Metamorphic Geology Of The High-grade Area Of The Larsemann Hills And Its Adjacent Region, East Antarctica

According to the synthetic study of the regional geology,strucrure,petrology,lnineralogy, geochemistry and geochronology of the high grade rocks of the Larsemann Hills and its adjacent region, East Antarctica, a brief sequence of geological events of the area is presented. The low-medium pressure granuttlite facies metamorphism(M₂) that occurred c.550Ma ago is a superimposition on the earlier meditum pressure granulite facies metalnorphism (M₁, c.1000Ma). The implication of the superimposition is stressed and it is pointed out that the second stage of metmnorphism(M₂) evolved without substantial crustal thickening. It Seems to bear no relation to collision tectonics,though the PT path of the later stage of metmnorphism is clockwise. Meanwhile, the superimposition intrigued local transposition of structure and petrology, mad resetting to some extent of the geochemical and isotopic systems,and the the formation of some metamorphic rocks that are different in composition from both sedimentay and igneous origin. The sillimabite-bearing gueiss of the khondalites is actually derived from greywack, subgreywack, quartz-sandstone and shale, not from clay or bauxite as conventionally believed. On the basis of the feature of metamnorphic rocks, the protolith formation and related deformation-metamorphism, it is argued that the discrimination of the basement and cover is not unambiguous in the study area. The difference essentially lies m the varying degrees of later superimposition at different part of the same sequence.