AN OVERVIEW ON REGIONAL GEOLOGY OF INNER MONGOLIA

1. Stratigraphy: （1） Platform. Precambrian successions are well developed on the platform. The lower Archean is mostly composed of various gneisses and granulites, with isotopic age between 2440 Ma（U-Pb） and 3218 Ma（Rb-Sr）. The upper Archean consists of gneisses and marble, U-Pb isochron measurement yielded an age of 2521 Ma. Middle-Upper Proterozoic is dominated by clastic sediments as well as carbonate which contains abundant Stromatolite. Cambrian-Ordovician is a typical platform type sedimentary sequence containing a lot of Trilobite, Actinoceras, Graptolite, etc. Carboniferous-Permian is characterized by of alternative sequences of marine and continental facies with fusulinids and plant remains, bearing coal and bauxites. （2） The geosynclinal belt. The Palaeozoic succession is dominated by marine clastic sediments and carbonate with variable sedimentary facies and biota. Lower Ordovician-Upper Silurian sequence contains volcanics, Upper Silurian is featured by the Tuvaella fauna. Devonian-Carboniferous accumulation is widely distributed and predominated by alternation of marine and continental facies, nevertheless continental clastic sediments and volcanics prevail in Dongwu County region, Angara fauna is pressent. Permian seguence is completely developed, the biota is exemplified by Misella for early period, Monodiexodina for the middle and Zhesi brachioPodous fauna for late period. Triassic system is continntal clastics, which is mostly developed in the Eerduosi district, The mamma such as Sinokannemeyeria is present in the lower-middle series and Coniopteris-Phoenicopsis in the upper series. The Lower and Middle Jurassic is coal-bearing clas itic sediments, while the upper contains Eusestheria-Ephemeropsis-Lyeoptera. The cretaceous is composed of fluvial and lacustrine sediments, its lower series contains coal and gypsum as well as lamellibranch, such as Ferganoconcha-Nakamuranaia-Sphaerium, Ostracods such as Cypridea- Lycopterocypris and plant remains like Segvoia and Cryptomeria, the upper series comprises Talicypridea-Cypridea-Candona and Reptilia. The Tertiary is dominated by red mudstone of fluvial-lacustrine facies, in the Erlian district are present abundant mammals including Sphenopsalis, Lophialetes, Rinotitan, Embolotheium, etc., The Miocene and Pliocene Consist of sandstone conglomerate and basalt with the presence of Hipparion. Pleistocene is widely distributed in both the Daxinanling and the four interglaeial stages, Rinoceras is present in the middle Pleistocene, Mammuthus-Coelodonta in the Upper Pleistocene. The Holocene occurs all over the region with diversified sedimentary facies. 2. Magmatism: Seven cycles and twelve subcycles have been summrized for igneous rocks of the region. Archean is predominated by migmatitie granite and gabbro with a U-Pb isotoptc age of 2470Ma. Proterozoic is mostly intermediate-acidic intrusion. The Caledonian period is subdivided into three episodes utrabasic-basic, intermediate and acidic respectively. Hercinian is dominated by intermediate-acid rocks, Indo-China and Yaanshanian are by granites. Jurassic volcanics, mostly developed in The Daxinanling-Yanshan district, are subdivided into three subbelts, the east and west are characteristic of alkaline-calc composition, the middle subbelt is calc-alkaline During Cretaceous-Cenozoic intervil, the volcanism was weak and dominated by intermediate-basic lavas. 3. Regional metamorphism: Six metamorphic periods, four first-order provinces and seventeen second-order units have been divided in the region. Lower Archean is characterized by granulite facies, Upper Archean is characterized by amphibolite facies. The lower Proterozoic is repesented by apolyphase progressive metamorphic belt, The Middle-Upper Proterozoic is featured by lower greenschist facies. In the geosynclinal belt, Caledonian and Hercinian terranes are characterized by lower greenschist facies with glaucophane Schist in Places.