Geochemistry and petrogenesis of gabbroic rocks from the Yap forearc in the western Pacific: Implications for early arc magmatism

Abstract

Herein, we analyze the elemental composition and Sr–Nd–Pb isotopic data of the gabbroic rocks (gabbros and gabbroic diorites) from the forearc of the Yap trench in the western Pacific to investigate the early arc magmatism. The gabbroic diorites, which exhibit a negative Eu anomaly and relatively flat REE patterns with slight LREE depletion, represent the evolved magma that has undergone varying degrees of fractional crystallization. In contrast, the gabbros, which exhibit a positive Eu anomaly and a significant depletion of LREE, can be further classified into two groups. The noncumulate Group 1 gabbros have composition closely resembles that of gabbroic diorites and the Group 2 gabbros represents cumulate rocks formed through melt crystallization. Most of the studied gabbroic rocks exhibit negative anomalies in high-field-strength elements, such as Nb, Zr, Hf, and Ti, accompanied by very low Ti/V and Nb/Yb ratios. Based on these features, they are suggested to originate from the partial melting of a depleted forearc mantle, which is supported by their high 143Nd/144Nd ratios that are similar to those of forearc basalts (FAB). The forearc origin of gabbroic rocks is further supported by their plagioclase composition, which exhibits a higher anorthite content (An: 86–99) compared to the plagioclase in backarc and mid-ocean ridge gabbros when coexisting with identical clinopyroxene composition. However, even though the major- and trace-element compositions of the Yap gabbroic rocks are similar to those of FAB, their large-ion lithophile element content (e.g., Cs, Rb, Ba, and Th) and 87Sr/86Sr ratios considerably exceed the values of FABs, highlighting their similarity to boninites. In addition, their Ba/Yb and Th/Yb ratios are higher than the mid-ocean ridge basalt values and plot between the FAB and boninite fields in Ba/Yb versus Th/Yb diagram. Consequently, we propose that the Yap gabbroic rocks evolved from an early arc magma, which was produced by a combination of decompression melting and fluid-flux melting. Thus, they exhibit a transitional composition that falls between FABs and boninites. The exposure of intrusive rocks related to the early arc magmatism in the Yap forearc is a consequence of the extensive tectonic erosion induced by the collision of the Caroline ridge with the trench. This collision resulted in the erosion of the volcanic crust, exposing the underlying plutonic rocks. Therefore, intrusive rocks likely hold a preserved record of the early arc magmatism in subduction zones exhibiting extensive tectonic erosion. The Yap arc system is characterized by multiple episodes of magmatism, and the collision with the Caroline ridge may prevent the Yap arc magmas from evolving into mature arc magmas.