山西云中山—关帝山地区界河口岩群岩石学特征、LA-ICP-MS锆石U-Pb年龄及其归属

山西吕梁山地区界河口岩群(后文简称界河口群)主要由一套变质砂泥质岩石组成,含有一定量的条带状大理岩、斜长角闪岩等。利用LA-ICP-MS对位于关帝山西榆皮、娄烦县杜交曲及云中山三角镇地区的含石榴黑云二长花岗片麻岩(12LL05-1)、片状黑云母石英岩(12LL09-2)和含石榴钾长石英岩(12LL15-1)进行锆石U-Pb年龄测定,获得的二长花岗片麻岩(12LL05-1)岩浆结晶年龄为1941±28Ma、片状黑云母石英岩(12LL09-2)碎屑锆石最小的峰值年龄为2519.5±6.7Ma及含石榴钾长石英岩(12LL15-1)碎屑锆石2个峰值年龄2143±15Ma和2599±11Ma。根据岩石组合和同位素测定结果推测,关帝山西榆皮地区的界河口群与界河口—汉高山一带的界河口群可以对比,沉积时代在1.94Ga之前;娄烦县杜交曲一带的界河口群可与五台岩群相对比,属新太古代;云中山三交镇一带的界河口群可与野鸡山群或岚河群相对比。沿岚县—方山—吕梁一线存在一条大型韧性剪切带,倾向NNW-NWW,表现为右行剪切的运动学性质,将吕梁山区近EW向构造带改造为NNE向。

Petrographic characteristics, LA-ICP-MS zircon U-Pb dating and affinity of Jiehekou Rock Group in Yunzhong Mountain-Guandi Mountain area, Shanxi Province

Lying in Lüliang Mountains area of Shanxi Province, the Jiehekou Rock Group is mainly composed of a suite of meta-argillaceous-arenaceous rocks, with certain amounts of banded marbles and amphibolites. The zircons from garnet-bearing biotite adamellitic gneiss (12LL05-1), schistose biotite quartzite (12LL09-2) and garnet-bearing feldspar quartzite (12LL15-1) from Xiyupi of Guandi Mountain, Dujiaoqu of Loufan and Sanjiaozhen of Yunzhong Mountain were dated by means of LA-ICP-MS. A magmatic crystalline age of 1941±28Ma was obtained for garnet-bearing biotite adamellitic gneiss (12LL05-1). The youngest peak age of the detrital zircons from schistose biotite quartzite (12LL09-2) is 2519.5±6.7Ma. Two significant peak ages of 2143±15Ma and 2599±11Ma were obtained from garnet-bearing feldspar quartzite (12LL15-1). It is inferred from the rock assemblages and geochronological data that the Jiehekou Group in Xiyupi can be correlated with the Jiehekou Group in Jiehekou-Hangao Mountain area, because both of them were formed before 1.94Ga. The Jiehekou Group in Dujiaoqu (Loufan) area might be the counterpart of the Wutai Group that was formed in late Archean. The Jiehekou Group in Sanjiaozhen (Yunzhong Mountain) can be probably correlated with the Yejishan Group and Lanhe Group. It is suggested that a large NNW-NWW dipping ductile shear zone characterized by dextral shearing kinematics occurred along Lanxian-Fangshan-Lüliang, which turned the EW-trending tectonic belt into NNE-trending one.