西藏马莜木埃达克质斑岩的^(40)Ar-^(39)Ar年龄与地球化学特征

^(40)Ar-^(39)Ar age and geochemical features of the Mayum adakitic porphyry in Tibet.

马莜木花岗闪长斑岩位于特提斯喜马拉雅构造带上的北喜马拉雅构造带,紧靠雅鲁藏布江深大断裂南侧产出。该岩体的黑云母^(40)Ar/^(39)Ar测年结果为17．68±0．15Ma,^(40)Ar/^(36)Ar初始比值为294±13,系中新世侵入产物。元素地球化学研究表明,该岩体具有高Al_2O_3、K_2O和低Y、Yb含量,富集轻稀土和大离子亲石元素,尤其是富集K和Sr,Eu为弱亏损,普遍具有较高的Sr/Y、(La/Yb)_N比值及较低的Y/Yb比值,具有与典型埃达克岩相似的地球化学特征。微量元素地球化学和Nd-Sr- Pb同位素研究表明,马莜木斑岩体可能是下地壳与富集地幔(EMⅡ)物质混合熔融的产物,熔融残余相为角闪石+石榴石+辉石+含钛矿物。区域对比发现,该岩体与冈底斯中新世含矿斑岩不仅具有非常相似的地球化学特征,而且形成时代近于一致,可能是在同一个大的区域构造背景下同期形成的产物。马莜木埃达克质花岗闪长斑岩的发现,不仅加深了人们对藏南地区地质构造演化的认识,为研究整个青藏高原的形成演化提供和积累第一手地质资料,而且也为在本区寻找与埃达克质岩有关的金属矿床提供了理论依据。

Close to the south Yarlung Zangbo suture zone, the Mayum granodiorite porphyry is located in the Northern Himalayan belt of Tethyan Himalayan tectonic zone. The ^40Ar/^39Ar dating on biotite from the granodiorlte porphyry yields a plateau age of 17.68 ± 0. 15 Ma, with initial ^40Ar/^39Ar value of 294 ± 13. It indicates that the porphyry intruded during Miocene. According to the element geochemistry, the Mayum granodiorite porphyry is characterized by high Al2O3 , K2O, Sr and Mg^#and low Y and Yb contents with high Sr/Y and （La/Yb）N ratios, and shows high-K calc-alkaline magmatic affinity. The porphyry has enrichment pattern of LREE and LILE with weak negative Eu anomalies, stronger negative HFSE spikes as Nb and Ti. The study shows that the Mayum granodiorite porphyry has the characteristics of adakite. Based on the results from trace elements and Nd-Sr-Pb analysis, the porphyry may be the product of the mixed melting of the lower crust and EM Ⅱ , and the melting relics is characterized by the presence of amphibole, garnet, clinopyroxene and titanate phase. The regional comparison studies show that the Mayum granodiorite porphyry and the orebearing porphyries in Gangdese not only have similar geochemical features, but also were emplaced at same time. It suggests that the Mayum granodiorite porphyry and the ore-bearing porphyries in Gangdese are the products formed simultaneously under a similar tectonic setting. The identification of the Mayum granodiorite porphyry is of great significance, not only for the deep understanding of the geologic evolution in the southern Tibet and the whole Tibet plateau, but also for prospecting the metallic ore deposit associated with adakite.