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《Chinese Journal of Geology》 1983-01
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THE ACIDIC VOLCANIC ROCKS IN BAYAN-OBO GROUP (MIDDLE PROTEROZOIC)

Li Jiliang (Institute of Geology,Academia Sinica,Beijing)  
The acidic volcanic rocks reported in this paper are exposed on both flanks of Bayan syucline near its western end.There are three types of the occurrence of these rocks: (1) hypabyssal dyke,(2) laminar pyroclastic rock,and (3) intraclastic pyroclastic rock.The hypabyssal rocks exhibit porphyritic texture.The albite and K-feldspar are the predominant minerals which occur as phenocrysts and the quartz is rarely found as phenocryst.In the rocks,the matrix recrystallized so often that few glass basis could be remained.Under the microscope,some rocks show fluidal micro-structure which is exhibited by the orentation of the acicular muscovite.These rocks have subjected to deformation that is indicated as kink bands in feldspar phenocrysts and undulatory extinction in quartz crystals.The lamilar pyroclastic rocks indicate laminar structure which is exhibited by acicular muscovite.Such rocks have the features of both lava and tuff.In these rocks,there is no phenocryst.The feldspar microcrystals that orentated along the laminar formed the pilotaxitic texture with the glass matrix.The quartz which makes up about 5 to 10 volum per cent of the bulk rocks spreads widely,reflecting that a considerable amount of pyroclasts fell into lava matrix.According to laminar and subaqeous slumping structure,it can be presumed that such rocks may be made up of lava matrix and volcanic clasts derived from transportation and deposition in marine current.The intraclastic pyroclastic rocks consist of the matrix and granuloclasts such as ooide,pellet and intra-basin grain.The pellets appear the radiating texture.Sometimes each of them has a nucleus and sometimes they are compound-pellets which have two or more nuclei.The ooides that exhibit concentric structure can be subdivided into three subtypes: (1) nucleusfree ooides,(2) nucleus-bearing ooide,and (3) compound ooide.It is clear that the intraclastic rocks were formed in a sedimentary environment with high energy.Based on the petrochemical analysis,these rocks all belong to rhyolitic composition.However,some of them have been altered into rhyolitic dacite by seawater metasomatism.After Miyashiro (1975),this rock suit belongs to straddle B-type alkaline series.From the major element chemistry,it may be seen that there is a magma-evolution relative between the acidic volcanic rock and the spirite in Bayan-obo Group.These rocks may be the products of different crystalline fractionation from same original magma.
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