新特提斯洋晚白垩世演化特点:来自泽当共国日二长花岗岩年代学、地球化学及Sr-Nd同位素证据

为加深对西藏泽当地区新特提斯洋演化的认识,对西藏泽当蛇绿混杂岩带内的共国日二长花岗岩进行了岩石学、岩石地球化学、同位素及年代学等研究,研究显示:共国日二长花岗岩岩体规模小、岩性稳定,LA-ICP-MS锆石U-Pb年龄为(90.40±0.68)Ma,属晚白垩世;岩石地球化学具高硅、富铝、富钾、低钛和准铝质钙碱性花岗岩特征;轻稀土富集、重稀土亏损,具明显的负Eu异常,微量元素表现为相对富集Rb、K、Ba、Th、U等大离子亲石元素,显著亏损Nb、P、Ti等高场强元素;(⁸⁷Sr/⁸⁶Sr)_i=0.705 708~0.706 284,(¹⁴³Nd/¹⁴⁴Nd)_i=0.512 689~0.512 716,ε_Nd(t)=2.00~2.51。以上特征表明,位于泽当蛇绿混杂岩带内的共国日二长花岗岩属于I型花岗岩,由正常岛弧岩浆形成,应为晚白垩世新特提斯洋向北俯冲形成的岛弧环境下俯冲带上部地壳部分熔融的产物,其不属于泽当蛇绿岩的组成部分,表明在90 Ma前泽当洋内弧已经消失。

Late Cretaceous Evolution of the Neo-Tethys:Evidence from Geochronology,Geochemistry, and Sr-Nd Isotopes of Gongguori Monzogranite in Zedang

To deepen our understanding on the evolution of the Neo-Tethys ocean in Zedang area, Tibet, a study of the petrology, petrochemistry, isotope and geochronology of the Gongguori monzogranite in the Zedang ophiolitic mélange belt is carried out. The result shows that it is small and has similar lithology. LA-ICP-MS zircon U-Pb dating on the pluton yielded Late Cretaceous crystallization age of (90.40±0.68) Ma. The Gongguori monzogranite is rich in Si, Al, and K and Ti-poor, and is metaluminous, calc-alkaline to alkaline. The rocks show LREE enrichments, HREE depletions, and negative Eu anomaly. The monzogranite is also enriched in LILEs (e.g.,Rb, K, Ba, Th, U) and depleted in HSFEs (e.g., Nb, P, Ti). The rocks have (⁸⁷Sr/⁸⁶Sr)_i=0.705,708 to 0.706,284, (¹⁴³Nd/¹⁴⁴Nd)_i=0.512,689 to 0.512,716, and positive ε_Nd(t)=2.00 to 2.51. Based on the geochronological and geochemical characteristics of the Gongguori pluton, this study demonstrates that the Gongguori monzogranite was formed by normal island arc I-type granitic magmatism. The Gongguori monzogranite may have been a partial melting product of the upper crust, which was formed by the Late Cretaceous north-dipping subduction of the Neo-Tethys. The Gongguori monzogranite is unlikely part of the Zedang ophiolite, and the Zedang intra-oceanic arc may have disappeared before 90 Ma.