华北板块北缘呼兰群^(40)Ar-^(39)Ar定年及其构造意义

^(40)Ar-^(39)Ar dating and its tectonic significance of the Hulan Group at the northern margin of the North China Plate

呼兰群发育于华北板块北缘,主要为一系列长英质火山岩和沉积岩组成的受到低绿片岩相-角闪岩相变质作用和多期变形作用改造的变质岩系,分布于吉林中部红旗岭-呼兰镇一带。大部分地区的呼兰群地层层序被严重破坏,野外观察的"层理"实际上是构造置换作用形成的构造面理,绝大多数的岩性界面为构造接触。呼兰群由一系列相互叠置的构造岩片构成,具有构造混杂岩性质。呼兰群经历了两期韧性变形作用:早期受到NNE-SSW向挤压作用,呼兰群表现为由NNE向SSW逆冲的运动学特征;晚期可能受到NE-SW向伸展作用,主要表现为由SW向NE倾向滑移的运动学特征。本次分别对采自红旗岭地区强变形带的白云石英片岩、石榴白云片岩中的白云母进行40Ar-39Ar测年,分别获得220.23±2.15Ma、221.31±2.6Ma的坪年龄,认为~220Ma年龄应代表华北板块北缘碰撞造山带造山后伸展作用发生的时间。

The Hulan Group, outcropped along the northern margin of the North China Plate in Hongqiling-Hulan area of central Jilin Province, consists mainly of felsic volcanic and sedimentary rocks with low greenschist to amphibolite facies metamorphism and multiple phases of deformation. Most parts of the stratigraphic sequence of the Hulan Group were severely damaged. The observed ＂bedding＂ in the field is actually mostly foliation formed by displacement. The majority of the lithologic boundaries are in fact structural interface. The Hulan Group is possibly a suit of melange consisting of a series of mutual tectonic slices in a stacked configuration. Hulan Group underwent two stages of ductile deformations： the early stage showed thrusting from NNE to SSW in the NNE-SSW trending compression; the late stage showed mainly dip-slip from SW to NE in the NE-SW extension. We have acquired 40Ar-39Ar plateau ages of 220. 23 ± 2. 15Ma and 221.31 ± 2. 6Ma for muscovite from the muscovite-quartz schist and garnet-muscovite schist, respectively, in strong deformation zone in the Hongqiling area. Consequently, the age of -220Ma could represent the post- orogenic extension time for the collision orogenic belt at the northern margin of the North China plate.