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

This article focuses on the petrology, geochemistry, and boron (B) isotope systematics of rare tourmaline-bearing blueschists associated with ultrahigh-pressure (UHP) serpentinites in southwestern Tianshan, China, to elucidate fluid–rock interactions during subduction zone metamorphism. Two stages of tourmaline (dravite) crystallization were identified: Tur-I formed during prograde metamorphism at ~430–470°C and 1.9–2.1 GPa, and Tur-II formed during initial exhumation at ~460–490°C and 2.1–1.7 GPa. Boron isotope data (δ¹¹B) show a decrease from +8‰ in tourmaline cores to −2‰ at rims, indicating that the B-rich fluid originated from partial dehydration of serpentinites (via antigorite + brucite = olivine + water) at forearc depths, rather than from surrounding metapelites or metabasites with lower δ¹¹B values. The study proposes that such tourmaline-bearing metabasites act as a new reservoir of heavy boron, potentially transporting ¹¹B-enriched signatures deeper into the mantle during subduction.