Geochemical characteristics of serpentinite types and their implications for tectonic environments

In eastern Taiwan, the metamorphic rocks of the Yuli Belt are mainly derived from subducted oceanic sediments, metabasite, arc-related volcanic rocks, and serpentinite bodies, which were subsequently exhumed from the subduction zone. This study focuses on serpentinites in Taiwan, including those from two distinct tectonostratigraphic units, the Yuli Belt and the Coastal Range, and aims to distinguish different types of serpentinites based on their mineral assemblages and geochemical characteristics. Based on mineralogy, microstructures, and geochemical features, serpentinites can be broadly classified into three major types in Taiwan. The first type, Eastern Taiwan Ophiolite serpentinites (ETO), is predominantly derived from oceanic crust, occurring as blocks within the mudstone of the Lichi Mélange. These serpentinites are mainly composed of mesh-textured fibrous chrysotile, sometimes containing incompletely serpentinized relict olivine. They lack subduction-related fluid signatures such as As and Pb, and display As/Ce ratios lower than 20. The second type located within the subduction zone of Yuli Belt. These serpentinites accompany with the schists, and are dominated by bladed antigorite. The subduction-related fluid metasomatism bring more As, Pb and Sb into serpentinite. Variations in As/Ce ratios reflect the shallower subduction depths in northern Yuli Belt and greater depths in the southern Yuli Belt. The third type comprises high-temperature metamorphic serpentinites. Their antigorite crystal morphology is distinctly different from the bladed form, having transformed into extremely fine-grained antigorite indicative of high-temperature recrystallization. New olivine porphyroblasts formed during high-temperature metamorphism, and magnetite aggregates developed around these olivine grains. They exhibit the lowest As/Ce ratios, and fluid-related elements such as As, Pb, and Sb are significantly depleted. The third type of high-temperature metamorphic serpentinite usually appears as large xenoliths in second type serpentinite in the northern part of the Yuli belt, while the metamorphic temperature of the surrounding schist is only 420-470°C, indicating that the third type of serpentinite was encapsulated and squeezed up in a state of plastic flow. This implies that the rheological behavior of serpentinites within the mantle wedge may be highly complex.