西藏冈底斯中段南木林地区始新世岩浆作用的厘定及其大地构造意义

选取南冈底斯中段南木林地区的二长花岗岩岩体进行了LA-ICP-MS锆石U-Pb测年和全岩地球化学分析。分析结果显示,研究区二长花岗岩体的锆石U-Pb年龄为50.24±0.68Ma,为始新世岩浆活动的产物,是冈底斯岩基的重要组成部分。岩石地球化学特征表明,里特曼指数为1.66～1.94,具有钙碱性特征,A/CNK=1.11～1.15,显示出过铝质的特征,同时具有高硅、高钾(可达钾玄岩系列)特征。微量元素强烈富集Rb、Th、U等大离子亲石元素(LILE)及La、Ce等轻稀土元素,亏损Nb、Ta、Zr等高场强元素(HFSE),表现出弧型或壳源岩浆岩的地球化学属性。Sm/Nd值为0.45～0.57,平均值为0.53,显示出岩浆的深源特征。稀土元素(La/Yb)N值较高,平均值为9.25,轻稀土元素相对富集,重稀土元素相对亏损,且具有较显著的负Eu异常特征。综合研究表明,南木林花岗岩体形成过程为俯冲至断离的特提斯洋壳发生脱水作用,导致上覆地幔楔发生部分熔融,玄武质岩浆形成,岩浆随后上涌底侵至莫霍面附近,巨大的热烘烤作用迫使下地壳(富黏土或泥质岩)发生部分熔融,之后玄武质与长英质岩浆发生了广泛的混合作用,最后侵位成岩形成南木林地区广泛的含基性包体的花岗岩体。这为特提斯洋闭合、印度-欧亚板块碰撞的时限提供了同位素年龄证据,也丰富了冈底斯岩体成岩模式和地球化学特征。

The identification of the Eocene magmatism and tectonic significance in the middle Gangdise magmatic belt, Nanmulin area, Tibet

The Nanmulin rock is located in the southern Gangdise center. In this study, the authors conducted systematical zircon LA-ICP-MS U-Pb dating and whole-rock geochemical analysis for the monzonitic granite in Nanmulin. The zircon U-Pb analyses demonstrate that the crystallization and emplacement age of the plutons is 50.24 ±0.68Ma, suggesting Eocene magmatic activities. It is an important part of the Gangdise batholith. Geochemical characteristics show that Rittmann index(σ) is 1.66~1.94, implying calc-alkaline series. The Al₂O₃ varies in the range of 11.82%~12.45%, whereas A/CNK varies in the range of 1.11~1.15, suggesting peraluminous characteristics. The granite is characterized by high SiO₂(77.4%~78.18%,averagely 77.82%), high K (shoshonite series). Rb, Th, U and LREE are enriched but Nb, Ta, Zr and HREE are depleted,suggesting arc-related or earth crust geochemical affinities. Sm/Nd varies from 0.45 to 0.57, averagely 0.53, suggesting the deep source characteristics of magma. (La/Yb)_N is high, averagely 9.25, suggesting enrichment of the LREE and depletion of HREE, with clear negative Eu anomaly. In summary, the process of granite petrogenesis resulted from subduction or breakoff of the Tethyan Ocean crust experiencing dehydration. This action resulted in partial melting of overlying mantle wedge and caused upward migration of basaltic magma near Moho. Huge thermal baking forces caused partial melting of the lower crust (clay-rich or argillaceous rocks). Magma mixing occurred from both basaltic magma and felsic magma. Nanmulin granite with MME was formed. This study provides isotope age evidence on the time of the closure of the Tethyan Ocean and Indian-Asian continental collision. Moreover, it enriches the diagenetic model and geochemical characteristics of Gangdise rock.