胶西北仓上、三山岛岩体二长花岗岩地球化学、锆石U-Pb年龄及Lu-Hf同位素研究

Geochemistry,Zircon U-Pb Age and Lu-Hf Isotopes of the Cangshang and Sanshandao Monzogranites in the Northwestern Jiaodong Peninsula,China

三山岛金成矿带位于胶西北金矿集区的西端,是胶东重要的金矿带,赋存有三山岛北部海域、三山岛、西岭、新立、仓上等大型、特大型金矿床,金矿的主要控矿围岩为二长花岗岩。本文通过对仓上金矿区片麻状黑云二长花岗岩、三山岛北侧似斑状黑云二长花岗岩的岩石地球化学、锆石LA-ICP-MS定年和Lu～Hf同位素组成测试和研究,讨论了该类岩石的地球化学类型、形成时代、岩浆源区与成因。岩石地球化学研究表明二长花岗岩具有高的SiO2(70.19%～76.36%)、全碱(Na_2O+K_2O=7.05%～8.19%)、Ba(>1573×10^(-6))、Sr(>738×10^(-6))、(La/Yb)N(>26.44)、Sr/Y(>121)、LILE/HFSE,低的Al_2O3(13.41%～15.61%)、MgO(0.1%～0.84%)、Y(<6.05×10^(-6))、Yb(<0.53×10^(-6))以及相对平坦的HREE,无明显的铕异常,明显亏损Nb、Ta、P、Ti等高场强元素,显示出典型的高Ba-Sr花岗岩所具有的地球化学特征,属高Ba-Sr花岗岩。锆石LA-ICP-MS定年结果表明仓上、三山岛二长花岗岩年龄分别为131±1Ma和127±2Ma,与郭家岭型花岗岩具有相似的主量元素组成及稀土和微量元素分布模式,表明仓上、三山岛岩体为郭家岭型花岗岩。仓上、三山岛二长花岗岩岩浆锆石εHf(t)值为-25.2～-18.1,对应的Hf二阶段模式年龄TDM 2为2.77～2.33Ga。从岩石学、地球化学和Lu-Hf同位素特征,并结合前人对郭家岭花岗岩成因研究成果,笔者认为仓上、三山岛二长花岗岩是陆壳酸性岩浆与幔源中基性岩浆混合而成。以壳源为主(主要来源于前寒武纪变质基底岩石,以新太古代-古元古代岩石为主),由幔源中基性岩浆混合后经分异作用而成的同熔深成型花岗岩,总体上,形成于较高温度、深度的环境。

Located at the western end of the gold ore cluster area in the northwestern Jiaodong Peninsula,the Sanshandao gold metallogenic belt is an important gold ore belt in the Jiaodong Peninsula,which hosts several large-scale and super-large gold ore deposits in Sanshandao and its northern part,Xiling,Xinli and Cangshang.Ore-controlling country rock of the gold deposits is monzogranite.This study carried out geochemistry analysis,zircon LA-ICP-MS U-Pb dating and Lu-Hf isotopic composition of Cangshang gneissic biotite monzogranite and Sanshandao porphyritic-like biotite monzogranite to discuss petrologic classification,forming age,magma source regions and genesis of the rocks.Lithogeochemical analysis shows that the monzogranites are high in SiO2（70.19%～76.36%）,total alkalis（Na2O＋K2O=7.05%～8.19%）,Ba（1573×10-6）,Sr（738×10-6）,（La/Yb）N（26.44）,Sr/Y（121）,LILE/HFSE,and low in Al2O3（13.41%～15.61%）,MgO（0.1%～0.84%）,Y（6.05×10-6）,Yb（0.53×10-6）,with relatively flat HREE,no obvious Eu anomaly.The rocks are characterized by prominent depletion of Nb,Ta,P and Ti,presenting geochemical feature of typical high Ba-Sr granite.All these features indicate that the rocks belong to the high Ba-Sr granites.LA-ICP-MS zircon dating yields the U-Pb ages of 131±1 Ma and 127±2 Ma for Cangshang and Sanshandao monzogranites,respectively.The Cangshang and Sanshandao monzogranites have similar major elements compositions,REE and trace element distribution patterns as the Guojialing-type granitoid granites, suggesting that Cangshang and Sanshandao monzogranites are Guojialing-type granitoid granites.The Cangshang and Sanshandao monzogranites have zirconεHf（t）values of-25.2 to-18.1,and corresponding two-stage Hf model ages（TDM2）range from 2.77 to 2.33 Ga.Based on previous studies on the genesis of the Guojialing granite and the results obtained in this study,it can be concluded that the Cangshang and Sanshandao monzogranites were the products of mixture between crustal felsic magma and mantle intermediate-mafic magma,i.e.syntexis plutonic granites formed by differentiation after the mixture of the crustal magma（mainly originated from the Precambrian metamorphic basement rocks consisting of Neoarchean-Palaeoproterozoic rocks）,and mantle intermediate-mafic magma in a high temperature and deep environment.