摘要
昌宁-孟连结合带是西南三江地区重要的古特提斯结合带,本文报道该带牛井山蛇绿混杂岩带内的早古生代英云闪长岩的锆石U-Pb年龄和Hf同位素、全岩地球化学特征,探讨其岩石成因,揭示早古生代(原)特提斯演化过程和恢复古地理格局。利用LA-ICP-MS对英云闪长岩进行锆石U-Pb定年,获得^(206)Pb/^(238)U加权平均年龄为468±2Ma(MSWD=1.4,n=22),代表岩浆岩结晶时代。对锆石进行原位微区Hf同位素分析获得锆石ε_(Hf)(t)=+1.6^+2.8。牛井山英云闪长岩具有高的SiO_2(61.6%~67.16%),Al_2O_3(15.4%~17.64%),Na_2O(2.93%~3.25%)含量,高的Na_2O/K_2O(3.9~5.4)比值。微量元素具有高Sr(469×10^(-6)~533×10^(-6)),低Yb(0.66×10^(-6)~1.22×10^(-6))和Y(8.5×10^(-6)~16.3×10^(-6)),Eu正异常(1.01~1.37)的特征。此外,在同等SiO 2条件下,还具有相对高的Mg~#和MgO(Mg~#=50~51,MgO=2.84%~3.58%)。这些特征表明英云闪长岩具有高镁埃达克岩的性质。岩石成因研究表明牛井山高镁埃达克岩很可能是俯冲洋壳加上部分大洋沉积物部分熔融并与上覆地幔橄榄岩反应的产物。早古生代洋壳俯冲成因的埃达克岩的发现证明昌宁-孟连特提斯洋演化历史可追溯至约468Ma。昌宁-孟连结合带早古生代岩浆作用与龙木错-双湖结合带早古生代洋壳残片的时代一致、构造属性相似,我们认为昌宁-孟连结合带与木错-双湖结合带可对比,它们代表了一个统一的古生代特提斯大洋残余。
The Changning-Menglian suture zone is well known as the Paleo-Tethys suture in the Nujiang-Lancangjian-Jiangshajiang area. Zircon U-Pb age, Hf isotope and whole-rock geochemistry of the Paleozoic adakites from the Niujingshan ophiolitic mélange in the Changning-Menglian suture zone were reported in this paper, aiming to reveal its petrogenesis and geological implications. LA-ICP-MS zircon U-Pb dating results yield a weighted mean 206Pb/238U age of 468±2Ma （MSWD=1.4, n=22） for the adakite. Zircon initial Hf isotopes range from 0.282524 to 0.282561, with εHf（t） values of 1.6～2.8. The studied rocks have high SiO2 （61.60%～67.16%）, Al2O3 （15.40%～17.64%） and Na2O （2.93%～3.25%） contents. They also have low Yb （0.66×10^-6～1.22×10^-6） and Y （8.5×10^-6～16.3×10^-6） concentrations, high Na2O/K2O （3.9～5.4） and Sr/Y （31～55） ratios, and obviously positive Eu anomaly （δEu=1.01～1.37）. Furthermore, they show relatively high Mg# and MgO （Mg#=50～51, MgO=2.84%～3.58%） at a given SiO2 contents. These geochemical and isotopic characters are similar to those of the high Mg adakites. Petrogenesis study of the high Mg adakites demonstrate that they were derived from oceanic slab and marine sediments melting and subsequently melts-mantle peridotite interaction. Occurrence of the oceanic subduction-related high Mg adakites in the Changning-Menglian suture reveal that the evolution history of the Changning-Menglian Paleo-Tethys can be traced back to about 468Ma. The chronology and character of the Paleozoic tectonic-magmatic events in Changning-Menglian suture can be well correlated with the Longmu Co-Shuanghu suture in the Qiangtang region. Thus, we suggest that the both Changning-Menglian and Longmu Co-Shuanghu sutures were probably transformed from the relic oceanic crust of the uniform Paleo-Tethys, which likely represents the original and main Paleo-Tethys oceanic basin.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2016年第8期2317-2329,共13页
Acta Petrologica Sinica
基金
国家重点研究发展计划项目(2015CB452601)
国家自然科学基金项目(41302157
41302170)
四川省基金项目(2014JQ0025)
中国地质调查局项目(121201010000150007
12120113049100
1212011220412)联合资助
