大兴安岭中南段桦杆子沟岩体锆石U-Pb年龄及其地质意义

文章对桦杆子沟岩体展开了岩相学、锆石U-Pb年代学的研究,探讨其成岩年代及其地质意义。桦杆子沟岩体中的岩石类型较为复杂,主要包括中细粒二长花岗岩、晶洞花岗岩、黑云母花岗岩、中细粒花岗岩、斑状花岗岩及含闪长质包体黑云母花岗岩等。而在岩体中见有大量含伟晶岩透镜体花岗岩、淡色花岗岩及晶洞花岗岩等高演化特征,并发现多处锡矿化线索或与锡矿化有关线索,这些证据指示出桦杆子沟岩体具有良好的成锡矿潜力。对中细粒二长花岗岩、晶洞花岗岩、中细粒黑云母花岗岩进行了LA-ICP-MS锆石U-Pb定年,其结果依次为(136.6±1.1)Ma、(137.03±0.55)Ma及(137.34±0.61)Ma,与区域上的哈力黑坝岩体、小井子岩体及花加拉嘎岩体等年龄基本一致,均在早白垩世期间形成,说明早白垩世阶段在大兴安岭中南段地区发生一次大规模的岩浆活动。在蒙古—鄂霍茨克洋造山伸展和古太平洋板块俯冲双重作用的伸展环境下,形成了以桦杆子沟岩体为代表的复杂岩体;同时在早白垩世区域上也形成了大量锡、铅锌银矿床,表明在区域上大规模岩浆活动带来大量成矿物质,形成大量矿床。

U-Pb age of zircon mineral in Huaganzigou pluton and its geological significance in central and southern part of the Da Hinggan Mountains

The petrographic study and geochronology of zircon the Huaganzigou intrusion are carried out to discuss the diagenetic age and geological significance in this paper. The rock types in Huaganzigou intrusion are complicated, mainly including intermediate-fine grain adamellite, miarolitic granite, biotite granite, intermediate-fine grain granite, granite porphyry and dioritic enclaves in biotite granite. Also many pegmatitic lenses in granite, leucogranite, miarolitic granite and other rocks are found in this intrusion, which are of high evolution characteristics. Meanwhile, a number of tin mineralization is discovered. All of these evidence indicate that there is great potential for tin mineralization in the Huaganzigou intrusion. Precise isotope ages of intermediate-fine grain adamellite, miarolitic granite and intermediate-fine grain biotite granite in Huaganzigou intrusion are obtained using single zircon-grain LA-ICPMS U-Pb dating method, the weighted average of ²⁰⁶Pb/²³⁸U ages are (136.6±1.1) Ma, (137.03±0.55) Ma and (137.34±0.61) Ma respectively, consistent with the age of regional Heiba intrusion, Xiaojingzi intrusion and Huajialaga intrusion, which are formed in the early Cretaceous, indicating that a large-scale early cretaceous magmatism occurred in central and southern part of the Da Hinggan Mountains region. In the spreading environment of the orogenic extension of the Mongolia-Okhotsk and the subduction of the ancient Pacific plate, Huaganzigou complex intrusion was formed. Simultaneously, a large amount of tin and Pb-Zn-Ag deposits were formed in this region during early Cretaceous, indicating that large-scale magmatism in this region brought large amount of ore-forming materials which formed Sn-Pb-Zn-Ag deposits.