西藏冈底斯南木林地区晚三叠世中酸性岩浆作用及构造意义

Petrogenesis and Tectonic Significance of the Late Triassic Nanmulin Intermediate-felsic Magmatic Rocks in the Gangdese Batholith,Southern Tibet

拉萨地块新特提斯初期演化过程是构建特提斯时空格架的重要环节,受制于研究对象的匮乏,其认识尚待深入。为此,本研究选取中拉萨地块南木林甲措乡地区花岗闪长岩和石英闪长岩,进行锆石LA-ICP-MS U-Pb年代学和岩石地球化学分析,探讨岩石成因和源区,为新特提斯初期演化提供新的制约。分析结果表明,花岗闪长岩和石英闪长岩的锆石U-Pb年龄分别为(214.3±1.1)Ma和(206.1±1.3)Ma,代表晚三叠世两期次岩浆事件。岩浆岩里特曼指数(σ)介于1.79~4.71,具有钙碱性-碱性特征,Al_(2)O_(3)含量为15.01%~17.57%,A/CNK=0.76~1.12,显示出准铝质-过铝质的特征,此外花岗闪长岩具有高硅(SiO_(2)含量61.87%~73.72%,平均65.76%)、高钾(最高达4.25%)、较低的Mg#(31~46,均值为41)特征。二者微量元素均表现为强烈富集Rb、U、K、Pb等大离子亲石元素(LILE)以及La、Ce等轻稀土元素,而亏损Nb、Ta、Ti等高场强元素(HFSE),表现出弧型岩浆岩的地球化学属性;Rb/Sr值(0.05~5.50,均值为2.54)指示壳源特征。(La/Yb)N值较高,平均12.05,反映了轻重稀土元素分馏显著的特征,Eu负异常不明显(δEu=0.57~1.01,平均0.73)。综合研究表明,南木林地区中酸性岩体为壳幔混合成因的I型花岗岩,其形成过程可能为:新特提斯洋北向俯冲导致地幔楔发生部分熔融,岩浆底侵形成新生下地壳,随后幔源岩浆热烘烤导致新生下地壳重熔,岩浆上升侵位形成甲措乡中酸性岩体。本研究指示新特提斯洋北向俯冲作用在晚三叠世(≈214 Ma)之前已经发生,早于新近发现的晚三叠世Be-Rb±(Nb-Ta)稀有金属矿化时限。

The initial Neo-Tethyan evolution in the Lhasa block is an important part for the reconstruction of the Tethys space-time framework.Due to the lack of research objective,its understanding needs to be deepened.The Nanmulin granodiorite and quartz diorite are located in the central Lhasa block.We conducted systematic zircon LA-ICP-MS U-Pb dating and whole-rock geochemical analyses for determining the petrogenesis and source region of these rocks.The results provide new constraints for the early Neo-Tethyan evolution.Zircon U-Pb dating shows that both lithologies were formed in the Late Triassic,with the granodiorite emplaced at(214.3±1.1)Ma and the quartz diorite at(206.1±1.3)Ma.Geochemically,these rocks have Rittmann index(σ)of 1.79-4.71 and are calc-alkaline.They are peraluminous with Al_(2)O_(3) varying from 15.01%to 17.57%,and A/CNK=0.76-1.12.Moreover,the granodiorite has high SiO_(2)(61.87%-73.72%,average 65.76%)and high K(up to 4.25%)and low Mg#(31-46,average 41).Rb,U,K,Pb and LREEs are enriched and Nb,Ta,Ti and HREEs are depleted,suggesting arc-related or crust-like geochemical affinities.Rb/Sr(0.05-5.50,average 2.54)values suggest that their parental magmas were crustal sourced.The granodiorite and quartz diorite have high(La/Yb)N values(average 12.05),with no clearly negative Eu anomalies(δEu=0.57-1.01,average 0.73).In summary,these data indicate that the Nanmulin intermediate-felsic rocks are I-type formed by the partial melting of juvenile crust.We propose that the northward subduction of the Neo-Tethyan was initiated before Late Triassic,which was before the formation of the newly-discovered Late Triassic Be-RB±(Nb-Ta)rare metal mineralization.