阶段，洋脊及海山的俯冲促进了俯冲侵蚀导致变质岩出露，新的俯冲引起雅浦海沟中年轻的岛弧火山岩产生。

以上研究结果表明，雅浦岛弧是西太平洋典型的洋底高原和大洋岛弧之间俯冲、碰撞形成的变质岩岛弧。雅浦俯冲带经历了复杂的构造演化过程，卡洛琳洋底高原的俯冲和碰撞作用对雅浦岛弧的构造演化历史产生了重要影响。雅浦岛弧变质岩的研究结果，对于理解雅浦沟弧体系的构造背景以及西太平洋俯冲带的演化具有极其重要的意义。

The Yap Subduction Zone is located at the southeast boundary of the Philippine Sea Plate in the Western Pacific. The shape and structure of the island arc are significantly distinct from other volcanic island arcs in the Western Pacific. The Yap Arc is the unique island arc with extensive outcrops of metamorphic rocks in the Western Pacific. Metamorphic rocks in the Yap Arc are closely related to the tectonic events, which can effectively indicate the structure and evolution history of the initiated island arc. Therefore, the study of metamorphic rocks in the Yap Arc can shed light on the development history and tectonic evolution of the boundary between the West Philippine Sea plate and the Pacific plate. Detailed mineralogy, chronology and geochemistry research were carried out on the metamorphic rocks from the Yap Islands and Y3 dredge hauls site in the middle section of the Yap Arc. this thesis studies the metamorphic process, age and composition of the metamorphic rocks in the Yap Arc, and focuses on the tectonic evolution history of the Yap Island Arc. The major findings include:

(1) The metamorphic rocks of the Yap Arc Y3 site and Yap Islands are principally composed of greenschists and amphibolites. The results of temperature and pressure calculation of amphibolite show that the outcrop of metamorphic rocks on the Yap Arc and Yap Islands results from subduction erosion. The seamounts on the Caroline Plateau promote the subduction erosion of the Yap Arc, leading to the erosion of the fore-arc area and even the main body of the island arc, eventually leading to the exposure of metamorphic rocks.

(2) There are significant differences in mineral assemblages, major elements, trace elements and isotopic compositions of amphibolite samples from the Y3 Site on the Yap Arc, and three kinds of protoliths have been recovered: FAB type basalt, OIB type alkaline basalt and IAB type tholeiite. These proto-rocks represent different tectonic settings: the forearc environment of the proto-Yap Arc, the Caroline oceanic plateau and the proto-Yap Arc volcanic rocks.

(3) According to the geochemical analysis results of metamorphic rocks on Yap Island, the protoliths of metamorphic rocks on Yap Island can be divided into two groups: high Mg calc-alkaline basalt and low Mg tholeiite. The trace element composition of the two groups of samples showed the enrichment characteristics of OIB type: enrichment of Nb, Ta, Zr, Hf, Ti; enrichment of light rare earth elements, the fractionation degree of the heavy and light rare earth elements is high. These results show that the protoliths of the metamorphic rocks probably represent the Caroline oceanic plateaus or seamounts.

(4) According to the ⁴⁰Ar/³⁹Ar dating of Amphiboles and the in-situ U-Pb dating of Titanites, the Yap arc amphibolites was formed at ~21 Ma, which records the time of the collision event between the Caroline Plateau and the proto-Yap Arc. The U-Pb dating of Rutiles and Titanites indicates that the metamorphism of the greenschist occurred at about 22.2 ± 5.7 Ma. The age of the greenschist in Yap Island is the same as that of the amphibolite in Y3 site, indicating the collision time between the Caroline Plateau and the Yap Arc.

 (5) The formation of metamorphic rocks in the Yap Arc is due to the collision between the proto-Yap Arc and the Caroline Plateau. The geochemical characteristics and chronological results of amphibolites in the Yap Arc accurately constrain the collision events between the Caroline Plateau and the initial Yap Arc from space and time. The Yap subduction system experienced at least three evolutionary stages: the early stage of subduction of the Caroline plate (the formation of volcanic rocks in the former Yap Island arc); The collision stage between the former Yap Arc and the Caroline Plateau (the formation of metamorphic rocks in the Yap Island Arc); Ultra-slow subduction of the Caroline Plateau towards the Yap Trench (development of subduction erosion and generation of young volcanic rocks in the trench).

The above research results indicate that the Yap Arc is a metamorphic arc formed by subduction and collision between a typical oceanic plateau and an island arc in the western Pacific. The Yap subduction zone underwent a complex tectonic evolution, and the subduction and collision of the Caroline plateau had an important influence on the tectonic evolution history of the Yap Arc. The study of the Yap arc metamorphic rocks is of great significance for understanding the tectonic setting of the Yap arc-trench system and the tectonic evolution of the subduction zones in the western Pacific.