History of arc magma and tectonic evolution on Southwest Mariana Island

Recently, the research group of Zhang Guoliang, a researcher at the Institute of Oceanography of the Chinese Academy of Sciences, has made new progress in the study of the structure, formation age and magmatic action of the southwest Mariana Island arc, and the team has revealed the important influence of the Caroline Ocean Floor Plateau on the magmatic and tectonic evolution of the Southwest Mariana Island arc based on the geochemistry and chronology of the volcanic rocks and metamorphic rocks of the Southwest Mariana Island arc. The results were published in the international marine science journal Deep-Sea Research Part I. 

When the ocean floor plateau moves to the subduction zone with the oceanic plate, it may have an important impact on the boundary morphology of the subduction zone and the tectonic evolution of the island arc, and cause large-scale metamorphism and magmatic activity. Zhang Guoliang’s team obtained samples of volcanic and metamorphic rocks through the unmanned cable-controlled submersible Discovery in the prearc zone at the southernmost tip of the Mariana Island arc. These volcanic and metamorphic rocks are ideal for studying the interaction mechanism of the peculiar and subduction zone of the ocean floor plateau, and may have documented the deformation and evolution history of the Caroline Ocean Floor Plateau and the southwest Mariana Island arc.

Schematic diagram of the topographic and tectonic background of the seabed in the western Pacific Ocean (a); Map of the sampling location of volcanic and metamorphic rock samples at station M2, the southernmost pre-arc zone of the Mariana subduction zone (b) Courtesy of the Institute of Oceanography

Seafloor sampling, micro-mineral composition, and backscattering photographs of volcanic rock samples (A-C) and metamorphic rock samples (D-F) Courtesy of the Institute of Oceanography

The results show that the metamorphic rock samples in this area are mainly greenschist facies metamorphic rocks, and the original rock is low silicon glass-amperite, with high silicon, high magnesium, low titanium, and very lossy characteristics of light rare earth elements. In situ U-Pb dating of Boan Greenstone showed metamorphic ages of 24.50 ± 0.77 million years and 23.9 ± 3.6 million years, respectively, indicating the regional metamorphism time caused by collisions on the Caroline Ocean Floor Plateau. The volcanic rock sample is a laban basalt with negative anomalies of Nb, Zr, Hf and Ti, and the results of extremely low loss of light rare earth elements, high V/Ti and low Yb/V indicate that the volcanic rock has geochemical characteristics similar to pre-arc basalt. According to the volcanic plagioclase feldspar 40Ar/39Ar dating, these volcanic rocks are 18.76 ± 0.79 million years old and 16.89 ± 0.70 million years old. The volcanic rocks apparently arose after the above-mentioned Boandite metamorphic event, representing magmatic action in the late tectonic period. 

These results suggest that the interaction between the Caroline Ocean Floor Plateau and the Mariana Island Arc began at least 24 million years ago, and may play an important role in shaping the super-trench water depth and island arc tectonic morphology of modern Southwest Mariana. The narrow trench arc distance of the modern Southwest Mariana Island arc may have an important relationship with the arrival of the Caroline Ocean Floor Plateau, which led to collisions and slow subduction, resulting in a large number of metamorphic rocks in front of the arc in this area, which largely inhibited the yield of southwest Mariana Island arc magma. The appearance of island arc volcanic rocks similar to pre-arc basalts 18 million years ago may be related to subduction deceleration and plate fragmentation caused by the ocean floor plateau. The arrival of the Caroline Ocean Floor Plateau has also reshaped the boundary shape of the Philippine Sea Plate and the Caroline Plate, helped the clockwise rotation of the Philippine Sea Plate, and caused a change in the direction of expansion of the Parisivera Basin. 

This study determined the formation age and tectonic evolution history of the Southwest Mariana Island Arc, and revealed the influence of Caroline Plateau subduction on the tectonic evolution of the Southwest Mariana Subduction Zone, which is of great significance for in-depth understanding of the plate dynamics mechanism of the interaction between the ocean floor plateau and the subduction zone. 

The research was supported by the Strategic Leading Special Project of the Chinese Academy of Sciences, the National Key Research and Development Program, and the National Natural Science Foundation of China. (Source: China Science News, Liao Yang, Wang Min)

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