拉萨南部始新世曲水辉长岩成因及其对同碰撞岩浆活动的指示意义  

Petrogenesis of the Eocene Quxu Gabbro in Southern Lhasa and Its Implications for Syn-collisional Magmatism

作　　者：耿啟凡 马林[1,2,3] 李奇维 GENG Qifan;MA Lin;LI Qiwei(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China;University of Chinese Academy of Sciences,Beijing 100049,China;CAS Center for Excellence in Deep Earth Science,Guangzhou 510640,Guangdong,China;School of Earth Sciences,China University of Geosciences,Wuhan 430074,Hubei,China)

机构地区：[1]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广东广州510640 [2]中国科学院大学,北京100049 [3]中国科学院深地科学卓越创新中心,广东广州510640 [4]中国地质大学(武汉)地球科学学院,湖北武汉430074

出　　处：《大地构造与成矿学》2024年第4期844-865,共22页Geotectonica et Metallogenia

基　　金：国家自然科学基金优秀青年科学基金项目(42122022)资助。

摘　　要：地球上的岩浆活动主要发生于大洋中脊、俯冲带和部分陆内或裂谷环境。大陆碰撞造山带在传统认识中通常是缺乏或不利于岩浆产生的。然而,青藏高原作为全球典型的陆陆碰撞造山带,在其南部的拉萨地块却发育有大量早始新世(51±3 Ma)岩浆作用,这些岩浆岩的成因和相关的深部动力学过程仍然是重要的未解之谜。本文对拉萨地块南部曲水地区一套始新世(53~50 Ma)辉长岩和辉石闪长岩进行岩石学、年代学和元素、同位素地球化学研究。结果显示,辉长岩由斜长石(约45%~55%)、单斜辉石(约20%~30%)、角闪石(约20%~30%)组成,辉石闪长岩主要由斜长石(约45%~55%)、角闪石(约30%~40%)、单斜辉石(约10%~20%)等组成。根据地球化学特征,样品可分为四组:第一组辉长岩样品(19QS03-11)稀土元素配分型式相对平坦,与E-MORB特征相似,可能源于亏损软流圈组分与岩石圈地幔之间的相互作用;相较第一组样品,第二组辉长岩轻重稀土元素分异显著((La/Yb)_(N)=4.10~6.61),指示其源区中软流圈组分参与减少;第三组样品主要为辉石闪长岩,具相对高的SiO_(2)和Th含量,可能受到一定程度地壳混染的影响;第四组样品表现为低硅高镁和极低的微量元素组成,主要受控于单斜辉石的堆晶效应。结合前人研究,研究区始新世岩浆岩中有软流圈组分的贡献。这些软流圈组分的参与可能与板片回转、岩石圈拆沉或板片断离等过程相关,并诱发了冈底斯大规模同碰撞岩浆活动。结合已有数据对上述模型进行了初步的合理性分析,提出板片断离可能是当前较为合理的模型,对于同碰撞岩浆活动的形成机制与深部动力学过程仍需进一步深入研究。Magmatism mainly occurs in mid-ocean ridges,subduction zones,and some intracontinental or rift environments.Continental collision orogenic belts usually lack magmatism from a traditional perspective.As a typical continental collision orogenic belt throughout the world,the Qinghai-Tibet Plateau has a large amount of Early Eocene(51±3 Ma)magmatism in Southern Lhasa block.However,the genesis of the magmatism there and related deep dynamics processes are still poorly understood.This paper reports the petrological,chronological,elemental and isotope geochemical results of a set of Eocene(53-50 Ma)gabbro and diorite in the Quxu area of the southern Lhasa.These gabbro and diorite are composed of plagioclase(45%-55%)+clinopyroxene(20%-30%)+amphibole(20%-30%)and plagioclase(45%-55%)+amphibole(30%-40%)+clinopyroxene(10%-40%),respectively.The samples are divided into four groups according to their geochemical characteristics:(1)The Group gabbroⅠis the most depleted and shares more similar features with E-MORB,which indicates that it may come from the interaction between upwelling depleted asthenosphere and lithospheric mantle;(2)The Group Ⅱ gabbro shows depleted characteristic of Sr-Nd isotope(εNd(t)>5)but relatively obvious differentiation of LREE and HREE((La/Yb)N=4.10-6.61),indicating the reduction of asthenosphere components involved;(3)the Group Ⅲ samples are mostly diorite affected by crustal contamination(Th/Nb>1.4);(4)the Group Ⅳ samples are mainly controlled by clinopyroxene and plagioclase cumulation.Based on the elemental and isotopic characteristics of the depleted gabbro,it can be concluded that asthenosphere material was involved in the formation of the Eocene rocks.The results show that magmatism in the Gangdese syncollisional zone may be related to slab rollback,lithosphere delamination,or slab breakoff.Combining our results with existing evidence,we argue that slab breakoff is relatively more feasible according to the time of asthenosphere upwelling and the spatial isotope distribution characteris

关 键 词：冈底斯 辉长岩 同碰撞岩浆作用 岩石成因 深部动力学 

分 类 号：P581[天文地球—岩石学]