陕西省丹凤地区商丹断裂南侧糜棱岩的岩石学研究

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

以往都将研究区内商丹断裂南侧的岩石定为角岩化碎屑岩或火山岩,划归泥盆系青石垭组或加里东蛇绿岩套。笔者认为这些岩石是一套糜棱岩,不是一个独立的地层单位。对该区的长英质、富铝质、斜长角闪质和碳酸盐质四类糜棱岩,分别做了岩相学特征描述。通过化学成分、矿物成分、碎斑矿物特征和变余构造恢复了各种糜棱岩的原岩。并讨论了这些原岩的时代归属,认为这些原岩分别来自秦岭群、丹凤群和中泥盆统,说明有不同时代、不同成因的地质体卷入了这条韧性剪切带。

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.