The retrograded eclogites have been discovered in the middle part of the northern margin of the North China Craton, which occur as lens or boudin within bio-tite-plagioclase gneisses in Paleoproterozoic Hongqiyingzi G...The retrograded eclogites have been discovered in the middle part of the northern margin of the North China Craton, which occur as lens or boudin within bio-tite-plagioclase gneisses in Paleoproterozoic Hongqiyingzi Group. The peak eclogite facies (P > 1.40—1.50 GPa, T = 680—730℃) mineral assemblage is composed of garnet, ompha-cite and rutile (±quartz), which was overprinted by the granulite facies mineral assemblage of vermicular symplec-tite of sodic clinopyroxene and plagioclase which replaced the precursory omphacite, and then amphibolite facies ret-rograded minerals with characterization of Amp+Pl ke-lyphitic rim and symplectite, and amphibole replaced clino-pyroxene. The protolith of retrograded eclogites is oceanic basalt formed at 438±11 Ma.The peak eclogite facies meta-morphic age of the retrograded eclogite is 325±4 Ma. These relict eclogites may be formed by the subduction of Pa-leo-Asian oceanic crust beneath the North China Craton during Late Paleozoic. The discovery of relict eclogite in this paper provides a new insight into farther understanding of tectonic evolution of the northern margin of the North China Craton, and the relationship between the Paleo-Asian Ocean and the North China Craton.展开更多
Abundant occurrences of quartz vein within eclogites in the Dabie-Sulu orogen provide us an opportu- nity to study metamorphic fluid flow during subduction and exhumation of continental crust. It is, however, usually ...Abundant occurrences of quartz vein within eclogites in the Dabie-Sulu orogen provide us an opportu- nity to study metamorphic fluid flow during subduction and exhumation of continental crust. It is, however, usually short of petrological constraints on pressure and tempera- ture of vein formation. This study focuses on kyanite-quartz veins within low-T/high-P eclogite in the Dabie terrane that contain unique polycrystalline aggregates, interpreted as pseudomorphs after porphyroblasts of lawsonite. Coesite pseudomorphs were found for the first time in garnet from eclogite, resulting in a revised estimate of peak P-T condi- tions at 670℃ and 3.3 GPa for the eclogite. This indicates a stability field at graphite/diamond transition, thus upgrading the HP unit to a UHP unit. Neither foliation texture, undulose extinction of quartz nor coesite were observed in quartz veins, although the peak P-T conditions were estimated the same as that in host eclogite in light of thermodynamic calculation based on mineral assemblage in kyanite-quartz veins. Therefore, the formation of the kyanite-quartz veins as well as the breakdown of lawsonite into the kyanite-quartz-zoisite assemblage took place during exhumation subsequent to the peak UHP conditions. In this regard, the continental subduc- tion not only brought the water of water-bearing mineral such as lawsonite into the deep mantle, but also released the water from the mineral during exhumation.展开更多
文摘The retrograded eclogites have been discovered in the middle part of the northern margin of the North China Craton, which occur as lens or boudin within bio-tite-plagioclase gneisses in Paleoproterozoic Hongqiyingzi Group. The peak eclogite facies (P > 1.40—1.50 GPa, T = 680—730℃) mineral assemblage is composed of garnet, ompha-cite and rutile (±quartz), which was overprinted by the granulite facies mineral assemblage of vermicular symplec-tite of sodic clinopyroxene and plagioclase which replaced the precursory omphacite, and then amphibolite facies ret-rograded minerals with characterization of Amp+Pl ke-lyphitic rim and symplectite, and amphibole replaced clino-pyroxene. The protolith of retrograded eclogites is oceanic basalt formed at 438±11 Ma.The peak eclogite facies meta-morphic age of the retrograded eclogite is 325±4 Ma. These relict eclogites may be formed by the subduction of Pa-leo-Asian oceanic crust beneath the North China Craton during Late Paleozoic. The discovery of relict eclogite in this paper provides a new insight into farther understanding of tectonic evolution of the northern margin of the North China Craton, and the relationship between the Paleo-Asian Ocean and the North China Craton.
基金supported by the National Natural Science Foundation of China(Grant No.40372034)the Ministry of Science and Technology of China(Grant No.1999075503).
文摘Abundant occurrences of quartz vein within eclogites in the Dabie-Sulu orogen provide us an opportu- nity to study metamorphic fluid flow during subduction and exhumation of continental crust. It is, however, usually short of petrological constraints on pressure and tempera- ture of vein formation. This study focuses on kyanite-quartz veins within low-T/high-P eclogite in the Dabie terrane that contain unique polycrystalline aggregates, interpreted as pseudomorphs after porphyroblasts of lawsonite. Coesite pseudomorphs were found for the first time in garnet from eclogite, resulting in a revised estimate of peak P-T condi- tions at 670℃ and 3.3 GPa for the eclogite. This indicates a stability field at graphite/diamond transition, thus upgrading the HP unit to a UHP unit. Neither foliation texture, undulose extinction of quartz nor coesite were observed in quartz veins, although the peak P-T conditions were estimated the same as that in host eclogite in light of thermodynamic calculation based on mineral assemblage in kyanite-quartz veins. Therefore, the formation of the kyanite-quartz veins as well as the breakdown of lawsonite into the kyanite-quartz-zoisite assemblage took place during exhumation subsequent to the peak UHP conditions. In this regard, the continental subduc- tion not only brought the water of water-bearing mineral such as lawsonite into the deep mantle, but also released the water from the mineral during exhumation.
