Fluid-rock interactions during ultra-high pressure metamorphism, Dabie Shan, China

Abstract

Oxygen isotope analyses of garnet separates and whole rock samples of gneiss- and marble-hosted eclogites from the “ultra-high” pressure (UHP) Dabie Shan terrain fall in the range −6.76‰ to 10.76‰. Comparison of the isotopic compositions of the eclogites with those of their host rocks leads to the following conclusions: 1) The basaltic protolith of the eclogites had isotopic compositions in the range 2.15 to 4.99‰. It is proposed that this is the result of contamination of the parent basaltic magma with hydrothermally-altered 18O-depleted source materials. 2) 18O-enriched eclogite compositions resulted from local-scale (<3 m) interaction between this basaltic eclogite protolith and fluids produced by decarbonation of impure marble lithologies during UHP metamorphism. 3) 18O-depleted eclogite (and gneiss) compositions resulted from regional-scale meteoric-hydrothermal alteration of the basalts and their host rocks prior to UHP metamorphism. Epidote separates and whole-rocks from the cores and rims of mineralogically-zoned eclogite boudins within amphibolite-grade biotite gneisses show no distinct isotopic differences. This suggests that the preferential recrystallisation of the gneisses and the associated amphibolite-grade overprint of the rims of eclogite bodies could not have resulted from regional-scale infiltration of significant volumes of fluid during the exhumation of the Dabie Shan terrain.