Geochemical And Chronological Constraints On The Tectonic Evolution History And Origin Of Metamorphic Rocks On The Yap Arc

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.