ON THE SURVIVAL OF INTERGRANULAR COESITE IN UHP ECLOGITE

The survival of intergranular coesite in UHP eclogite at Yangkou (Sulu belt, eastern China) is surprising and implies low a_(H2O) and minimal interstitial fluid throughout exhumation. The dominant structures in the eclogite are a strong D₂ foliation associated with tight-to-isoclinal F₂ folds overprinted by close-to-tight F₃ folds. The coesite-bearing eclogite occurs as rootless isoclinal F₁ fold noses wrapped by a composite S₁–S₂ foliation in interlayered Ph-bearing quartz-rich schist. Samples from coesite eclogite, Ph-bearing coesite eclogite (2–3 vol.% Ph) and quartz eclogite (6–7 vol.% Ph) were selected to evaluate controls on the survival of intergranular coesite at Yangkou. Since the variation in modal Ph implies higher a_(H2O) and higher structural water (SW) concentrations from coesite to quartz eclogite, we determined the number density of intergranular coesite grains per cm² and measured the amount of SW in Grt and Omp. As the coesite decreases in amount the SW in eclogite increases from 116/197 ppm H₂O in the coesite eclogite to 174–290 ppm in the Ph-bearing coesite eclogite to 386/432 ppm in the quartz eclogite. If the SW contents reflect original differences locked in during the late prograde evolution, this trend implies that the amount of fluid stored in the NAMs was higher outside of the F₁ fold noses. During exhumation, dehydroxylation of NAMs may generate a fluid phase. Strong D₂ deformation likely promoted interconnection of the fluid and migration along the developing S₂ foliation, enabling conversion of the intergranular coesite into quartz outside the F₁ fold noses. By contrast, the eclogite forming the F₁ fold noses behaved as independent rigid bodies within the composite S₁–S₂ foliation of the surrounding schist. Thus, any exsolved fluid remained isolated in pores and the fold noses remained immune to fluid penetration from outside, and dehydroxylation during exhumation did not produce sufficient fluid to convert coesite to quartz. This inherited drier environment in the F₁ fold noses was maintained during exhumation by deformation partitioning and strain localization in the schist, allowing coesite to survive. Thus, variations in the amount of inherited SW and the effects of strain partitioning are important in the survival of coesite during exhumation of deeply subducted continental crust.