阿尔金南缘中段清水泉斜长花岗岩同位素年龄及成因研究

Isotopic age and genesis of plagiogranite from Qingshuiquan area in the middle of South Altyn Tagh

阿尔金南缘清水泉地区与基性-超基性岩伴生的花岗岩为斜长花岗岩。岩石地球化学显示该花岗岩高硅、富铝和钠,低镁和钾;轻稀土富集,具有Eu的正异常（δEu为1.01～2.01）。岩石富Rb、Ba,特别高Sr（779×10^-6～864×10^-6）,低Y（1.17×10^-6～1.51×10^-6）及Yb（0.15×10^-6～0.20×10^-6）,强烈亏损Nb、Ta等。斜长花岗岩锆石振荡环带清晰,Th/U和Nb/Ta比值分别为0.38～0.52,2.92～5.04;具有明显的Ce正异常和Eu负异常,为典型的岩浆锆石,利用LA-ICP-MS微区原位定年获得该花岗岩206Pb/238U-207Pb/235U谐和年龄为465Ma,206Pb/238U加权平均年龄为451±4Ma。锆石饱和温度计和锆石Ti温度计演算结果显示锆石的结晶温度分别为783～811℃和693～821℃。推测花岗岩源区压力范围为1.8～2.0GPa,形成深度在60km以上。综合分析清水泉花岗岩主、微量元素地球化学特征,并结合区域地质,认为该花岗岩属＂I＂型花岗岩,由地幔基性岩浆上侵分异形成,产于伸展环境。

Qingshuiquan area plagiogranite associated with mafic-ultramafic rocks distributed in the middle of South Altyn Tagh.This plagiogranite is characterized by relatively high Si O2,Na2 O,Al2O3and low in K2 O,Mg O contents. Its geochemical data shows that the granite are enriched in LREE with weakly positive Eu anomalies（ δEu = 1. 01 ～ 2. 01）. The primitive mantle-normalized spider-grams are characterized by Rb,Ba enrichments and Y（ 1. 17 × 10^- 6～ 1. 51 × 10^- 6）,Yb（ 0. 15 × 10^- 6～ 0. 20 × 10^- 6）depletion,especially for a significantly negative Nb-Ta anomaly and positive Sr（ 779 × 10^- 6～ 864 × 10^- 6） anomaly. Zircons from plagiogranite are typical magmatic zircon crystals with negative Eu and positive Ce anomalies. Their ratios of Th / U and Nb / Ta are 0. 38～ 0. 52 and 2. 92 ～ 5. 04,which is typical for magmatic zircon. The LA-ICP-MS U-Pb isotopic dating on zircons yield206 Pb /238U-207 Pb /235 U concordia age and206 Pb /238 U weighted mean age of 451 ± 4Ma and 465 Ma,respectively. According to the calculation results on zircon saturation and Ti thermometer,the zircons are indicated being formed under the temperature of ca. 693 ～ 821℃ and pressure1. 8 ～ 2. 0GPa. Based on comprehensive analysis of the petrology,mineralogy,geochemistry and regional geology, it may be preliminarily determined that the Qingshuiquan granite is I-type granite with adakite features, which formed in the geological background of extension and related to mafic magma differentiation.