拉萨地块西部直若地区中侏罗世花岗闪长岩锆石U-Pb定年、岩石成因及其构造意义

    Zircon U-Pb dating, petrogenesis and tectonic significance of the Middle Jurassic granodiorite in Zhiruo area, western Lhasa block.

    • 摘要: 青藏高原拉萨地块西部的侏罗纪岩浆作用对于讨论新特提斯洋的俯冲演化历史具有重要的约束意义。本文报道了拉萨地块西部直若地区花岗闪长岩的LA-ICP-MS锆石U-Pb年龄、锆石Hf同位素和全岩地球化学组成。研究表明,两件花岗闪长岩样品的锆石206Pb/238U年龄的加权平均值分别为165.9 ±1.3 Ma、160.3±1.2 Ma,属于中侏罗世。样品具有高SiO2、Al2O3,较富碱,低P2O5、TiO2的特征,铝饱和指数A/CNK为0.9~1.3,其中样品(S19T13)属于弱过铝质钾玄岩系列I型花岗岩,样品(S21T12)属于准铝质中钾钙碱性I型花岗岩。样品的稀土元素含量总体在(98~186)×10-6之间,有较明显的Eu负异常(Eu/Eu*值为0.18~0.28),亏损Nb、Ta、P、Ti等高场强元素,富集Rb、Th、U、Pb等大离子亲石元素和轻稀土元素,具有弧岩浆岩的特征。花岗闪长岩具有亏损的锆石Hf同位素成分(εHf(t)=-15.2~ -13.0),地壳模式年龄TDMC 为2030~2171 Ma,表明岩浆物质来源于古老地壳的部分熔融,且在上升过程中发生了以角闪石为主的分离结晶作用,最终形成直若地区的一系列中酸性岩石。综合研究表明,该套岩石的形成可能与新特提斯洋北向俯冲有关。

       

      Abstract: The Jurassic magmatism in the western Lhasa block of the Qinghai-Tibet Plateau is of great significance for discussing the subduction evolution history of the Neo-Tethys Ocean. This paper reports LA-ICP-MS zircon U-Pb age, zircon Hf isotope and whole-rock geochemical composition of granodiorite in Zhiruo area, western Lhasa block. The results show that the weighted average zircon 206Pb/238U ages of the two granodiorite samples are 165.9±1.3 Ma and 160.3±1.2 Ma, respectively, belonging to the Middle Jurassic. The samples have the characteristics of high SiO2, Al2O3, rich alkali, low P2O5, TiO2, and the aluminum saturation index A/CNK is 0.9~1.3. The sample (S19T13) belongs to the weakly peraluminous shoshonite series I-type granite, and the sample (S21T12) belongs to the quasi-aluminum medium potassium calc-alkaline I-type granite. The content of rare earth elements in the samples is generally between (98~186) × 10-6, with obvious negative Eu anomaly (Eu/Eu* value of 0.18~0.28), loss of high field strength elements such as Nb, Ta, P, Ti, enrichment of large ion lithophile elements such as Rb, Th, U, Pb and light rare earth elements, with the characteristics of arc magmatic rocks. The granite diorite has depleted zircon Hf isotope composition (εHf(t)= -15.2~-13.0), and the crustal model age (TDMC) is 2030 ~ 2171 Ma, indicating that the magmatic material is derived from the partial melting of the ancient crust, and the fractional crystallization dominated by hornblende occurred during the ascending process, and finally formed a series of intermediate-acid rocks in Zhiruo area. Comprehensive studies have shown that the formation of this set of rocks may be related to the northward subduction of the Neo-Tethys Ocean.

       

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