阿尔金南缘塔特勒克布拉克花岗岩的地球化学特征、锆石U-Pb定年及Hf同位素组成

Geochemical,zircon U-Pb dating and Hf isotope compositions studies for Tatelekebulake granite in South Altyn Tagh

地球化学研究显示,阿尔金造山带南缘瓦石峡河附近塔特勒克布拉克二长花岗岩具有高硅(SiO2=63.62%～72.06%)、高碱(ALK=6.39～7.83)、过铝质(ASI=1.27～1.43)的特征,其稀土元素含量中等,Eu负异常较明显,轻稀土相对富集;在原始地幔标准化蛛网图上,大部分样品相对富集LREE、Rb、Th、U等大离子亲石元素,亏损Nb、Ta、Sr、P、Ti等高场强元素,指示其为壳源型高钾钙碱性系列的强过铝质S型花岗岩。锆石Hf同位素研究结果表明,其εHf(t)=-9.1～-10.5,两阶段Hf同位素模式年龄tDMC为1457～1553Ma,反映该花岗岩的源岩主要来自于地壳物质的重熔,其来源也相对单一。岩石中Nb/Ta、Zr/Hf、Th/U比值及高的CaO/Na2O比值(>0.3),高的Al2O3、CaO和FeOT+MgO+TiO2总量,暗示该花岗岩是由中下地壳的杂砂岩经过黑云母脱水熔融形成的。较低的Al2O3/TiO2比值以及高的Y和Yb含量表明,该花岗岩的源岩是在较高的温度及较低的压力条件下部分熔融产生的,熔融的残留相可能为斜长角闪岩。阴极发光图像显示,该岩石中的锆石呈自形的长柱状,具有明显的岩浆环带结构,LA-ICP-MS微区原位U-Pb定年获得462±2Ma一组年龄,结合阴极发光图像和锆石微量元素特征(Th/U>0.1),推断该年龄值为花岗岩的形成年龄。根据CaO/Na2O-Al2O3/TiO2图解和R1-R2构造判别图解,确定该花岗岩可能形成于碰撞造山后抬升初期。对比花岗岩形成年龄(462Ma)与阿尔金造山带南缘已获得的高压-超高压岩石变质年龄(～500Ma),可推断阿尔金瓦石峡南二长花岗岩形成于阿尔金造山带俯冲碰撞造山后应力释放的初级阶段,在该阶段地壳物质由于压力的降低从而发生部分熔融。

Geochemical researches indicate that the monzogranite from Washixia area in the South of Altyn Tagh is rich in silica (SiO2=63.62%~72.06%) and alkali (ALK=6.39~7.83),and is peraluminous. The rock has moderate REE concentration and negative Eu anomaly,showing the relative enrichment of LREE pattern,which is a typical high-K calc-alkaline S-type granite with the character of enrichment of LILE (Rb,Th and U) and depletion of Nb,Ta,Sr,P and Ti. All spots of zircons from the rock have εHf(t)=-9.1^-10.5 and tDMC=1457~1553Ma,suggesting that the source rock of the granite is mainly from crust which was accreted in Mesoproterozoic. The element ratios,such as Nb/Ta,Zr/Hf,Th/U,combining with the high CaO/Na2O and low Al2O3/ TiO2,high Al2O3,CaO,FeOT+MgO+TiO2 and Y,Yb concentrations,indicate the rock is formed by the biotite partial melting of greywacke from middle to lower crust in the condition of lower pressure and higher temperature,with plagioclase amphibolite as the residue. Zircons from the granite are euhydral and preserve magmatic zones on the CL images. Based on the CL images and the characters of trace element (Th/U >0.1) of zircons,LA-ICP-MS in situ U-Pb dating obtains the age of 462±2Ma should be the formation age of the granite. According to CaO/Na2O-Al2O3/TiO2 and R1-R2 diagram,the granite is formed in original period after collision. Comparing the formation age (462Ma) with the metamorphic ages (~500Ma) of South Altyn Tagh,the monzogranite from Washixia area in Altyn Tagh is produced in the initial stage of stress relief after collision,when the crustal materials were partial melted due to the lessening of stress.