西藏班公湖-怒江结合带中段早白垩世花岗闪长斑岩年龄、Hf同位素及地球化学特征

西藏班公湖-怒江结合带中段分布大量早白垩世岩浆岩,其岩浆源区和岩石成因与班公湖-怒江洋的演化密切相关。对班公湖-怒江结合带中段东卡错地体发现的花岗闪长斑岩进行了系统的岩相学、年代学和地球化学研究。锆石U-Pb年龄结果显示,花岗闪长斑岩成岩时代为早白垩世(109.4±1.9 Ma)。样品显示出较高的SiO 2(67.40%~69.48%)和Al2O 3(15.66%~15.81%)含量及较低的Mg#值(19.03~21.48),具有高钾钙碱性系列岩浆岩特征;稀土元素总量较低(ΣREE=196.45×10^-6~207.6×10^-6),轻稀土元素富集,重稀土元素亏损,轻、重稀土元素分异明显,且具有较明显的负Eu异常(δEu平均值为0.64);微量元素蛛网图总体呈右倾的锯齿状,富集大离子亲石元素,亏损高场强元素;锆石εHf(t)均为负值且变化范围不大(-4.21^-10.59),对应的Hf模式年龄t2DM在1438~1842 Ma之间,显示古老地壳的特征。综合以上特征并结合区域资料分析,认为东卡错地体发现的花岗闪长斑岩为I型花岗岩,来源于古老下地壳的部分熔融,可能是东卡错地体与北拉萨地体或聂荣地体/南羌塘碰撞过程中板片断离岩浆活动的产物。

Age,Hf isotopes and geochemistry of Early Cretaceous granodioriteporphyry in the middle segment of Bangong Co-Nujiang suture zone of Tibet

Early Cretaceous igneous rocks are widely distributed in the middle segment of Bangong Co-Nujiang suture zone,and the nature of magma source and petrogenesis of these rocks are of great significance for the closure of the Bangong Co-Nujiang Ocean.The authors carried out systematic petrographic,geochemical and isotopic studies of the granodiorite-porphyries from Dongkaco microcontinent,middle segment of the Bangong Co-Nujiang suture zone.The results show that the granodiorite-porphyries have the age of 109.4±1.9 Ma by zircon U-Pb dating method,and granodiorite-porphyries are characterized by high K calc-alkaline series,enrichment of SiO2,Al2O3 and depletion of Mg#.The content ofΣREE is relatively low(196.45×10^-6~207.6×10^-6)and the rocks are relatively enriched in LREEs and depleted in HREEs.In the spider diagram the trace elements are inclined to the right in zigzag form and display enrichment of large-ion lithophile elements and relative depletion of high field strength elements.The granodiorite-porphyries have negative zirconεHf(t)values(-4.21^-10.59),corresponding to crustal Hf model ages of 1438~1842 Ma and showing the characteristics of the ancient crust.Combining the above characteristics with regional information,it is believed that the granodioriteporphyries were found in the Dongkaco microcontinent and belonged to I-type granite,which may be the product of magmatic activity caused by the oceanic fragmentation during the collision between the Dongkaco micro-continent and the Central Lhasa or Nie Rong terrane.