内蒙古北山小狐狸山钼多金属矿区志留纪安山玢岩的发现及其地质意义

Discovery of the Silurian andesitic porphyry in the Xiaohulishan Mopolymetallic deposit,the Beishan district,Inner Mongolia,and its geological significance

小狐狸山钼多金属矿床位于内蒙古北山地区的东北部,矿区内出露的地层主要为奥陶系咸水湖组火山岩。本次工作显示,小狐狸山矿床赋矿火山岩地层存在安山玢岩呈岩株状侵位于奥陶系咸水湖组安山岩中,测得该安山玢岩(XHLS-01)的锆石LA-ICP-MS U-Pb同位素年龄为(419±5) Ma,对应晚志留世,晚于奥陶纪安山岩的成岩年代(489±11) Ma,研究成果更正了对矿区仅发育奥陶纪火山岩的传统认识。小狐狸山矿区晚志留世安山玢岩w(SiO2)为57.18%~58.19%,w(K2O+Na2O)为5.78%~6.97%,w(TiO2)为0.57%~0.90%,为富钾贫钠、准铝质岩石,与奥陶纪低钾钙碱性的安山岩不同;微量元素显示安山岩和安山玢岩均呈大离子亲石元素富集、高场强元素亏损特征,安山玢岩稀土总量(∑REE含量)为71.9×10^-6~72.24×10^-6,低于安山岩96.89×10^-6,Eu呈较弱负异常。以上显示小狐狸山矿区晚志留世安山玢岩的母岩浆来自俯冲洋壳板片交代的亏损地幔源区,上升过程中经历了较弱的分离结晶和同化混染作用。小狐狸山志留纪安山玢岩和奥陶纪安山岩均形成于岛弧环境。综合区域地质、年代学和地球化学研究显示,内蒙古北山地区早古生代具有相似的构造演化过程,同属西伯利亚板块与塔里木板块之间的岛弧增生带。

The Xiaohulishan Mo polymetallic deposit is located in the northeastern part of the Beishan district,Inner Mongolia,in which the Ordovician Xianshuihu Formation volcanic rocks occur.In this study,the authors discovered andesitic porphyry intruding into the andesite as stocks during the ore-bearing volcanic rock section observation.The zircon LA-ICP-MS U-Pb dating result of the andesitic porphyry is 419±5 Ma,corresponding to Late Silurian epoch and much younger than the Ordovician andesite 489±11 Ma.The dating results rectify the viewpoint that there are only Ordovician volcanic rocks distributing in the Xiaohulishan deposit.The geochemical characteristics of the Silurian andesitic porphyry in the Xiaohulishan deposit show that the SiO2 contents are between 57.18%and 58.19%,the K2OH+Na2O contents are between5.78%and 6.97%,and the TiO2 contents are between 0.57%and 0.90%,which indicate the andesitic porphyry is K-Al-rich,Na-poor rocks,and obviously different from the Ordovician low calc-alkaline andesite.The Silurian andesitic porphyry exhibits enriched in LILE(e.g.Rb,Ba,U,K,and Sr),but depleted HFSE(e.g.Nb,Ta,and Ti).The∑REE concentrations of the andesitic porphyry are from 71.9×10^-6 to 72.24×10^-6,which is lower than andesite(96.89×10^-6).The LREE/HREE ratio of Silurian andesitic porphyry is from 4.68 to 5.91,showing a weak right-leaning trend with weak negative Eu anomalies,which probably experienced lower degree differentiation than andesite.The Silurian andesitic porphyry in the Xiaohulishan deposit is derived from crust-mantle mixing zone,and underwent weak differentiation and contamination during its emplacement.Based on regional geological setting,geochronology,and geochemical characteristics,it is showed that the Beishan district,Inner Mongolia underwent same geological evolution during the Early Paleozoic era,which probably consistently belongs to the island-arc accretion zone between the Siberia and the Tarim plates.