Petrology of Tourmaline-Bearing Blueschist from SW Tianshan, China, and Its Implication in B-Rich Fluid Migration in Subduction Zone

Abstract

Boron geochemistry can track fluid-rock interaction during subduction zone metamorphism. Rare tourmaline-bearing blueschists, which are associated with ultrahigh-pressure (UHP) serpentinites are first recognized in SW Tianshan, China. Detailed petrology, whole-rock and mineral chemistry, B isotope analysis, and modeling characterized two consecutive stages of tourmaline crystallization (Tur-I, Tur-II). Tourmaline included in, or intergrown with, garnet and the cores of tourmaline in rock matrixes and veins are Tur-I, which grew during prograde metamorphism at 430–460/470 °C, ~1.9–2.1 GPa. The rims of tourmaline in rock matrixes and veins are Tur-II, which formed during initial exhumation at 460–490 °C, ~2.1–1.7 GPa. Variable ẟ11B values of tourmaline (+8‰, Tur-I to –2‰, Tur-II) point to a 11B-rich signature of the fluid infiltrating at Stage I. With progressing metamorphism, ẟ11B decreased in the fluid. The high-ẟ11B Tur-I (up to +8‰) could not have crystallized from fluid released from the high-pressure metapelites (–12 to –7‰) and metabasites (–15 to –5‰) surrounding the tourmaline host rocks given the lower ẟ11B values. Modeling of B isotope fractionation yields the ẟ11B values of –9 to –5‰, –11 to –1‰, and +8 to +17‰ for the fluids equilibrium with the restitic metapelites, metabasites, and serpentinites, respectively. The tourmaline and whole-rock B isotope data, along with the tourmaline compositions, point to the associated serpentinites as source of the fluid that infiltrated the metamorphic rocks. This fluid was released by the partial dehydration of serpentinites through the reaction antigorite + brucite = olivine + water at forearc depth. We propose that metabasites in subduction zones can acquire 11B-rich signatures through interaction with serpentinite-derived fluids, leading to the formation of robust tourmaline minerals at shallow levels. As a new reservoir of heavy boron, these metabasites can then transport this signature to greater depths.