摘要
青藏高原中部班公湖-怒江缝合带的两侧在晚白垩世经历了大规模的岩浆活动,但其深部动力学机制至今仍存有较大争议。本文对藏北盐湖地区新识别出的辉长岩进行了系统的锆石U-Pb定年、原位Hf同位素以及全岩主微量元素分析,旨在为该缝合带的构造演化过程提供新的约束。分析结果显示,辉长岩形成于~100 Ma,其地球化学特征与洋岛玄武岩(OIB)相似,具有明显的Nb-Ta元素亏损,εHf(t)值介于6.60~8.33之间。这些特征表明,辉长岩具有板内基性岩的地球化学属性,是软流圈地幔部分熔融熔体混染地壳物质的产物。结合区域上近同期基性岩脉和大规模高Mg埃达克质岩浆活动,本文认为盐湖辉长岩的形成于碰撞后伸展环境,是拉萨-羌塘地块碰撞后增厚榴辉岩相下地壳拆沉过程中,深部软流圈地幔上涌减压熔融的产物。盐湖辉长岩的发现为青藏高原中部晚白垩世碰撞后增厚下地壳拆沉模型提供了岩石学上的证据。
Late Cretaceous magmatic rocks are extensively exposed along the flanks of the Bangong-Nujiang Suture Zone in central Qinghai-Tibet Plateau.However,the geodynamic mechanism has always been controversial due to the lack of constraints on the mafic rocks.Here,we report zircon U-Pb age,whole-rcok major and trace element geochemistry,and zircon in situ Hf isotopes of gabbros in the Yanhu area,Northern Xizang,to provide new constraints on the geological evolution of the Bangong-Nujiang Suture Zone.Zircon U-Pb dating results indicate that gabbros formed at 100 Ma with ε_(Hf)(t)values of 6.60-8.33.The results show that the gabbros have geochemical affinity to OIB-type rocks but have obvious depletion of Nb-Ta.These geochemical features indicate that the gabbros belong to intraplate mafic rocks,derived from the partial melting of the upwelling asthenosphere with various degrees of crustal contamination.Our results,combined with those of the regional coeval mafic dykes and high-Mg adakitic rocks,demonstrate that the gabbros were formed in post-collisional extension setting,which was triggered by the delamination of the over thickened eclogitic lower crust after the Lhasa-Qiangtang continental collision.The identification of the Yanhu gabbros provides direct petrological evidence of mantle convection induced by post-collisional delamination in Northern Xizang.
作者
周高宇
祁诚雪
万牧钦
吴浩
黄煜奇
李宾铭
ZHOU Gaoyu;QI Chengxue;WAN Muqin;WU Hao;HUANG Yuqi;LI Binming(Collaborative Innovation Center for Exploration of Nonferrous Metal Deposits and Efficient Utilization of Resources in Guangxi,Guilin University of Technology,Guilin 541004,Guangxi,China;Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi,Guilin University of Technology,Guilin 541004,Guangxi,China)
出处
《大地构造与成矿学》
EI
CAS
CSCD
北大核心
2024年第4期866-878,共13页
Geotectonica et Metallogenia
基金
广西科技基地和人才专项(桂科AD21220033)
山东省自然科学基金项目(ZR2020QD045)
中国地质调查项目(DD20190167)
桂林理工大学人才引进科研启动项目(RD2000002384)联合资助。
