西藏努林晚白垩世花岗闪长岩地球化学特征及成因

Geochemistry and Genesis of the Late Cretaceous Granodiorite in Nulin, Tibet

关键词:  南冈底斯  晚白垩世  埃达克岩  洋脊俯冲  努林

Key words:  south gangdese  late cretaceous  adakitic  subduction of New Tethys ridge  Nulin

摘要:

南冈底斯晚白垩世岩浆岩的成因及地球动力学机制一直存在争议。本文对冈底斯南缘努林花岗闪长岩开展地球化学、锆石U-Pb年代学及同位素示踪研究。结果显示，该岩体具有富SiO₂（66.62%～67.81%）、高Al₂O₃（15.11%～15.66%）、高Sr（>481×10^-6），低Y（≤8.13×10^-6）和低Yb（≤0.73×10^-6）特征，Sr/Y比值达59～111，显示埃达克岩的特征；岩石轻稀土富集，重稀土亏损，具有显著的Eu正异常；富集大离子亲石元素，亏损高场强元素。（⁸⁷Sr/⁸⁶Sr）_i=0.704011～0.704244，ε_Nd（t）=＋3.61～＋5.75，总体反映地幔源区的Sr、Nd同位素特征。锆石U-Pb LA-ICP-MS测年显示存在83Ma和89Ma两组年龄。结合地质及地球化学分析，认为努林花岗闪长岩是新特提斯洋洋脊俯冲引起的镁铁质新生下地壳部分熔融的产物。

Abstract:

There are different understandings of the genesis and geodynamic mechanism for the Late Cretaceous magmatic rocks of the South Gangdese. In this paper, we present geochemistry, zircon U-Pb and Sr-Nd isotopic data for the granodiorite from Nulin area. The results show that the rocks are characterized by high SiO₂(66.62%~67.81%)、Al₂O₃(15.11%~15.66%)、Sr(> 481×10^-6),low Y(≤ 8.13×10^-6)and Yb(≤ 0.73×10^-6) and high Sr/Y ratio(59~111),implying adakitic geochemical features. These rocks are enriched in LREE and depleted in HREE, together with enrichment in large-ion lithophile elements(LILE) and depletion in high field strength elements(HFSE),as well as significant positive Eu. (⁸⁷Sr/⁸⁶Sr)_i=0.704011~0.704244,ε_Nd(t)=3.61~5.75,indicating mantle source of Sr and Nd isotopic characteristics. The results of zircon U-Pb LA-ICP-MS of the granodiorite show that, the magmatic activities was 83Ma and 89Ma respectively. Based on the geological and geochemical analysis, we infer that the Nulin granodiorite may be the product of melting of the thickened juvenile lower crust which was triggered by the subduction of New Tethys ridge.