西藏中拉萨地块门巴二长花岗岩年代学、岩石地球化学特征及地质意义  

Geochronology,geochemistry characteristics and geological significance of Menba monzonitic granite,central Lhasa subterrane,Xizang

作　　者：解鸿儒 郎兴海[1] 邓煜霖 何青 李宸 王兆帅 吴伟哲 王涌滔 XIE Hong-ru;LANG Xing-hai;DENG Yu-lin;HE Qing;LI Chen;WANG Zhao-shuai;WU Wei-zhe;WANG Yong-tao(College of Earth and Planetary Sciences,Chengdu University of Technology,Chengdu 610059,China)

机构地区：[1]成都理工大学地球与行星科学学院,四川成都610059

出　　处：《岩石矿物学杂志》2024年第6期1553-1577,共25页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金项目(41972084,42230813);国家重点研发计划(2022YFC2905000);四川省科技计划项目(2020JDJQ0042);成都理工大学珠峰科学研究计划(2020ZF11407)。

摘　　要：在西藏中拉萨地块中-西部发育着一系列早白垩世A型花岗岩,但目前在东段尚未发现相关的A型花岗岩,制约了对中拉萨地块深部动力学的认识。本文基于详实的野外地质调查,对门巴地区的二长花岗岩开展了岩相学、锆石U-Pb年代学、锆石Hf同位素地球化学和全岩地球化学研究。通过LA-ICP-MS定年测得的该二长花岗岩的锆石206Pb/238U加权平均年龄为120.9±0.8 Ma,表明门巴二长花岗岩形成于早白垩世。门巴二长花岗岩具有较高含量的SiO_(2)(69.69%~75.81%,平均73.84%),较高的总碱(Na_(2)O+K_(2)O)含量(8.22%~9.34%,平均8.92%),中等含量的Al_(2)O_(3)(12.77%~15.60%,平均13.74%),较低含量的MgO(0.01%~0.12%,平均0.05%),A/CNK的值为1.08~1.22,A/NK的值为1.12~1.27,表明门巴二长花岗岩为过铝质的高钾钙碱性-钾玄岩系列岩石。门巴二长花岗岩相对富集Zr、Hf、Ce、Y等微量元素,相对亏损Ba、Ti、Sr、P等微量元素,强烈的负Eu异常(δEu=0.04~0.11,平均0.07),具有较高的10000 Ga/Al值(1.99~4.17,平均3.06)、FeO^(T)/MgO值(18.45~69.96,平均47.05)、Y/Nb值(1.07~2.42,平均1.77)、Rb/Nb值(6.70~12.16,平均9.83)以及向右缓倾的"V"型稀土元素配分曲线,表明门巴二长花岗岩为形成于后碰撞伸展环境中的A2型花岗岩。另外,由负到正的ε_(Hf)(t)值(-2.7~+2.6)、相对古老的锆石Hf地壳模式年龄(tDM2=1352.8~1016.8 Ma)以及与地壳接近的Nb/Ta值,指示门巴二长花岗岩起源于古老下地壳并混有少量地幔物质。结合前人研究成果,本文认为门巴二长花岗岩形成于早白垩世拉萨地块和羌塘地块的碰撞造山背景,其成因为俯冲板片发生断离导致软流圈物质上涌并诱发了地壳部分熔融,起源于地壳部分熔融并有少量地幔物质加入的岩浆,经历了以钾长石为主的分离结晶作用。A series of Early Cretaceous A-type granites are developed in the central-western part of the Central Lhasa subterrane in Xizang,but no related A-type granites have been found in the eastern section,which constrains the understanding of the deep dynamics of the central Lhasa subterrane.In this paper,petrographic,zircon U-Pb chronology,zircon Hf isotope geochemistry and whole-rock geochemistry studies were carried out on the monzonitic granites in the Menba area based on detailed field geological investigations.The zircon 206Pb/238U weighted average age of this monzonitic granite measured by LA-ICP-MS dating is 120.9±0.8 Ma,indicating that the Menba monzonitic granite was formed in the Early Cretaceous.The Menba monzonitic granite has a high content of SiO_(2)(69.69%~75.81%,average 73.84%),a high content of total alkali(Na_(2)O+K_(2)O)(8.22%~9.34%,average 8.92%),a medium content of Al_(2)O_(3)(12.77%~15.60%,average 13.74%),and a low content of MgO(0.01%~0.12%,average 0.05%),values of 1.08~1.22 for A/CNK and 1.12~1.27 for A/NK,suggesting that the Menba monzonitic granite is a peraluminous,high-K calc-alkaline-potassium basaltic series rock.The Menba monzonitic granite is relatively enriched in trace elements such as Zr,Hf,Ce,and Y,and relatively deficient in trace elements such as Ba,Ti,Sr,and P.It has strong negative Eu anomalies(δEu=0.04~0.11,average 0.07),high 10000 Ga/Al values(1.99~4.18,average 3.06),FeO^(T)/MgO values(18.45~69.96,average 47.05),Y/Nb(1.07~2.42,average 1.77),Rb/Nb(6.70~12.16,average 9.83),and a rightward gently dipping"V"rare earth element distribution curve,indicating that the Menba monzonitic granite is an A2-type granite produced in a post-collisional and extensional environment.In addition,the negative to positiveε_(Hf)(t)values(-2.7~+2.6),relatively old zircon Hf crustal model ages(tDM2=1352.8~1016.8 Ma),and Nb/Ta values close to those of the crust indicate that the Menba monzonitic granite originated from the ancient lower crust with a small amount of mantle material.Combining the resu

关 键 词：西藏 中拉萨地块 门巴地区 A型花岗岩 后碰撞 

分 类 号：P588.121[天文地球—岩石学] P59[天文地球—地质学]