滇西剑川始新世粗面岩及其包体成因:印度-欧亚大陆碰撞过程的约束和大地构造意义  

Petrogenesis of Early Eocene Trachytes and Enclaves in the Jianchuan Basin,Western Yunnan:Constraints on Tectonic Processes of the India-Asia Collision and Tectonic Implications

作　　者：董晓涵 彭头平[1,3] 范蔚茗 刘兵兵[1,2] 彭世利 武利民 DONG Xiaohan;PENG Touping;FAN Weiming;LIU Bingbing;PENG Shili;WU Limin(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;Institute of Tibetan Plateau Research,Chinese Academy of Sciences,Beijing 100101,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]中国科学院青藏高原研究所,北京100101

出　　处：《大地构造与成矿学》2024年第4期820-843,共24页Geotectonica et Metallogenia

基　　金：第二次青藏科考项目(2019QZKK0702);国家自然科学基金项目(92055207、42072263、41490613)联合资助。

摘　　要：新生代以来,印度-欧亚大陆碰撞导致青藏高原发生大规模隆升和岩石圈挤出,这一过程引发了金沙江-哀牢山-红河构造带等深大断裂带的大规模剪切走滑,并沿该构造带发生强烈的碱性岩浆作用。这些始新世碱性岩记录了印度-欧亚大陆的早期碰撞过程,因而可以作为研究青藏高原早期隆升和岩石圈挤出过程及其深部动力学机制的重要“探针”。本文针对剑川盆地甸南地区始新世粗面岩及其二长岩包体和辉长岩包体开展了系统的锆石U-Pb年代学、Lu-Hf同位素和全岩主-微量元素和Sr-Nd同位素组成研究。锆石U-Pb定年结果显示,剑川盆地甸南粗面岩形成时代为36Ma;其内部包含的两件二长岩包体的形成年龄分别为41.6±3.4 Ma和35.7±0.3 Ma,一件辉长岩包体的形成时代为35.8±0.2 Ma。地球化学研究结果显示,粗面岩与二长岩包体K_(2)O含量均较高,属于钾玄质系列岩石,且富集轻稀土元素(LREE)和大离子亲石元素(LILE),亏损高场强元素(HFSE)。结合锆石Hf同位素和全岩Sr-Nd同位素组成,表明甸南地区寄主粗面岩和二长岩包体来自相同的交代富集岩石圈地幔的部分熔融,并经历了不同程度的橄榄石和辉石分离结晶作用。包体研究显示,~41Ma二长岩包体为寄主粗面岩岩浆上升过程中捕获自早期围岩,而~36Ma二长岩包体为同源岩浆房内与围岩接触淬冷的早期结晶产物,它们被寄主粗面岩岩浆裹挟而存在其内部。堆晶辉长岩包体较寄主粗面岩和二长岩包体具明显低的K2O、稀土和微量元素含量,其全岩Nd同位素组成与剑川盆地二龙山橄榄玄武岩相似,但高于寄主粗面岩,表明其源自弱富集地幔的部分熔融,并在壳-幔过渡带堆晶形成。此外,~41 Ma碱性岩包体和碱性岩内46~38 Ma捕获锆石年龄,以及剑川盆地内近源沉积岩内碎屑锆石的同期年龄记录,均表明金沙江-哀牢山-红河碱性岩带的岩浆作用并非存在Since the Cenozoic,the India-Eurasia continental collision has caused large-scale uplift and lithospheric extrusion of the Qinghai-Tibet Plateau.Meanwhile,it also resulted in intra-crustal shortening and strong strike slip of the Jinshajiang-Ailaoshan-Red River tectonic zone,and the related alkali-rich magmatism along this tectonic belt during the Eocene.These Eocene alkaline rocks recorded the tectonic processes of the early Indian-Eurasian collision.Therefore,they provide a crucial“probe”to shed light on the mechanisms of early uplift and lithospheric extrusion of the Qinghai-Tibet Plateau.In this paper,we present zircon U-Pb geochronology,Lu-Hf isotopic compositions,and REE element contents,together with whole-rock major and trace elements,and Sr-Nd isotopic compositions of the Eocene trachytes and monzonite and gabbro enclaves at Diannan in the Jianchuan Basin.The zircon U-Pb dating results indicate that the Diannan trachytes,as well as one monzonite enclave and one gabbro enclave hosted within,formed at ca.36 Ma,whereas another monzonite enclave crystallized at 41.6±3.4 Ma.The geochemical signatures show that the trachytes and monzonite enclaves have similarly high K_(2)O contents and shoshonite affinity.Moreover,they are enriched in light rare earth elements(LREE)and large-ion lithophile elements(LILE)and depleted in high field strength elements(HFSE).Such geochemical signatures along with the enriched Nd isotopic compositions reflect that these shoshonitic rocks were derived from the partial melting of enriched lithospheric mantle,which was modified by subducted fluids/melts.Among them,the ca.41 Ma monzonite enclave was captured from the early alkaline rock during the ascending of the host trachyte magmas,whereas the ca.36 Ma monzonite enclave represents the product of the early quenching crystallization due to the contacting of the trachyte magmas with country rocks.Then,it was subsequently broken and carried by the host trachyte magma.Unlike the host trachyte and monzonite enclaves,the cumulate gabb

关 键 词：青藏高原 金沙江-哀牢山-红河构造带 粗面岩 包体 岩石圈挤出 

分 类 号：P581[天文地球—岩石学]