西藏措勤尼雄矿田滚纠铁矿金云母矿物学特征及^(40)Ar-^(39)Ar年代学

Mineralogical characteristics and ^(40)Ar-^(39)Ar dating of phlogopite from the Gunjiu iron deposit in the Nixiong Ore Field,Coqen,Tibet

尼雄矿田位于隆格尔-工布江达断隆带,是措勤-申扎铁铜多金属成矿带的重要组成部分。滚纠铁矿位于矿田西北端,矿体主要产于花岗闪长岩和二长花岗岩与二叠系敌布错组的接触带及敌布错组层间破碎带中。矿区主要金属矿物有磁铁矿、穆磁铁矿、赤铁矿、磁赤铁矿及少量的褐铁矿、针铁矿等,夕卡岩矿物有石榴石、透辉石、金云母、蛇纹石、绿帘石、阳起石等,其中金云母与磁铁矿密切伴生,本文对其进行电子探针成分分析和40Ar-39Ar同位素测年。电子探针成分分析表明金云母富镁贫铁,Mg/(Fe+Mg+Mn+Ti)介于0.90~0.94之间;金云母40Ar-39Ar同位素测年获得总气体年龄(Total age)为112.3 Ma,与成矿相关的花岗闪长岩(113.6±1.6 Ma)和二长花岗岩(112.6±1.6 Ma)年龄在误差范围内一致,指示矿床形成于早白垩世晚期(113 Ma)。与铁矿化时代不同,矿田内铜矿化主要发生在晚白垩世早期(87 Ma)。结合区域地质资料,认为滚纠铁矿的形成与班公湖-怒江洋壳的向南俯冲关系密切,冈底斯陆壳和羌塘陆壳在约113 Ma对接碰撞,已俯冲的班公湖-怒江洋壳在俯冲惯性和/或重力拖拽作用下发生板片回转,导致软流圈地幔流体上涌,热的软流圈地幔流体携带的巨大热能引发岩石圈地幔和上覆地壳发生部分熔融,形成以壳源为主的壳幔混源岩浆,在弧后拉张区上侵形成花岗闪长岩和二长花岗岩。同时,遭遇下拉组和敌布错组地层,与之发生接触交代作用并形成磁铁矿床。

The Nixiong ore field,located in the Longgeer-Gongbujiangda fault uplift zone, is an important part of Coqen-Shenzha iron and copper metallogenic belt.The Gunjiu iron deposit lies in the northwest of the Nixiong ore field.The ore bodies mainly occur in the contact zone,which lie between the intrusive granodiorite and monzogranite and the Permian Dibucuo Formation,with some also located in the interlayer fracture zone of Dibucuo Formation.The main metallic minerals include magnetite,hematite and maghemite,with small amounts of limonite and goethite,whereas the skarn minerals such as grossular,andratite,diopside,sahlite,ferrosalite,serpentine,phlogopite,actinolite,epidote and chlorite are developed in the Gunjiu iron deposit.Among these minerals the phlogopite is closely associated with magnetite.Electron microprobe analysis and 40Ar-39Ar dating of phlogopite were conducted in this paper.Electron microprobe analyses show that the phlogopite from the Gunjiu iron deposit is rich in magnesium but low in iron,with Mg/（Fe ＋Mg＋Mn＋Ti） value being 0.90～0.94.The 40Ar39Ar dating of phlogopite does not yield a plateau age or isochron age but a total age of 112.3 Ma.The value represents the time that magnetite started to precipitate and is well consistent with the published LA-ICP-MS zircon U-Pb age of 113.6±1.6 Ma and 112.6±1.6 Ma from granodiorite and monzogranite. The result suggests that the iron mineralization took place in late Early Cretaceous at about 113 Ma.Different from iron mineralization,the copper mineralization mainly occurred in early Late Cretaceous at about 87 Ma.Combined with regional geology and the published data,the authors hold that the Gunjiu iron deposit is related to the southward subduction of Bangong Co-Nujiang Ocean crust.In late Early Cretaceous（113 Ma）,the Gangdise and the Qiangtang continental crust started to collide.The subducted oceanic crust of Bangong Co-Nujiang began to rotate under the diving inertia and/or gravity drag.Because of the rotation of the oceanic crust,the asthenospheric fluids were upwelling and caused partial melting of the lithospheric mantle and the overlying crust due to the tremendous heat.At last there formed the mixed magma,which intruded upward under the back-arc extensional condition,forming intrusive rocks and fractionated a large number of ore-forming fluids.Meanwhile,the ore-forming fluids met the stratigraphic formation.There occurred contact metasomatism and eventual formation of the magnetite deposit.