山西云中山地区~2.53Ga碱性花岗岩的成因和地质意义

Petrogenesis and geological implications of the~2.53Ga alkaline granites in Yunzhong Mts,Shanxi

在山西云中山遨湖地区进行野外填图时,识别出一个新太古代碱性花岗岩岩体。该岩体东西向展布,长约3km,宽约1km,岩石类型为花岗闪长岩和正长花岗岩。岩石具有较高的SiO 2(66.33%~75.01%)、Na 2O(3.78%~7.62%)、碱(Na 2O+K 2O=7.89%~12.56%)和Al 2O 3(13.95%~18.08%)含量;贫CaO(0.14%~2.20%)和MgO(0.03%~0.23%);另具较高的AR(2.72~5.44)、里特曼指数(δ=2.22~6.66)和FeO T/MgO值(3.75~30.39);其稀土总量较低,富集轻稀土和大离子亲石元素(Rb、Th、K),相对亏损Nb、Ta、Sr、Ti、P等元素,与I型花岗岩的特征相似。上述全岩主、微量元素结果显示这套岩石与云中山地区新太古代的花岗闪长岩成分相似,指示碱性花岗岩的母岩浆来自新生地壳(花岗闪长岩)的部分熔融。相较于正长花岗岩,花岗闪长岩的SiO 2含量偏低,但Na 2O含量更高。因此,我们推测在岩石形成过程中经历了分离结晶作用,早期形成花岗闪长岩,晚期形成正长花岗岩。结合区域地质及前人的研究成果,我们认为该新太古代碱性花岗岩形成于板块俯冲碰撞之后的伸展环境中。此外,SHRIMP锆石U-Pb测年结果显示其侵位时代为~2527Ma。锆石原位Lu-Hf同位素分析显示该碱性花岗岩具有正的εHf(t)值(+3.5~+6.6),二阶段模式年龄(t DM2)为2520~2710Ma,在误差范围内与岩石的结晶年龄一致。云中山地区从超镁铁质岩-镁铁质岩、TTG向正长花岗岩的转变,记录着华北克拉通新太古代晚期由板块俯冲碰撞向碰撞后伸展的转变。新太古代碱性花岗岩的出现代表一次重要的热事件,标志着克拉通化的完成。

During a detailed field geological mapping,a suit of Neoarchean alkaline granites is newly identified in the Aohu area of Yunzhong Mts,Shanxi.This intrusion outcropped in a nearly EW extension and sized by 3km in length and 1km in width.The main rock types are granodiorite and syenogranite.The anlysized samples have high SiO 2(66.33%~75.01%),Na 2O(3.78%~7.62%),Al 2O 3(13.95%~18.08%)and total alkali(Na 2O+K 2O=7.89%~12.56%)contents together with relatively high AR(2.72~5.44),Rittman index(δ=2.22~6.66),FeO T/MgO(3.75~30.39)values,but low CaO(0.14%~2.20%)and MgO(0.03%~0.23%)concentrations.They are enriched in light rare earth elements(REEs)and large ion lithophile elements(LILEs)such as Rb,Th,K,but depleted in such elements as Sr,Ba,Ti and P,which is affinitive to the I-type granite.SHRIMP zircon U-Pb analyses give an emplacement age of~2527Ma.The in-situ zircon Lu-Hf isotopic analysis results show that these Neoarchean alkaline granites have positiveεHf(t)values(+3.5~+6.6)with two-stages depleted mantle model Hf ages(t DM2)of 2520~2710Ma,which are consistent with the crystallization age within errors.In addition,the measured values of whole-rock major and trace elements show that the alkaline granites are similar to the Neoarchean granodiorites in Yunzhong Mts,suggesting that their parent magma is derived from partial melting of a juvenile crust(granodiorite).Compared to syenogranite,the granodiorite has lower SiO 2 but higher Na 2O contents.It is implied that the magma had experienced a fractional crystallization to generate the granodiorites at the early stage,and the syenogranites at the late stage.Combined with regional geology and previous studies,we proposed that the alkaline granites were formed in a post-collision extensional environment.The series of mafic-ultramafic rocks,TTG,monzogranite,granodiorite and syenogranite record the tectonic transition from subduction-collision to post-collision extension in the Late Neoarchean in the central North China Craton.The presence of the Neoarchean alkaline granite represents an important thermal event,marking the completion of cratonization.