Abstract
The Yap Arc was considered to have derived from the proto-Yap-Izu Bonin-Mariana arc system. It is a rare example of subduction of an oceanic plateau with short arc–trench distance and consists mainly of metamorphic rocks, which are distinct from the Izu Bonin-Mariana arc system. However, it is not known on the origin and formation time of the basement rocks of the Yap Arc. For the first time, in this study we provide the results of radiometric age-dating and whole-rock major element, trace element and Sr–Nd–Pb–Hf isotope analyses for the metamorphic basement rocks (amphibolites) of the Yap Arc. These rock samples can be classified into three groups based on their major and trace element compositions. Group 1 samples have strong depletion of LREEs and a Dupal-type isotopic signature, and they represent forearc basalts (FABs) of the nascent Yap forearc. Group 2 samples have an alkali basalt-like geochemistry and oceanic island basalts (OIBs) type trace element patterns, and they represent the basalts of the Caroline Plateau. Group 3 samples have island-arc-type negative anomalies of Nb–Ta–Zr–Hf, and they may represent the volcanic rocks of the proto-Yap Arc. These amphibolites were formed at 21 Ma according to 40Ar/39Ar amphibole dating and in situ SIMS UPb titanite dating, and they record the timing of the collisional event between the Caroline Plateau and the proto-Yap Arc. We propose that the Yap arc has been evolving distinctly from the Izu-Bonin-Mariana Arc since this collision time. These rocks were metamorphosed and brought onto the present Yap Arc as a result of the collision between the proto-Yap Arc and the Caroline Plateau. Based on the new results of age-dating and geochemistry of this study, we propose three stages of tectonic evolution for the Yap Arc as a result of collision from the Caroline plateau: an early stage (proto-Yap Arc) of subduction of the Caroline Plate, collision of the proto-Yap Arc with the in-coming young Caroline Plateau, and rejuvenated plate subduction at the Yap Trench with limited volcanism.
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