SUCGESTIONS ON THE STRATIGRAPHIC DIVISION OF THE ARCHEAN IN CENTRAL NEI MONGGOL （INNER MONGOLIA）

Central Nei Monggol is a terrain where the Archean strata are best ex- posed in the entire region. This terrain extends in a near-E-W direction for about 500 km, covering an exposed area of over 6,000 sq km. From the top downwards the Archean may be divided into the upper Archean Erdao’ao and Urals Groups and the lower Archean Jining Group, as shown in the "Region- al Stratigraphic Table of North China （Net Monggol Volume）" published in 1978. According to the results obtained through recent work, a stratigra- phic division of the Archean has been made on the basis of the characteris- tics of the major rock associations （metamorphic formations, protolith forma- tions, tectonic settings and isotopic age data）, and the following suggestions are put forwards: the original Erdao’ao Group should be assigned to the mid- dle and upper Archean; the marble rock series of the Donggoucun Formation of the original Jining Group is equivalent to the Tao’erwan Formation of the original Wulashan Group, so should be assigned to the presently proposed Wulashan Group; the original Jining Group （the Donggou Formation exclud- ed） may be further divided into two major rock associations, i. e. the original Shajucun Fromation and the gneissic rock series of the Lower Baiyao Forma- tion in the upper part are collectively called the Upper Jining Group, while the granulitic rock series of the original Dashiyaogou Formation in the lower part is named the Lower Jining Group; the gneissic and granulitic rock series in the original Wulashan Group should be separated and correlated respectively with the Upper and Lower Jining Groups if they really lie at the stratigraphic horizon of the original Jining Group. Therefore, the Archean metamorphic complexes in central Nei Monggol may be divided into three rock associations, which are named （in descending order） the upper Archean Wulashan Group, the lower Archean Upper Jining Group and the lower Ar- chean Lower Jining Group. The Wulashan Group is in faulted contact with the Upper Jining Group, but their relationship awaits further clarification. The Upper Jining is inferred to be in angular unconformity with the Lower Ji- ning Group. This paper systematically summarizes the characteristics of the associa- tion of the above-mentioned groups, and discusses problems on the criteria for the classification of rock series, correlation between the Upper and Lower Jining Groups in the central region and assignment of the rock masses as- signed originally to the Archean and some rock masses in the Zha’ertai Mo- untains originally assigned to the Proterozoic. It is pointed out that the 3, 108.3 m. y. Rb-Sr isochron age may largely represent the lower limit of the age for the deposition of the prototiths of the Wulashan Group and its corresponding rock series in the region and may also be regarded as the boundary of the early and late Archean. The granu- ire-facies metamorphic event at the end of the early Archean in the region should occur 3,100 m. y. ago, and the amphibolite-facies metamorphic event at the end of the late Archean should occur about 2,500 m. y. ago.