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PETROLOGIC STUDY OF THE MYLONITES ON THE SOUTH SIDE OF SHANGDAN FAULT, DANFENG, SHAANXI

Hu Nenggao An Sanyuan ( Xi'an College of Geology )  
Up to now the rocks on the south side of the Shangdan Fault, in the Danfeng area have been regarded as metavolcanites or hornfelses, which belong to Caledonian ophiolite or Qingshiya group of the Devonian system.This study shows that these rocks are mylonites, but they are not classified into the stratigraphic unit.According to the mineral contents of the rocks authors divide these mylonites into four types, felsic, aluminous, amphibolitoid and carbonatoid,each of which shows apecific petrographic characteristics because of the ductile contrast between the minerals the mylonites consist of Quartz is easy to be deformed and the rich-quartz rocks are easy to become ultra-myl-onites.The augen structure often appears in the rich-plagioclase and the late penetrative cleavage develops in the rich-mica rocks.The brittle deformation in the maphibolite rocks is quite obvious. According to the chemical compositions, the mineral contents, the characteristics of porphyroclasets and the blastobedded structure, this paper discussed the classification of the parent rocks of mylonites.The age of the parent rocka is discussed, too.The authors think that some of the felsic mylonites are transformed from clastic rocks of the Devonian age and others from the biotite-plagioclase gneiss and pegmatite of the Qinling group.The parent rocks of carbonatiod and aluminous mylonites respectively are marbles and sill-imanite-garnit-plagioclase gneiss of the Qinling group.Because the trace elements and the REE concentration of amphibolitoid mylonites are similar to those of amphibolites of the Dangeng group, it is possible that the parent rocks are amphibolites of the Danfeng group.These show that the rocksof diferent ages and origins were drawn into the ductile shear zone during the ductile shear deformation.
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