藏南冈底斯中段谢通门始新世复式岩体锆石U-Pb年代学、Hf同位素特征及其地质意义

Zircon Geochronology and Hf Isotopic Composition of Eocene Granite Batholith from Xaitongmoin in Middle Gangdise and its Geological Significance

本文对冈底斯谢通门地区的三个花岗岩体进行了LA-ICP-MS锆石U-Pb定年、Hf同位素分析以及岩体侵位深度研究。研究结果表明,三个岩体的锆石U-Pb同位素年龄分别为43.5±0.6 Ma、44.9±0.5 Ma和48.3±1.4 Ma。锆石的Hf同位素研究显示,样品xy911的εHf（t）值介于–5.1~0.35,为典型的壳幔混合型, Hf同位素的二阶段模式年龄平均值为1142 Ma,表明其主要岩浆源区以古老地壳基底的部分熔融为主,在此过程中可能伴有幔源岩浆或新生下地壳的贡献。而样品xy957和样品xy961具有相似的Hf同位素特征,εHf（t）均为正值；样品xy957的εHf（t）值介于0.23~7.31,平均值为5.07,样品xy961的εHf（t）值介于2.12~5.82,平均值为4.70,其二阶段模式年龄平均值分别为764 Ma、792 Ma,表明它们的源岩以新生地壳的部分熔融为主或有大量幔源物质的加入,同时,也混有少量古老陆壳的成分。根据角闪石压力计计算得出,样品xy911、xy957、xy961侵位深度分别为9.36 km、9.53 km和10.89 km,为中深成侵入相。综合分析表明,谢通门复式岩体主要以新生地壳物质的部分熔融为主,在岩浆演化或上升过程中混染了部分拉萨地体的古老物质；此外该复式岩体的形成可能和新特提斯洋板片的断离引起地幔物质上涌有关。

LA-ICP-MS U-Pb dating of zircon from the granites in the Gangdise magmatic belt at the Xaitongmoin area yielded weighted mean ages of 43.5±0.6 Ma, 44.9±0.5 Ma, and 48.3±1.4 Ma for the samples xy911, xy957 and xy961 respectively. Zircons from sample xy911 show εHf（t） values of –5.1 to 0.35 with mean Hf tDM2 age of 1142 Ma, suggesting the remelting of ancient crust and participation of little mantle material or juvenile crust. In contrast, sample xy957 and xy961 have positiveεHf（t） values of 0.23-7.31 with a mean value of 5.07, and 2.12-5.82 with a mean of 4.70, respectively, corresponding to mean tDM2 ages of 764 Ma and 792 Ma. The positive εHf（t） values of samples xy957 and xy961 demonstrate significant input of depleted mantle material or juvenile crust, which means that the mantle material played a vital role in the formation of the granites at the Xaitongmoin area. Horn blende pressure analyses show that the intrusion depth of these granites is 9 to 11 km. In summary, the Xaitongmoin granites were mainly derived from the reworking of juvenile crust with minor ancient crust blended in during the ascent of magma. This process may indicate geological events such as basaltic magma underplating and represent crustal growth in the Xaitongmoin region, which is coincident with the magmatic explosion during the Eocene in southern Tibet due to the break-off of the Neo-Tethys slab.