Petrographic Characteristics and Metamorphic Evolution of Ultrahigh-Pressure Eclogites in Plate-Collision Belts

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Abstract
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 terrenes within certain major Phanerozoic continental plate—collision belts. Unequivocal identification of ultrahigh-pressure eclogites depends on the presence of relict coesite or of polycrystalline quartz pseudomorphs after earlier coesite. However, preservation of such distinctive petrographic evidence is at best sporadic, leading to serious problems over whether or not particular eclogite samples have experienced “peak” pressures consistent with coesite stability. Accordingly, we also review here other petrographic, mineral-assemblage, and mineral-chemistry features that may aid in the identification 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 garnet and omphacite phases. Various aspects of the mineral compositions are taken to indicate that the “peak” metamorphic pressures were most likely attained at lower than “peak” temperatures. Hence it is concluded that tight, hairpin-style P-T-time paths for the prograde and retrograde stages in the metamorphic evolution of ultra-high pressure eclogites that assume synchronous achievement of Pmax and Tmax conditions, as deduced in many recently published studies, may be invalid. A rather more open style, clockwise P-T-time path, as deduced for the metamorphic evolution of a typical ultrahigh-pressure phengite-bearing eclogite sample from the Dabie Shan in central China and characterized by a significant further temperature rise during the initial phase of relatively rapid exhumation, is considered to be more realistic.