内蒙古额尔古纳乌尔根矿田富碱斑岩体成因及构造启示:地质年代学和地球化学证据

Petrogenesis and Tectonic Implications of Alkali-rich Porphyry in the Urgen Ore Field,Erguna Block,Inner Mongolia:Constraints from Geochronology and Geochemistry

乌尔根矿田主要由东珺浅成热液型银铅锌矿床和大加布果斯斑岩型钼矿床组成。野外地质调查揭示富碱斑岩岩浆热事件与浅成热液型银铅锌矿床时空关系密切,获得成矿期正长斑岩和成矿前石英二长斑岩锆石U-Pb年龄分别为125 Ma和165 Ma。鉴于大加布果斯斑岩型钼矿床成矿时代为147 Ma,而东珺银铅锌矿床成矿作用发生在早白垩世,进而得出两者为相对独立成矿体系。岩石地球化学特征表明,正长斑岩和石英二长斑岩属偏铝质钾玄岩系列,与A型花岗岩特征相似。岩石富集大离子亲石元素(LILE)和轻稀土元素(LREE),亏损高场强元素(HFSE)和重稀土元素(HREE),Eu异常不显著(Eu/Eu^(*)=0.65~0.83),表明岩浆源自富集型岩石圈地幔的部分熔融,经历了分异结晶并遭受有限的地壳混染;暗示矿田岩浆‒成矿作用与洋壳俯冲有关,结合区域地质背景和矿田相关研究认为中侏罗世矿田处于弧后引张环境,而晚侏罗世‒早白垩世处于蒙古‒鄂霍次克洋闭合之后的伸展环境。

The Urgen ore field,situated in the northern part of the Erguna Block,hosts the Dongjun epithermal Ag-Pb-Zn deposit and the Dajiabuguosi porphyry Mo deposit.The syenite porphyry,whose zircon U-Pb age is 125 Ma,is spatially and genetically related to the Dongjun deposit.This finding indicates that the Dongjun deposit was formed in the Early Cretaceous,later than the formation of the Dajiabuguosi porphyry Mo deposit(mineralization age is 147 Ma).The zircon U-Pb age of the quartz monzonite porphyry is 165 Ma(the Middle Jurassic),suggesting it is the product of pre-metallogenic magmatism.Owing to the different mineralization ages of the Dajiabuguosi porphyry Mo deposit and Dongjun epithermal Ag-Pb-Zn deposit,we concluded that they are two independent metallogenic systems.The intrusive rocks in the Urgen ore field show petrological,mineralogical,geochemical,and geotectonic characteristics of metaluminous shoshonite series,and are related to A-type granites.The rocks show enrichment in large-ion lithophiles and light rare earth elements,and depletion in high-field-strength elements and heavy rare earth elements,with weak negative Eu anomalies(Eu/Eu^(*)=0.65 to 0.83).The data indicate that the intrusive rocks were derived from partial melting of enriched lithospheric mantle,and underwent various degree of fractional crystallization and limited crustal contamination during magma ascent.The magmatism and mineralization were related to slab subduction.Combined with the regional research and previous studies on the Urgen ore field,we suggest that the Urgen ore field was in a back-arc tension setting in the Middle Jurassic,whereas in a post-collisional lithospheric extensional setting resulting from the closure of the Mongol-Okhotsk Ocean during the Early Cretaceous.