The dynamics of the Sorol Trough magmatic system: Insights from bulk‐rock chemistry and mineral geochemistry of basaltic rocks

Abstract

The Yap trench‐island arc system, where three tectonic plates (i.e., the Philippine Sea Plate, Pacific Plate, and Caroline Plate) interact, has undergone a relatively complicated geological evolution. East of the system, the Caroline Plateau are being subducted, and the plateau has been rifted into east and west Caroline ridges by the development of the Sorol Trough. However, until now, knowledge of the nature of the basement of the Sorol Trough is lacking, which limits our understanding of the complete tectonic evolution of the Yap island arc‐trench system and the Caroline Plateau. In this study, we performed petrological and mineral geochemical (in situ major element compositions and trace element compositions) analysis of newly acquired subducting basalt samples from the Sorol Trough segment, which is being subducted along the Yap Trench, and K‐Ar age of the Sorol basalts is 23.8 ± 0.7 Ma. The results show that phenocryst minerals of basalt from Sorol Trough are mainly olivine and plagioclase, with minor clinopyroxene microlites. The forsterite (Fo) values of the olivine phenocrysts in these rocks vary from 84.97 to 86.85, and plagioclase (Pl) phenocrysts are predominantly labradorite, with a few andesine plagioclase microlites. Bulk‐rock chemical compositions confirm that these rocks are intraplate alkaline basalts and show ocean island basalt (OIB)‐like geochemical characteristics. The mantle source of these basement basalts could be peridotite, and these Sorol Trough basalts may have been formed by 5–10% partial melting of mantle spinel lherzolite. The potential temperature of the mantle beneath the Sorol Trough was inferred to be 1,331–1,393°C, shows that there is no thermal anomaly in the mantle source. In addition, the crystallization temperatures of the clinopyroxenes (1,068–1,263°C) and plagioclase (702–1,216°C) and their compositional characteristics indicate that the magmatic processes have a conspicuously fast rate of magma upwelling. In conclusion, the basalts in the Sorol Trough were the products of low‐degree partial melting of a relatively enriched mantle source that passively upwelled as the Sorol Trough separated the Caroline Ridge into its western and eastern parts.