小秦岭地区古元古代晚期花岗岩的LA-ICP-MS U-Pb年龄、地球化学特征及其构造意义

LA-ICP-MS U-Pb ages and geochemical characteristics of the Late Paleoproterozoic granitoids in Lesser Qinling Mountins and their tectonic significance

古元古代晚期的花岗岩类在华北克拉通南缘广泛分布,记录了区域岩石圈演化的重要信息。通过对小秦岭地区南部沿小河断裂出露的花岗岩类的锆石LA-ICP-MS U-Pb年代学、岩石地球化学和Nd—Hf同位素地球化学研究,为小秦岭地区古元古代晚期的地壳演化提供了依据。自西向东,小河中粗粒二长花岗岩、小河细粒二长花岗岩和贵家峪花岗闪长岩的结晶年龄分别为1831±32 Ma、1860±13 Ma和1811±10 Ma。3个花岗岩体均以富硅(67.4%~76.0%)和高碱(7.47%~9.33%)为特征,属于高碱钙碱性到钾玄岩类。地球化学特征表明两个小河花岗岩体属I型花岗岩,具有碰撞型花岗岩特征,其中形成年龄为1831 Ma花岗岩具有高的Sr/Y(26.56~49.52)和(La/Yb)N(31.35~236.71)值、Eu负异常(0.42~0.68),表明源区残留有石榴子石和少量的斜长石;贵家峪花岗闪长岩具有较高的FeO^(T)/MgO值(17.7~29.2)和10000Ga/Al值(2.95~3.17),表现出造山后A2型花岗岩特征,与1831 Ma的地壳相比,地壳厚度明显减薄。3个岩体的岩浆锆石ε_(Hf)(t)=-10.5~-4.5,两阶段模式年龄T_(DM2)=2781~3122 Ma,全岩的ε_(Hf)(t)=-7.5~-4.5,T_(DM2)=2667~2888 Ma,均指示了其物质来源于中到新太古代古老基底的部分熔融。这期岩浆事件可能是小秦岭区域对1.85 Ga吕梁运动的响应,反映了碰撞造山作用地壳增厚向碰撞后地壳伸展减薄这一演化过程。

Widespread Late Paleoproterozoic magmatism has been identified in the southern margin of North China Craton(SMNCC)especially in Lesser Qinling Mountains(Xiaoqinling area).The Xiaohe and Guijiayu granitoids are located in the SMNCC along the Xiaohe fault.A combined LA-ICP-MS zircon U-Pb geochronology,whole-rock geochemistry and Nd-Hf isotope study of the granitoids in Lesser Qinling Mountains provides the insights to discuss the Late Paleoproterozoic crustal and tectonic evolution of SMNCC.From west to east,the crystallization ages of Xiaohe mediumcoarse grained monzonitic granite,Xiaohe fine-grained biotite monzonitic granite and Guijiayu granodiorite are 1831±32 Ma,1860±13 Ma and 1811±10 Ma,respectively.All samples are characterized by high SiO2(67.4%~76.0%)and high alkali(7.47%~9.33%),plot in the calc-alkali to shoshonite fields.Geochemical characteristics show that XH-2 and WJY plutons are Ⅰ-type granite,with collision granite characteristics.XH-2 pluton,whose forming age is 1831 Ma,has high Sr/Y(26.56~49.52)and(La/Yb)N(31.35~236.71)ratios,Eu negative anomalies(0.42~0.68),indicating that the residue is garnet and plagioclase.The Guijiayu granodiorite has high FeO^(T)/MgO(17.7~29.2)and Ga/Al(2.95~3.17),showing the characteristics of A2-type granite formed in a post-collisional environment,and the crustal thickness is significantly reduced compared to that of 1831 Ma.The zircons have negativeε_(Hf)(t)(-10.5 to-4.5),and T_(DM2)=3122~2781 Ma,with a consistentε_(Hf)(t)(-7.5 to-4.5)and T_(DM2)(2888~2667 Ma).It is recognized from Hf and Nd isotopes,the granitoids formed by the partial melting of the Meso-to Neoarchean crystalline basement.This magmatism in Lesser Qinling Mountains may be related to the 1.85 Ga Lüliang movement,reflecting the crustal evolution process from collisional orogeny to post-orogenic extension.