Reorgnization Of Zhalantun Group In Northern Daxinanling Mountains And Its Tectonic Implications

Zhalantong Group was identified and named in 1976 during 1 : 200000 geologic survey in Buteha-qi. It is consisted of chlorite-quartz schist, crystalline limestone and hematite-quartz. It was put into Lower Proterozoic in the same year. Regional geology of Inner Mongolia Autonomous Region (1982-1985) placed it into Lower Proterozoic Xinha-dukou group. An unconformity within the previous so-called Zalantong Group was recognized in north of Zhalantun city in 2004. Beneath this unconformity there developed strong deformed and metamorphosed schists. Above the unconformity, however, basal conglomerate were found at the bottom of the sequence, which derived from the underlined schist. Primitive sedimentary structures were preserved quite well in the metamorphic conglomerate, sandstone and calcareous siltstone. Microfossils were discovered in calcareous siltstone: Leiominuscula sp., Leiosphaeridia sp., Prototracheites porus in 2005. Based on these new findings, the schists complex beneath the unconformity was treated to be Paleoproterozoic Baoyintu Group. Meanwhile the metamorphic conglomerate, sandstone, meta-basalt, lamellar quartzite assemblage above the unconformity was defined to be late Neoproterozoic Ailegemiao Formation. Geochemical research on the meta-basalt within the Ailegemiao Formation indicate that it formed in a continental rift. The finding of this unconformity and the reognization of the previous so-called Zhalantun Group are of important implications toward a better understanding of the tectinic evolution in northern Daxinganling Mountains during Late Proterozoic.