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地球学报 1987
U-Pb AND Rb-Sr ISOTOPIC AGE STUDY OF THE JINING GROUP FROM NEL MONGOL OF CHINA
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Abstract:
The Archaean Jining Group, of dominant granulite fades metamorphism, is stratigraphically the oldest metamorphic rock series in the south Nel Mongol of China.According to the metamorphic rock assemblages and rock chracteristics, the Jining Group can be subdivided into two rock formations.The lower part of the lower formation is mainly composed of pyroxene-plagioclase-granulite, intercalated with biotite hypersthene-plagioclase-gneis-ses, pyroxene-bearing plagioclase-amphibolite and pyroxenite, the upper part of the same formation mainly consists of hypersthene-(garnet ±) biotite-pla-gioclase-gneisses, intercalated with pyroxene-plagioclase granulite, and occasionally also thin-beded pyroxene-bearing magnetite-quartzite. The protoliths correspord to those of a basic-intermediate (-acidic) volcanic-sedimentary formation. The upper formation of this group is mainly composed of sillimanite-garnet-K-feldspar- (or two feldspars) gneisses, biotite-plagioclase-gneisses, garnet-quartzite, diopside-marble, and leuco-leptite, intercalated with pyroxene plagioclase-granulite in the lower part and with a graphite ore bed of several meters thick, in the basal part. The protoliths are those of a graphite-bearing semipelitic-cart onate formation, which is similar to these of the Khondalitic rocks of India.The U-Pb zircon and Rb-Sr whole rock iscchron method have been applied to dating this group.Cn the six U-Pb zircon and two Rb-Sr whole rock isochion results, con-bired with geologic features of this group, conclusions have been made as follows: 1. Zircon discordent dating for biotite-bearing hornblende-plagioclase-gne-iss of this group gives an age of 2467-35+54Ma. It is the oldest dating that has been obtained so far in this group. This result can be interpreted as the minimum age for late Archaean metamorphic event which may be correlated with the same event recognized in the Qianxi Group, eastern Hebei. 2. The lower limit of metamorphic age for the upper formation of the Jining Group seems to be older than 2316 Ma. (Rb-Sr) or 2359 Ma (U-Pb, allanite). 3. The other three zircon discordent data for gneisses and amphibolite of the upper formation from this goup give the ages of 1821, 1962 and 1957 Ma. They are interpreted as the age of early Proterozoic metamorphic event which is super-imposed on the late Archaean metamorphic event. The formation of the rather widespread pegmatites in this terrain is closely related to this metamorphic event. 4. The age of protolith of Jining Group should be older than 2500 Ma.
