鲁西金岭矽卡岩型铁矿区早白垩世石英闪长岩成因:年代学和地球化学证据

THE PETROGENESIS OF EARLY CRETACEOUS QUARTZ DIORITE IN SKARN IRON DEPOSITS,JINLING AREA,LUXI:EVIDENCE OF GEOCHRONOLOGY AND GEOCHEMISTRY

金岭岩体是鲁西地区代表性的中生代侵入杂岩体,与金岭矽卡岩型铁矿的成因密切相关。为了解石英闪长岩的成因及与矽卡岩铁矿成矿关系,本次研究对金岭杂岩体中的石英闪长岩进行了年代学、地球化学及锆石Lu—Hf同位素和角闪石矿物微区成分的系统研究。LA-ICP-MS锆石U-Pb年代学显示石英闪长岩加权平均年龄为(128±1) Ma,侵位时代为早白垩世;样品具有较低的w(SiO_2)(58.34%~65.22%),和高的w(Al_2O_3)(14.95%~15.72%),w(Na_2O)为4.44%~5.16%,w(K_2O)为2.26%~5.17%,w(CaO)为3.39%~6.30%,w(Fe_2O_3~T),w(MgO)含量变化范围较大,分别为2.21%~6.86%和2.32%~5.21%,此外,样品具有较高的Sr含量(682~834×10^(-6))、Ba含量(1 130~1 595×10^(-6)),表现出高Ba, Sr花岗岩的岩石地球化学特征。石英闪长岩中角闪石为岩浆成因角闪石,具有壳源岩浆成因角闪石的特征,其结晶时的氧逸度为△NNO+0.83~△NNO+1.87,温压条件为849~1 092℃和89~279 MPa。石英闪长岩含有较高的Ni含量、Co含量、Cr含量以及Mg~#,其Nb/Ta, Zr/Hf, Rb/Sr和Ba/Rb比值接近原始地幔平均值。样品含有~2.5Ga的捕获锆石,所有锆石的ε_(Hf)(t)值为负值。锆石Hf同位素和岩石地球化学特征表明石英闪长岩侵位深度为3.9~10.3 km,岩浆源区为岩石圈地幔并受到地壳物质的混染。

Jinling pluton is a representative Mesozoic intrusive complex in the west Shandong area and it is closely related to the genesis of Jinling skarn type iron ore deposit.In this paper,the geochronology and geochemistry of quartz diorite in Jinling complex,as well as the composition of zircon Lu-Hf isotope and hornblende mineral microzone are systematically studied in order to understand the origin of acid magmatic activity and its relationship with skarn ore mineralization.LA-ICP-MS zircon U-Pb geochronology shows that the weighted mean age of the quartz diorite is 128±1 Ma and the emplacement age is early Cretaceous.The samples have low w(SiO2)content(58.34%-65.22%),high w(Al2O3)content(14.95%-15.72%),w(Na2O)content(4.44%-5.16%),w(K2O)content(2.26%-5.17%),and w(CaO)content(3.39%-6.30%).While the contents of w(Fe2O3T)and w(MgO)have a wide range from 2.21%to 6.86%and from 2.32%to 5.21%,respectively.In addition,the samples have high Sr content(682-834×10^(-6))and Ba content(1130-1595×10^(-6)),showing the characteristic of high Ba and Sr granites.The amphibole in quartz diorite is of magmatic origin and has the characteristics of crustal magmatic origin.The oxygen fugacity during crystallization is△NNO+0.83-△NNO+1.87,and the temperature and pressure conditions are 849-1092℃and 89-279 MPa respectively.The quartz diorites in the area contain high Ni,Co,Cr and Mg#contents,and their Nb/Ta,Zr/Hf,Rb/Sr and Ba/Rb ratios are close to the original mantle average.Meanwhile,the samples contain-2.5 Ga captured zircons,all of which have negativeεHf(t)values.Zircon Hf isotopes and lithogeochemical characteristics indicate that the quartz diorite emplacement depth is 3.9-10.3 km,and the magma source area is lithospheric mantle and is mixed with crustal materials.