J. Gao et R. Klemd, Primary fluids entrapped at blueschist to eclogite transition: evidence from the Tianshan meta-subduction complex in northwestern China, CONTR MIN P, 142(1), 2001, pp. 1-14
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.