小兴安岭西北部卧都河地区落马湖岩群年代学及构造背景

GEOCHRONOLOGY AND TECTONIC SETTING OF LUOMAHU COMPLEX IN WODUHE AREA,NORTHWESTERN DAXINGANLING MOUNTAINS

出露于小兴安岭西北部的“落马湖岩群”是多宝山矿集区重要的地质建造之一,由于缺少化石及顶底不全,其形成时代一直存在争议,制约了对区域构造演化及成矿背景的认识.为了探讨其形成时代及古构造背景,采用锆石U-Pb(LA-ICP-MS)方法对卧都河乡附近的“落马湖岩群”变粉砂质泥岩的碎屑锆石进行了年代学研究,85个测点中68个有效测点年龄介于216~2636 Ma,分为8个年龄组:216~347 Ma(n=8)、430~443 Ma(n=2)、463~502 Ma(加权平均485±5 Ma,n=21)、513~572 Ma(加权平均528±7Ma,n=16)、596~613 Ma(加权平均606±12 Ma,n=7)、718~739 Ma(加权平均730±12 Ma,n=3)、830~882 Ma(加权平均850±26 Ma,n=5)、1769~2636 Ma(n=6),以第Ⅲ—第Ⅶ年龄组构成明显的峰值.综合分析锆石晶态、CL图像、U含量特征及地质体宏观岩相学特点,认为第Ⅲ年龄组代表样品层位沉积下限.结合区域研究成果,认为“落马湖岩群”形成于奥陶纪至志留纪罗德洛世;其碎屑锆石主要来源于“额尔古纳-兴安联合地块”及奥陶纪弧火山岩区,形成于弧后盆地构造背景;其与多宝山弧岩浆岩带共同构成早古生代弧-盆系.

Due to the lack of fossils and incomplete roof and bottom,the forming age of Luomahu Complex,one of the important geological formations in Duobaoshan ore concentration area,has been controversial,restricting the cognition of regional tectonic evolution and metallogenic background.The geochronology of detrital zircons in meta-silty mudstones from Luomahu Complex near Wuduhe Township are studied by zircon U-Pb LA-ICP-MS dating method to investigate the formation age and paleotectonic setting.Among the 85 survey points,68 effective ones range in age from216 to 2636 Ma and can be divided into eight age groups:216-347 Ma,430-443 Ma,463-502 Ma,513-572 Ma,596-613 Ma,718-739 Ma,830-882 Ma and 1769-2636 Ma,with the third to seventh groups forming obvious peak values.Based on the comprehensive analysis of zircon crystalline state,CL image,U content characteristics and macro petrographic features of geological body,it is considered that the third age group represents the maximum sedimentary age of sample layers.Combined with the regional research results,it is concluded that Luomahu Complex was formed in Ordovician-Silurian Ludlovian,with the detrital zircons mainly derived from the Erguna-Xing’an block and Ordovician arc volcanic rocks and formed in the tectonic setting of back-arc basin.The Luomahu Complex,together with Duobaoshan arc magmatic belt,formed the Early Paleozoic arc-basin system.