Primary fluids entrapped at blueschist to eclogite transition: evidence from the Tianshan meta-subduction complex in northwestern China

The Tianshan is the first locality worldwide where primary fluids at bluesc hist to eclogite transition have been documented. Veins containing high-pre ssure minerals in massive host eclogites transitional to blueschists, or ec logite boudins surrounded by blueschists, indicate that a free fluid phase was present at the time of eclogitization. The high-pressure veins are pred ominantly composed of omphacite fibers with minor quartz or calcite. The tr ansition from blueschist- to eclogite-facies parageneses occurs as "dehydra tion" halos around these veins. Clinozoisite, paragonite, glaucophane, and omphacite inclusions preserved in garnet porphyroblasts in wall eclogites d ocument the transformation of blueschist to eclogite. C-axis-parallel, non- planar populations of fluid inclusions were trapped during the growth of om phacite in high-pressure veins and dehydrated wall rocks. Low salinity H2O + NaCl +/- solid-bearing inclusions are preserved in omphacite fibers in ve ins and matrix omphacite of wall rocks. None of the isochores of these low salinity aqueous fluid inclusions intersect peak eclogite-facies metamorphi c conditions, suggesting that, although the textural evidence constrains th e entrapment of fluid inclusions to peak metamorphic conditions, their dens ities must have been modified during exhumation. The fluids are interpreted to have been derived from the host blueschist as a result of dehydration r eactions such as 13 Gln + 5 Czo = 9 Prp + 26 Jd + 12 Di + 19 Qtz + 15 H2O a nd Gln + Pg = Prp + 3 Jd + 2 Otz + 2 H2O. The similarity of vein and wall r ock mineral compositions, fluid inclusion characteristics and O-isotope dat a also favor an internal source for the fluids. The major element compositi on of veins indicate that Si, Na, and Ca-rich aqueous fluids were released during dehydration at a depth of 50 +/- 10 km within a Paleozoic subduction zone.