A Study of Metamorphic Characteristics of Laoling Group in Southern Jilin Province
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Metamorphic minerals are widely developed in Laoling Group of southern Jilin Province.Based on a detailed analysis of the metamorphic minerals in rock samples,the authors hold that the main minerals in lower sub-group of Laoling Group can be divided into two generations:M1 and M2,which represent two metamorphic stages.Researches on metamorphic assemblage,distribution characteristics and division of metamorphic facies show that M1 is characterized by low-to-moderate temperature regional dynamic heat flow metamorphism which can be divided into two metamorphic facies:lower greenschist facies and high greenschist facies.By contrast,M2 is represented by local thermal metamorphism which can be divided into three metamorphic facies:lower greenschist facies,high greenschist facies and lower amphibolitic facies.Comprehensive studies show that metamorphism of the sub-group of Laoling Group in southern Jilin Province is characterized by reduction of influencing area and decrease of intensity from early to late.This tendency suggests hat the earth's crust became stable in Paleo Proterozoic.Keywords:
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Abstract New U– Pb ages of zircons from migmatitic pelitic gneisses in the Omuta district, northern Kyushu, southwest Japan are presented. Metamorphic zonation from the Suo metamorphic complex to the gneisses suggests that the protolith of the gneisses was the Suo metamorphic complex. The zircon ages reveal the following: (i) a transformation took place from the high‐ P S uo metamorphic complex to a high‐ T metamorphic complex that includes the migmatitic pelitic gneisses; (ii) the detrital zircon cores in the Suo pelitic rocks have two main age components ( ca 1900–1800 Ma and 250 Ma), with some of the detrital zircon cores being supplied (being reworked) from a high‐grade metamorphic source; and (iii) one metamorphic zircon rim yields 105.1 ±5.3 Ma concordant age that represents the age of the high‐ T metamorphism. The high‐ P to high‐ T transformation of metamorphic complexes implies the seaward shift of a volcanic arc or a landward shift of the metamorphic complex from a trench to the sides of a volcanic arc in an arc–trench system during the Early Cretaceous. The Omuta district is located on the same geographical trend as the Ryoke plutono‐metamorphic complex, and our estimated age of the high‐ T metamorphism is similar to that of the Ryoke plutono‐metamorphism in the Yanai district of western Chugoku. Therefore, the high‐ T metamorphic complex possibly represents the western extension of the Ryoke plutono‐metamorphic complex. The protolith of the metamorphic rocks of the Ryoke plutono‐metamorphic complex was the Jurassic accretionary complex of the inner zone of southwest Japan. The high‐ P to high‐ T transformation in the Omuta district also suggests that the geographic trend of the Jurassic accretionary complex was oblique to that of the mid‐Cretaceous high‐ T metamorphic field.
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This paper presents the petrologic characteristics of the granulite facies metamorphic rocks found for the first time in the eastern Nanling, South China. Their protoliths are sedimentary rocks based on the mineral assemblages and compositions. Petrogenetic grid and T-P calculations indicate that the peak metamorphism took place at temperature of 750 -880℃ and pressure of ~ 1.1GPa. It is suggested that these surficial sediments were carried into lower crust by some tectonic processes, and underwent granulite facies metamorphism. P-T-t path of isothermal decompression indicates rapid exhumation history, which was likely related with Mesozoic extension. The compositions of the Nanling granulites are evidently different from those of mafic granulite xenoliths in the coastal region of southeastern China, implying that the lower crust components beneath both regions are possibly different. Accordingly, the distinct type of granites in both regions resulted probably from the difference of lower crustal composition.
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The metamorphic rocks exposed in the Sefuri and Tenzan areas, northwestern Kyushu, are composed mainly of mafic rocks with trace amounts of pelitic, ultramafic and calc-silicate rocks and crystalline limestone occurring as lenses or blocks. These rocks underwent regional metamorphism prior to the Cretaceous intrusive rocks. The metamorphic rocks in the Tenzan area, south of the Sefuri area, underwent amphibolite-facies metamorphism and estimated metamorphic P-T conditions of these rocks reach up to 700 °C and 4.0 kbar. On the other hand, the metamorphic conditions in the Sefuri area have been reported to be up to 850 °C and 5.5 kbar. The metamorphic P-T-t path of northwestern Kyushu is interpreted as a clockwise trajectory. These P-T conditions and lithological features suggest that the metamorphic rocks of northwestern Kyushu can be correlated with those of the Higo metamorphic terrane, which is thought to be a candidate of the eastern extension of collision zone between the North China and the South China cratons during Late Permian to the Triassic periods.
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Garnet is one of the common metamorphic minerals in sedimentary metamorphic iron deposits of eastern Hebei. This work collected garnetbearing samples from typical iron deposits in the Qianan-Luan area to carry out electron microprobe analysis(EMPA) and X-ray powder diffraction analysis. The results show that the Sima area belongs to amphibolite facies,the Xingshan-Huangbaiyu belt belongs to amphibolite-granulite facies and Shuichang-Dashihe belt belongs to granulite facies. It is consistent with the previously determined metamorphic facies by analysis of paragenetic assemblage of primary minerals. In addition,end member components and unit cell parameters(a0) of garnets from sedimentary metamorphic iron deposits in the Qianan-Luan area regularly changes with the elevated degree of metamorphism: from almandite and grossularite in the Sima area to almandite in Xingshan-Huangbaiyu belt,to almandite and pyrope in Shuichang-Dashihe belt,as well as the unit cell parameters( a0) values decrease. Therefore,it is of great significance to study the chemical composition and unit cell parameters of the metamorphic mineral garnet,as it has indication meanings for metamorphic degrees of different regions.
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