内蒙古荷尔勿苏铅锌矿区二叠纪侵入岩成因:锆石U—Pb年代学与岩石地球化学制约

Petrogenesis of Permian intrusive rock in Heerwusu Pb—Zn deposit,Inner Mongolia:constraints from zircon U—Pb dating and geochemistry

内蒙古荷尔勿苏铅锌矿是华北板块北缘少郎河多金属成矿带内新发现的矿床。笔者对矿区黑云母石英闪长岩进行了LA—ICP—MS U—Pb年代学和岩石地球化学研究,以期揭示其岩石成因、成矿背景及对成矿的贡献。黑云母石英闪长岩锆石为自行晶-半自形晶,内部具有清晰的生长环带,Th/U比值为0. 43～1. 20,具有岩浆成因的特点。测年结果显示其形成于古生代中二叠世(268. 1±2. 6 Ma)。岩石地球化学分析显示,黑云母石英闪长岩SiO_2含量59. 23%～61. 07%,Al_2O_3含量16. 24%～16. 53%,K_2O/Na_2O比值0. 62～0. 63。稀土元素配分曲线为右倾轻稀土富集型,轻重稀土分异明显,Eu弱亏损(δEu=0. 84～0. 90),Ce弱异常(δCe=0. 91～0. 94),说明源区极少残留斜长石。富集大离子亲石元素K、Rb、Ba,亏损高场强元素Nb、Ti,并且Nb、Ti具有"TNT"负异常特征。荷尔勿苏铅锌矿黑云母石英闪长岩是在古生代中二叠世华力西晚期板块挤压环境下,火山弧岩浆作用的产物,其形成的动力学机制受古亚洲洋闭合作用的影响。

Heerwusu lead-zinc deposit is a newly found deposit in the Shaolang River polymetallic metallogenic belt of the northern margin of the North China Plate,Inner Mongolia. The LA-ICP-MS U-Pb chronology and rock geochemistry of the biotite quartz diorite in the mining area are studied to reveal its petrogenesis,metallogenic background and contribution to the mineralization. The zircons of the biotite quartz diorite are euhedral-subhedral in shape and there is a clear growth loop inside. The Th/U ratio ranges between 0. 43~1. 20,indicating that it has the characteristics of magmatism. The dating results show that the samples were formed in the Paleozoic Middle Permian( 268. 1 ± 2. 6 Ma). The rock geochemical analysis shows that the content of SiO2 is 59. 23%~61. 07%,Al2O3 is 16. 24%~16. 53% and the K2O/Na2O ratio ranges between 0. 62~0. 63. The distribution curve of rare earth elements is characterized by right inclining type with enrichment of light LREE,distinct differentiation of LREE and HREE,weak Eu loss(δEu = 0. 84~0. 90) and weak Ce anomaly(δCe = 0. 91~0. 94),indicating that there are few plagioclase in the source area. The primitive mantle-normalized trace element spider diagram also shows the enrichment of large ion lithophile element K,Rb,Ba and the loss of high field strength elements Nb and Ti,which have the "TNT"negative anomaly characteristics. The biotite quartz diorite in Heerwusu lead-zinc deposit is the product of the magmatism of the volcanic arc under compressional environment during Late Variscan of Middle Permian. The dynamic mechanism of its formation is influenced by the closure of the Paleo Asian Ocean.