Petrographic characteristics and metamorphic evolution of ultrahigh-pressure eclogites in plate-collision belts

Ultrahigh-pressure eclogites are metabasic rocks that have equilibrated at pressures within the coesite P-T stability field, This paper reviews their occurrences and petrographic characteristics in exhumed ultrahigh-pressure metamorphic terranes within certain major Phanerozoic continental plate-col lision belts. Unequivocal identification of ultrahigh-pressure eclogites depends on the p resence of relict coesite or of polycrystalline quartz pseudomorphs after e arlier coesite. However, preservation of such distinctive petrographic evid ence is at best sporadic, leading to serious problems over whether ol not p articular eclogite samples have experienced "peak" pressures consistent wit h coesite stability. Accordingly, we also review here other petrographic, m ineral-assemblage, and mineral-chemistry features that may aid in the ident ification of ultrahigh-pressure eclogites, In addition, we discuss the: quantitative evaluation of P-T conditions for formation based on various reaction equilibria, including the dominant garn et and omphacite phases. Various aspects of the mineral compositions are ta ken to indicate that the "peak" metamorphic pressures were most likely atta ined at lower than "peak" temperatures. Hence it is concluded that tight, h airpin-style P-T-time paths for the prograde and retrograde stages in the m etamorphic evolution of ultra-high pressure eclogites that assume synchrono us achievement of P-max and T-max conditions, as deduced in many recently p ublished studies, may be invalid. A rather more open style, clockwise P-T-t ime path, as deduced for the metamorphic evolution of a typical ultrahigh-p ressure phengite-bearing eclogite sample from the Dabie Shan in central Chi na and characterized by a significant further temperature rise during the i nitial phase of relatively rapid exhumation, is considered to be more reali stic.