Assemblages and Textural Evolution of UHP Eclogites from the Chinese Continental Scientific Drilling Main Hole

Coesite-bearing eclogite samples from the Chinese Continental Scientific Drilling main hole (100–2000 m) were examined. Three major evolutionary stages are distinguished from mineral assemblages and textural relations. (1) The pre-peak stage is indicated by inclusion assemblages in rutile eclogite such as amphibole and paragonite, with pre-peak P-T conditions of ~13 kbar and ~910ºC. (2) The peak ultrahigh-pressure (UHP) metamorphic stage is characterized by the mineral assemblage garnet-omphacite-phengite-rutile-apatite-coesite/quartz pseudomorph ± kyanite; P-T conditions reached ~910ºC and ~37 kbar for rutile eclogite, and ~850ºC and ~35 kbar for phengite eclogite. (3) The retrograde stage produced decompression textures via back reactions, such as thin coronas of amphibole or zoisite on garnet, fine-grained amphibole-plagioclase symplectites on omphacite and biotite-plagioclase, or K-feldspar-albite on phengite, ilmenite or titanite on rutile, at P-T conditions of 530–560ºC and 7–9 kbar. The presence of K-feldspar–albite symplectite on phen-gite and minor interstitial K-feldspar implies that post-peak decompressional partial melting occurred locally during rapid exhumation of the subducted UHP slab. Two alternative P-T paths may be constructed: (1) the first possesses a different pre-peak stag