内蒙古黄岗锡铁矿床花岗岩深部源区特征与构造岩浆演化:Sr-Nd-Pb-Hf多元同位素制约

Deep Source Characteristics and Tectonic-Magmatic Evolution of Granites in the Huanggang Sn-Fe Deposit,Inner Mongolia:Constraint from Sr-Nd-Pb-Hf Multiple Isotopes

黄岗锡铁矿是大兴安岭南段成矿带的一个重要矿床,其广泛分布的花岗岩类与成矿有密切的成因联系。采用热电离质谱仪和LA-ICP-MS系统研究了黄岗花岗岩的Sr、Nd、Pb和Hf同位素组成特征,结果显示其[N(87Sr)/N(86Sr)]i值在0.702 11～0.707 29间,接近现代大洋玄武岩而小于大陆地壳的[N(87Sr)/N(86Sr)]i值,εNd(t)值在-0.8～0.9间,Nd模式年龄TDM介于993～855Ma间;Pb同位素值较高,N(206Pb)/N(204Pb)介于18.974～26.107间,N(207Pb)/N(204Pb)介于15.554～15.914间,N(208Pb)/N(204Pb)主要在38.894～39.890间,铅具有混合来源特征;N(176 Hf)/N(177 Hf)比值介于0.282 744～0.282 922间,εHf(t)值变化范围较大,为1.9～18.3,两阶段Hf模式年龄(TDM2)变化范围为888～561Ma。同位素组成特征表明,黄岗花岗岩的源区为起源于亏损地幔的初生下地壳物质的部分熔融,可能存在少量古老陆壳物质的混染。年轻的Nd、Hf同位素模式年龄暗示本区新元古代时期曾发生一次重要的地壳增生事件。结合区域晚中生代构造背景,黄岗花岗岩的形成机制为:被俯冲洋壳析出流体交代的地幔楔或亏损地幔减压部分熔融作用形成的基性岩浆分离后,诱发岩石圈拆沉、幔源岩浆上涌和底侵,促使镁铁质的初生地壳物质重熔并不断分异演化,从而产生了大量花岗岩浆,其地球动力学背景可能与太平洋板块的NNW斜向俯冲作用有关。

Huanggang tin-iron deposit,Inner Mongolia,is an important deposit of the South Daxinganling metallogenic belt,and its widely distributed granitoids have close genetic relationship with mineralization.This paper studies systematically Sr,Nd,Pb and Hf isotopic compositions.The results show that（87Sr/86Sr）i values range from 0.702 11 to 0.707 29,close to the（87Sr/86Sr）i of modern oceanic basalts and lower than the（87Sr/86Sr）i of continental crust.The εNd（t） values and Nd model ages（TDM） vary from-0.8 to 0.9 and 993 to 855 Ma respectively.The Pb isotope values are relatively high,with the 206Pb/204Pb values range from 18.974 to 26.107,207Pb/204Pb values between 15.554 and 15.914,and 208Pb/204Pb values between 38.894 and 39.890,which suggested the lead with a mixed source.The 176Hf/177Hf values ranging from 0.282 744 to 0.282 922,with the corresponding εHf（t） values ranging from 1.9 to 18.3,and Hf model ages（TDM2） from two-stage between 888 and 561 Ma.The isotopic composition characteristics reveal that Huanggang granites are derived from the partial melting of newborn lower crust originated from depleted mantle,and could be with the contamination of a small amount ancient continental crust.Young Nd,Hf isotope model ages imply that this area could have experienced an important crustal growth event during the Late Proterozoic.Combining with regional tectonic setting of Late Mesozoic,we could conclude that the formation mechanism of Huanggang granites as following： the separation of mafic magma,produced from the partial melting of mantle wedge metasomatized by the precipitated fuilds from subducted oceanic crust or the decompressed depleted mantle,could induce lithospheric delamination,mantle upwelling and magma underplating,which promoted the remelting,differentiation and continued evolution of mafic primary crust,resulting in a large number of granitic magma,and its geodynamic setting may be related with NNW Pacific plate subduction.