CHARACTERISTICS OF BASEMENT REWORKED COMPLEX AND IMPLICATION FOR DAQINGSHAN OROGENIC BELT
Liu Xishan(Department of Geology,Changohun University of Earth Sciences,Changchun,130061)
By this study,the basement reworked complex(BRWC) is considered as one toctolithologic unit besides the Archean garnet pyroxene granulite-charnockite and early Proterozoic khondalite-S-type granite units(Fig.1).The BRWC is mainly composed of high temperature mylonite gneisses,partial anatectic granitic rocks and relict blocks of Archean crust in Daqingshan orogenic belt.Studies on the characteristics of compositions,deformtion structures and overprinted reaction textures indicate that the first two-parts of the BRWC do not belong to both Archean supracrustal and plutonic rocks and early Proterozoic metamorphc supracrustal rocks.The existance of the Archean relict blocks in the BRWC indicates that by early Proterozoic reworking,the Archean crust had been reworked and most of the crust had been changed into pyroxene-mylonite gneisses and migmatized mylonite gneisses within ensailic orogenic belt.There are two-type reaction textures in the large Archean relict blocks of the BRWC complex.One represents the IBC tectonic regime and is characterized by the corona texture that fine grained symplectite of Grt+Qz±Cpx surrounds the medail grained aggregates of Opx+Cpx+PI(Fig.2) or occures in the interfaces of the minerals above(Fig.2a,c).The characteristics of the corona texture looks like the "red-eye ring" on the hand speciman.Because the "red-eye" texture can be found in the Guyang-Wucuan and Miyun-Jidong-Liaoxi areas where the Archean graunlites have not been reworked by early Proterozoic ensailic orogenic event,it is called" red-eye ring" event which took place at the later Archean.Another represents the decompression with heating or ITD tectonic regime and is characterized by the fine grained symplectite of Opx2+ Pl2 which surrounds the corona composed of garnet +quartz or garnet porphyroblast(Fig.2).The inclusions of plagioclase with clinopyroxene or orthopyroxene can be found in the garnet porphyroblast.The feature of this corona texture looks like" white-eyering" on the hand speciman.The H white-eye ring" texture can be only found in the granulite relict blocks of the BRWC and is similar to those textures developing in overlying Al-rich gneisses(khondalite series) of early Proterozoic.In khondalite series,however,the "white-eye ring" texture is characterized by the cordierite or aggregates of Pl +Qz surrounding the garnet porphyroblast(Fig.2b).It can be called "white-eye ring" event which took place at the early Proterozoic.It is clear that the "white-eye ring" event is overprinted on the "red-eye ring" event,only within the ensailic orogenic belt,that is the ITD event of the early Proterozioc overprinted on the Archean IBC event.The early Proterozoic ensailic orogeny involves both the early Proterozoic cover and Archean basement.The action involving both progressive deformation-metamorphism for cover and reworking for basement can be implication for interpreting the evolution of Precambrian ensailic orogeny.
【CateGory Index】： P588.3
【CateGory Index】： P588.3