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
High-pressure metamorphic eclogite, as a window of study for depth level of lowermost crust and mantle, has an important indicating significance to composition, property and accretion of continental lower crust. Archa...High-pressure metamorphic eclogite, as a window of study for depth level of lowermost crust and mantle, has an important indicating significance to composition, property and accretion of continental lower crust. Archaean high-pressure metamorphic rocks of eclogite facies are very rare. According to this fact, it is suggested that there is a difference in principle between the mechanisms of formation and evolution of early Precambrian and Phanerozoic continental crust. Smelov and Beryozkin first reported that some展开更多
The rheology of rocks at depth remains a key point in earth sciences. Deformation of high-temperature–highpressure rocks from the subduction zone has not been fully studied; in particular, the deformation behavior of...The rheology of rocks at depth remains a key point in earth sciences. Deformation of high-temperature–highpressure rocks from the subduction zone has not been fully studied; in particular, the deformation behavior of eclogites remains poorly understood. This research is focused on the microstructure and fabric of retrograded eclogites from the Hongqiyingzi Complex in Chicheng, north Hebei, China, based on photomicrography, scanning electron microscopy(SEM) and electron backscattered diffraction(EBSD) analysis. The analytical results show that plastic deformation occurred in the garnets during exhumation, but they do not show an obvious lattice preferred orientation(LPO). This can be interpreted as being caused by the simultaneous activation of multiple slip systems in the garnets during deformation by dislocation creep. The plagioclases have a special fabric; the(001) LPO presents a maximum in Z direction which can be correlated with a new [100](001) slip system. The misorientation angle distribution(MAD) of the plagioclases in the deformed retrograded garnet augens shows a special bimodal distribution with peaks in both a low-angle range(<40?) and a high-angle range(>140?). In the retrograded garnet augens(i.e. "white eye socket" garnets) the maximum peak moves from a high-angle range to a low-angle range when we analyze only those plagioclases surrounding the residual garnet porphyroclast. Deformation behavior is controlled by the crystallographic orientation of the host grain and the grain boundary sliding process. Hornblendes in different layers and the retrograded garnet augens show almost the same strong LPO patterns, which are correlated with the [001](010) slip system; and MAD diagrams show a peak in a low-angle range(<40?). Integrating recent studies on metamorphism and geochronology, we argue that ductile deformation occurred during the exhumation periods together with retrograded metamorphism.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.
文摘High-pressure metamorphic eclogite, as a window of study for depth level of lowermost crust and mantle, has an important indicating significance to composition, property and accretion of continental lower crust. Archaean high-pressure metamorphic rocks of eclogite facies are very rare. According to this fact, it is suggested that there is a difference in principle between the mechanisms of formation and evolution of early Precambrian and Phanerozoic continental crust. Smelov and Beryozkin first reported that some
基金supported by the National Natural Science Foundation of China(Grant No.41430207)the National Basic Research Program of China(Grant No.2013CB429804)the Special Fund for Basic Research of Central Universities(Grant No.310827161010)
文摘The rheology of rocks at depth remains a key point in earth sciences. Deformation of high-temperature–highpressure rocks from the subduction zone has not been fully studied; in particular, the deformation behavior of eclogites remains poorly understood. This research is focused on the microstructure and fabric of retrograded eclogites from the Hongqiyingzi Complex in Chicheng, north Hebei, China, based on photomicrography, scanning electron microscopy(SEM) and electron backscattered diffraction(EBSD) analysis. The analytical results show that plastic deformation occurred in the garnets during exhumation, but they do not show an obvious lattice preferred orientation(LPO). This can be interpreted as being caused by the simultaneous activation of multiple slip systems in the garnets during deformation by dislocation creep. The plagioclases have a special fabric; the(001) LPO presents a maximum in Z direction which can be correlated with a new [100](001) slip system. The misorientation angle distribution(MAD) of the plagioclases in the deformed retrograded garnet augens shows a special bimodal distribution with peaks in both a low-angle range(<40?) and a high-angle range(>140?). In the retrograded garnet augens(i.e. "white eye socket" garnets) the maximum peak moves from a high-angle range to a low-angle range when we analyze only those plagioclases surrounding the residual garnet porphyroclast. Deformation behavior is controlled by the crystallographic orientation of the host grain and the grain boundary sliding process. Hornblendes in different layers and the retrograded garnet augens show almost the same strong LPO patterns, which are correlated with the [001](010) slip system; and MAD diagrams show a peak in a low-angle range(<40?). Integrating recent studies on metamorphism and geochronology, we argue that ductile deformation occurred during the exhumation periods together with retrograded metamorphism.
