STRATIGRAPHIC DIVISION OF THE ORIGINAL WULASHAN GROUP IN THE WULA MOUNTAINS,NEI MONGGOL（INNER MONGOLIA）

The Archean strata in the Wula Mountains, Nei Monggol, were called the Sanggan Group in the early 1930s by sun Jianchu. Based mainly on the data of Sheet Serial No. K-49-XXVI （Shetai Town Sheet）,the strata are formally named the Wulashan Group in the "Regional Stratigraphic Tables of North China" （Nei Monggol Uolume）published in 1978. Its age is assumed to be late Archean and its stratigraphic position lies above the Jining Group （so called by Mr. Li Pu） in the Jining area, but both show no direct contact. In the recent subject study of the early Precambrian metamorphic rock series in Nei Monggol, the authors et al. have conducted a detailed regional investigation of the original Wulashan Group in the Wula Mountains and measured its sections. The results have shown that at the base of the original Mulashan Group there are not only a granulite association corresponding to the upper part of the Dashiyao Formation of the original Jining Group but also a gneiss association characterized by silimanite-garnet gneiss, leuco-granulitite and graphitic gneiss corresponding to the Dabaiyao and Shajucun Formations of the original Jining Group. Based on the actual data and from analyses of the episodicity and cyclicity of crustal evolution, rock-forming mannerand characteristics of original rock formations,the two above-mentioned associations are divorced from the original Wulashan Group. The "granulite association" is renamed the Lower Jining Group and the "gneiss association" is renamed the Upper Jining Group, while the rest is still called the "Wulashan Group". The disintegrated original Wulashan Group is composed in descending order of the Wulashan Group, the Upper Jining Group and Lower Jining Group. The Contacts between the newly established Upper and the Lower Jining Groups are inferred to be unconformable; the contacts between the Upper Jining Group and the Wulashan Group are found to be faulted in the Wula Mountains. The age of the earlier metamorphic phase in the area ranges from 2.4-2.5 b.y. It is evident that the rocks of the Wulashan Group were formed earlier than 2.5 b.y. ago, and its age undoubtedly belongs to Archean.