冈底斯带别若则错地区美苏组高Nb-Ta酸性火山岩锆石U-Pb年龄、地球化学特征及地质意义

Zircon U-Pb age,geochemistry and geological significance of high Nb-Ta acid volcanic rocks from Meisu Formation,Bieruo-Zecuo area,Gangdise belt

别若则错地区美苏组构造上处于北冈底斯南缘,其中流纹岩的LA-ICP-MS锆石^(206)Pb/^(238)U年龄为39.62±0.77Ma,时代为晚始新世。岩石具高Si O_2(72.85%~78.54%)、富碱(Alk=6.36%~8.47%)、贫Ca O(0.33%~1.56%)和Mg O(0.08%~0.58%)特征,A/CNK=0.99~1.35,属于弱铝质-过铝质碱性系列。稀土元素总量为122.52×10^(-6)~208.35×10^(-6),(La/Yb)_N值为7.27~16.11,δEu=0.13~0.26,在球粒陨石标准化稀土元素分布模式上表现为Eu的右倾V字形。微量元素Nb(38.1×10^(-6)~88.7×10^(-6))、Ta(3.36×10^(-6)~6.35×10^(-6))高,Rb、K、Th、Ce、Zr、Hf相对富集,Ba、Sr、P、Ti强烈亏损。岩石地球化学特征表明,别若则错地区美苏组酸性火山岩"集壳幔特性于一身",是初始熔体为富碱和富Nb、Ta的幔源玄武质岩浆与壳源岩浆以某一特定比例混合/混染的产物。初步认为,美苏组酸性火山岩产出于大陆板内伸展(裂谷)环境,其地球动力学背景与南侧新特提斯洋壳向北俯冲,导致高原内部的陆内俯冲、走滑剪切与地壳缩短有关。

Meisu Formation in Biemo-Zecuo area is tectonically located on the sooth margin of nor th Gangdise belt. LA-ICP-MS zircon 26Pb/23SU dating shows that rhyolite from Meisu Formation has an age of 39.62±0.77Ma （n= 14 ,MSWD= 0.99）, corresponding to Late Eocene. Geochemistry shows that the rhyolite belongs to weakly aluminous to peraluminous alkaline series （A/CNK=0.99=1.35） with characteristics of high SiO2 （72.85%-78.54%）,high alkali （Aik=6.36%-8.47%）, low C aO （0.33%=1.56%） a （0.08%=0.58%）.The chondrite-normalized REE patterns are characterized by the significant enrichment of LREE （LaN/YbN=7.27- 16.11） and strong negative Eu anomalies （8Eu：0.13-0.26） with total REE content of 122.52×10^6-208.35×10^6. The primitive man- tie-normalized trace element spider diagram shows high content of Nb （38.1×10^6-88.7×10^6） and Ta （3.36×10^6-6.35×l0^6）,rela- tive enrichment of Rb,K,Th,Ce,Zr and Hf and strong depletion of Ba,Sr,P and Ti. These geochemical characteristics show thatthe Meisu rhyolite is the product of mixing/contamination by a specific proportion of alkali-rich,Nb-Ta enriched,mantle-derivedbasaltic magma and crust^ derived magma. The authors hold that the Meisu rhyolite was formed in an intraplate extension environ-ment （rift）. Geodynamically, the formation of those rocks was associated wi th the on-plateau intracontinental subduction, strike-slipshear, and crust shortening triggered by nor thward subduction of Tethys Oceanic crust on the southern side.