ON THE NATURE OF METAMORPHIC SULFIDE MELT INCLUSIONS IN SUBDUCTION-RELATED ULTRAMAFIC ROCKS

The process of metamorphism is capable of redistributing chemical elements and creating new minerals through solid state diffusion and partial melting. With an increasing grade of metamorphism, the change in mineral assemblage and likelihood of partial melting increases among silicate phases while the effect on non-silicate phases (i.e. sulfides, oxides and salts) is less known. Current experimental data constrain sulfide melting during anatexis to granulite (720° C at 5kbars) and peak amphibolite facies (~500° C). However, a record of such melts in natural samples is scarce. We present petrographic and mineralogical data from the Bay Islands Accretionary Complex (BIAC), an exhumed accretionary terrain composed of granites, amphibolites, meta- ultramafic and meta-mafic rocks. The BIAC is situated in the Bay Islands archipelago chain along the Motagua-Swann Islands fault zone in the Caribbean Sea, north of Honduras. Extensively deformed meta-ultramafic rocks composed of pyroxene, biotite and amphibole host parallel and intersecting trails of spherical, elliptical and dumbbell shaped sulfide globules up to 10 microns residing in pyroxene. These globules are composed of chalcopyrite, pyrrhotite, pentlandite, and Co-rich pentlandite. The distribution, shape and composition of these sulfide globules suggest their derivation from a homogenous sulfide melt that percolated along grain-fracture boundaries during brittle deformation under greenschist-amphibolite facies conditions that were annealed during later stages of metamorphism. The exhumation of the BIAC served as a catalyst for adiabatic partial melting assisted by presence of water-rich fluid (abundant biotite selvages). This process essentially isolated newly formed sulfide assemblages during the otherwise brittle-ductile deformation of the accretionary complex in a typical subduction environment. The metamorphic processes documented in BIAC meta-ultramafic rocks facilitate extraction of sulfide-forming components, their transport and accumulation in the form of sulfide melts in the crust and appear to be underappreciated in the formation of sulfide mineralization in subduction-related and other tectonic settings.