Division and Age of Hulan Group in Hulanzhen Area of Central Jilin
吉中呼兰镇地区呼兰群的划分及时代

The metamorphozed rocks in Hulanzhen area of central Jilin are named Hulan group, which was divided into Huangyingtun, Xiaosangedingzi and Beichatun formations. However, this division and its age has always been difference of opinion. This paper redefines that the Huangyingtun formation is composed mainly of gneiss, granulite, schist intercalated with marble and quatzite, and dated as 524Ma with Rb-Sr method; that the Xiaosangedingzi formation consists mainly of marble intercalated with schist and granulite, containing Ectomaria; and that the Beichatun formation is composed mainly of marble, calcareous sandstone and slate, yielding Thamnopora and Cladopora . The Beichatun formation is different from the Huangyingtun and Xiaosangedingzi formations in metamorphic grade, lithologic association and age. So, the authors propose that the Hulan group includes only Huangyingtun and Xiaosangedingzi formations and that this group is Middle Cambrian Ordovician in age, and that the Beichatun formation is Ludlow of Silurian in age.