摘要
拉萨地块中部措勤盆地广泛发育酸性火山岩,对该区火山岩的形成时代和产出动力学背景以往研究存在不同认识。本文以区内塔诺错-措勤断裂南北两侧的火山岩为对象,进行了锆石U-Pb年龄与Hf同位素组成,以及全岩元素地球化学组成的系统测定,旨在精确限定火山岩的形成时代,并探讨火山岩的源区组成及可能的成岩动力学制约机制。锆石LA-ICP-MS U-Pb定年结果表明,塔诺错-措勤断裂南北两侧火山岩的成岩年龄相近,均介于110~120Ma,说明它们均应归为早白垩世则弄群(K1z),并非以往区调资料所认为的断裂带南侧火山岩属古新世典中组(E1d),而北侧火山岩属早白垩世则弄群(K1z)。区内火山岩主要为一套英安质、流纹质为主体的酸性岩石组合,均可归为高钾钙碱性岩系。火山岩富轻稀土和Rb、Ba等大离子亲石元素,贫Nb、Ta、Ti等高场强元素,具有与俯冲作用有关弧型岩浆岩的典型特征。它们均具有负的锆石εHf(t)值(=-12.9^-1.6)和偏老的二阶段模式年龄(tDM2=1.28~1.99Ga),岩石应主要起源于中部拉萨地块古—中元古代基底的熔融,但成岩过程中有亏损地幔组分的参与。根据对火山岩地质地球化学特征与产出背景的综合分析,表明它们应形成于具较厚陆壳背景的大陆边缘弧环境,早白垩世中晚期班公湖-怒江洋板片的南向俯冲断离是诱发区内火山岩成因的主要动力机制。
The acidic volcanic rocks are widespread in the Coqen basin, central Lhasa subterrane. Although numerous studies have been conducted on theses volcanic rocks, many uncertainties and controversies still remain as to their petrogenesis and geodynamic setting. In this paper, we take the volcanic rocks outcropped in both south and north sides of the Tanuochuo-Coqen fault as an example, and conducted an integrated study including zircon U-Pb dating and Hf isotopes, and whole-rock elemental geochemistry, aiming to obtain the precise ages, and to discuss magma source and geodynamic mechanism for their formation. Previously regional survey suggests that the volcanic rocks in the north side of the fault belong to the Lower Cretaceous Zenong Group and those in the southern side belong to the Paleocene Dianzhong Formation. However, our new zircon LA-ICP-MS U-Pb dating results demonstrate that volcanic rocks from both sides of the fault share similar ages of 110~120 Ma, indicating that they all belong to the Lower Cretaceous Zenong Group. Lithologically, volcanic rocks in the studied region are mainly a set of dacitic and rhyolitic rock association. They are chemically belong to high-K calc-alkaline series, and are enriched in LREEs and LILEs （e. g., Rb, Ba, etc.）, depleted in HFSEs （e. g., Nb, Ta, Ti, etc.）, showing typical signatures of arc-related igneous rocks under a subduction setting. All samples have enriched zircon Hf isotope compositions, with εHf（t） values of -12.9- -1.6 and tDM2 values of 1.28-1.99 Ga, indicating that these volcanic rocks were derived mainly from anatexis of the Paleoproterozoic to Mesoproterozoic basement of central Lhasa subterrane, with contributions of depleted mantle components. Based on a synthesis of geology, geochemistry and tectonics, we suggest that these volcanic rocks were likely generated under a continental marginal arc setting with a relatively thick crust, and the slab break-off of the southward subducted Bangong-Nujiang oceanic lithosphere occurred during the m
出处
《地质学报》
EI
CAS
CSCD
北大核心
2016年第11期3173-3191,共19页
Acta Geologica Sinica
基金
中国地质调查局工作项目(编号12120115026801)
国家自然科学基金项目(编号41572041)联合资助成果
