吉中呼兰镇地区呼兰群的划分及时代

吉中呼兰镇一带分布的轻变质岩系称呼兰群，曾划分为黄莺屯组、小三个顶子组和北岔屯组，对其划分和时代一直分岐较大。本文重新厘定了黄莺屯组，其主要为片麻岩、变粒岩、片岩等夹大理岩和石英岩，小三个顶子组由大理岩夹片岩和变粒岩组成；北岔屯组由大理岩、钙质砂岩和板岩组成，时代为晚志留世。北岔屯组在变质程度、岩性组合和时代上与另两组都有很大差别。因此，呼兰群仅包括黄莺屯组和小三个顶子组，时代为寒武－奥陶纪。

Division and Age of Hulan Group in Hulanzhen Area of Central Jilin

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.