It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Easte...It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Eastern Hebei, trondhjemites occur as small dikes, intrusions or leucosomes in tonalitic gneisses, suggesting their origin of in-situ partial melting. Based on the petrological analysis of a tonalitic gneiss sample from Eastern Hebei, in combination with zircon U-Pb dating, we investigated the petrogenesis of trondhjemite through simulating anatectic reactions and the major and trace element characteristics of the product melt at different pressures(0.7, 1.0 and 2.0 GPa). The results indicate that hornblende dehydration melting in a tonalitic gneiss at 0.9–1.1 GPa and 800–850°C, corresponding to the high-T granulite facies, with melting degrees of 5–10wt.% and a residual assemblage containing 5–10wt.% garnet, can produce felsic melts with a great similarity, for instance of high La/Yb ratios and low Yb contents to the trondhjemitic rocks from Eastern Hebei. However, the modelled melts exhibit relatively higher K2 O, and lower CaO and Mg~# than those in the trondhjemitic dikes and leucosomes from Eastern Hebei, suggesting that the leucosomes may not only contain some residual minerals but also be subjected to the effect of crystal fractionation. The zircon U-Pb dating for the tonalitic and trondhjemitic rocks in the Eastern Hebei yields a protolith age of 2518±12 Ma and a metamorphic age of 2505±19 Ma for the tonalitic gneiss. The latter age is consistent with a crystallization age of 2506±6 Ma for the trondhjemitic rock, confirming a close petrogenetic relation between them.展开更多
The early Precambrian high-grade metamorphosed basement in the Xi Ulanbulang area, central Inner Mongolia of China, is composed mainly of intermediate granulites and charnockitic gneisses. Both types of the rocks are ...The early Precambrian high-grade metamorphosed basement in the Xi Ulanbulang area, central Inner Mongolia of China, is composed mainly of intermediate granulites and charnockitic gneisses. Both types of the rocks are closely associated spatially and temporally, with a gradual variation between them. In order to understand timing of the high-grade metamorphism, we carried out SHRIMP U-Pb dating of zircons of the rocks. Zircons from the granulites and charnockitic gneisses are similar in structure and age. Zircon cores show magmatic zoning and have ages of 2507-2545 Ma. The ages are interpreted as the forming time of protolith of the granulites and charnockitic gneisses, indicating that a strong magmatism existed at that time in the Yinshan Block. The zircon mantles and rims show homogeneous structures and record a strong granulite facies metamorphism event around 2500 Ma, with a time interval between the metamorphism and magamatism being less than 50 Ma. These suggest that the Western Block was similar to the Eastern Block in tectono-thermal timing at the end of the Neoarchean.展开更多
基金supported by the National Natural Science Foundation of China (Grant No. 41430207)
文摘It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Eastern Hebei, trondhjemites occur as small dikes, intrusions or leucosomes in tonalitic gneisses, suggesting their origin of in-situ partial melting. Based on the petrological analysis of a tonalitic gneiss sample from Eastern Hebei, in combination with zircon U-Pb dating, we investigated the petrogenesis of trondhjemite through simulating anatectic reactions and the major and trace element characteristics of the product melt at different pressures(0.7, 1.0 and 2.0 GPa). The results indicate that hornblende dehydration melting in a tonalitic gneiss at 0.9–1.1 GPa and 800–850°C, corresponding to the high-T granulite facies, with melting degrees of 5–10wt.% and a residual assemblage containing 5–10wt.% garnet, can produce felsic melts with a great similarity, for instance of high La/Yb ratios and low Yb contents to the trondhjemitic rocks from Eastern Hebei. However, the modelled melts exhibit relatively higher K2 O, and lower CaO and Mg~# than those in the trondhjemitic dikes and leucosomes from Eastern Hebei, suggesting that the leucosomes may not only contain some residual minerals but also be subjected to the effect of crystal fractionation. The zircon U-Pb dating for the tonalitic and trondhjemitic rocks in the Eastern Hebei yields a protolith age of 2518±12 Ma and a metamorphic age of 2505±19 Ma for the tonalitic gneiss. The latter age is consistent with a crystallization age of 2506±6 Ma for the trondhjemitic rock, confirming a close petrogenetic relation between them.
基金supported by National Natural Science Foundation of China (Grant Nos. 90814006, 40972135)Geological Survey of China (Grant No. 1212010510515)
文摘The early Precambrian high-grade metamorphosed basement in the Xi Ulanbulang area, central Inner Mongolia of China, is composed mainly of intermediate granulites and charnockitic gneisses. Both types of the rocks are closely associated spatially and temporally, with a gradual variation between them. In order to understand timing of the high-grade metamorphism, we carried out SHRIMP U-Pb dating of zircons of the rocks. Zircons from the granulites and charnockitic gneisses are similar in structure and age. Zircon cores show magmatic zoning and have ages of 2507-2545 Ma. The ages are interpreted as the forming time of protolith of the granulites and charnockitic gneisses, indicating that a strong magmatism existed at that time in the Yinshan Block. The zircon mantles and rims show homogeneous structures and record a strong granulite facies metamorphism event around 2500 Ma, with a time interval between the metamorphism and magamatism being less than 50 Ma. These suggest that the Western Block was similar to the Eastern Block in tectono-thermal timing at the end of the Neoarchean.
