PARTIAL MELTING OF UHP ECLOGITES DURING EXHUMATION, SULU BELT, CHINA—FLUID–MELT EVOLUTION, DEFORMATION AND IMPLICATIONS FOR EXHUMATION DYNAMICS

We summarize results of the past 12 years of integrated research on eclogites of the Sulu belt, one of the most deeply-subducted (possibly > 7 GPa, Ye et al., 2000) UHP eclogites known in the world. Our studies have determined a peak pressure > 5.5 GPa, confirming that continental crust was subducted to and exhumed from great depth, and developed a new understanding of the role of fluid–melt associated with deformation, and the implications for exhumation dynamics in collisional belts.

Detailed structural mapping in Yangkou Bay, central Sulu belt has revealed that intergranular coesite is preserved in rootless isoclinal F1 fold noses. Comparison of the water content of nominally anhydrous minerals (NAMs), plus microstructural, petrological and structural features within and outside fold noses, indicates these structures inherited a drier environment that limited fluid connectivity along grain boundaries in the fold noses, which themselves acted as rigid low strain sites and remained immune to grain-scale fluid infiltration. Strain localization combined with these serendipitous conditions allowed coesite to survive and may provide a guide to discover preserved peak metamorphic mineral assemblages in other orogens.

Migmatites formed by partial melting of UHP eclogite and gneiss was first discovered after detailed mapping at General’s Hill, central Sulu belt. Deeply subducted continental eclogites develop a solute-rich supercritical fluid or melt along grain boundaries by dehydroxylation of nominally anhydrous minerals during the early stage of decompression, before partial melting by breakdown of phengite and/or omphacite during later stages of exhumation. The dominant deformation mechanism of eclogite changed from dislocation creep during the near peak metamorphism to diffusion creep with increasing fluid–melt mobility during exhumation.

Dehydroxylation of garnet and omphacite promoted the generation of supercritical fluid/melt, while the associated weakening activated deformation by diffusion creep in eclogite. This is consistent with the hypothesis that fluid exsolution and partial melting were important in facilitating exhumation of UHP metamorphic rocks from mantle depths, as indicated by numerical modelling and experimental petrology studies.