【摘要】 青藏高原南拉萨地体晚白垩世岩浆岩是揭示新特提斯洋北向俯冲动力学过程的重要岩浆记录。前人对东西向展布的冈底斯岩基不同区段出露的各类晚白垩世岩浆岩进行了大量研究,但对该时期岩石成因和新特提斯俯冲的深部过程仍存在一些争议。本文报道的冈底斯岩基南缘南木林地区的辉长岩体及侵入其中的基性和酸性脉岩,形成于晚白垩世早期,在辉长岩体（92～94Ma）就位后不久基性和酸性脉岩（91Ma）同时侵入,野外露头较好地展示了它们的穿切关系。寄主辉长岩和基性脉岩均属于高钾钙碱性系列,SiO₂含量不均一（49.60%～57.99%）,高铝（Al₂O₃,16.41%～18.67%）,无明显Eu负异常,低Cr、Ni,表明可能经历过角闪石分离结晶。酸性脉岩属于弱过铝质钾玄质系列,普遍高硅（SiO₂>70%）、高全碱（8.06%～9.44%）,与岩体无成分演化关系。三类岩石的微量元素均显示弧岩浆的特征,表现为轻稀土富集、重稀土亏损,Th、U、K、Pb、Sr等大离子亲石元素相对富集,高场强元素Nb、Ta、Ti等相对亏损。辉长岩和酸性脉岩锆石Hf同位素均表现亏损特征(ε_Hf（t）为+7.8～+11.4),一阶段模式年龄和地壳模式年龄均低于506Ma。综合辉长岩和基性脉岩的Nb/La、Nb/U和Ce/Pb特征,表明岩浆应来源于受流体交代的弧下地幔楔源区,而酸性岩脉具有高Sr/Y比和（La/Yb）_N比的埃达克质特征,可能来自弧下加厚的新生镁铁质下地壳。考虑到区域上95～85Ma岩浆活动广泛分布于整个冈底斯岩基中,本文认为在冈底斯岩浆弧的中段,晚白垩世早期北向俯冲的新特提斯洋发生了板片回转,同时导致了地幔和下地壳的部分熔融作用。更多还原

【Abstract】 The Late Cretaceous magmatic rocks in the southern Lhasa sub-terrane are the best records in revealing the dynamic evolution of the northward subduction of the Neo-Tethys.Many researches have been carried out on various types of Late Cretaceous magmatic rocks in different sections of the west-east trending Gangdese batholith,but there are still some disputes about the rock genesis and deep process of the Neo-Tethys subduction during this period.The gabbro in the Namling area of the southern margin of the Gangdese batholith were emplaced in the Late Cretaceous（92～94Ma）,and was intruded shortly by the basic and acidic dykes（91Ma）,with clearly crosscut relationships on outcrops.Both gabbro and basic dykes belong to high-K calc-alkaline series,displaying characteristics of heterogeneous silica（49.60%～57.99%）and high Al₂O₃（16.41%～18.67%）,slightly Eu negative anomaly,low Cr and Ni,indicating a possible origination from the fractional crystallization of amphibole.Acidic dykes belong to weak peraluminous shoshonitic series,with high SiO₂（>70%）and total alkalis（8.06%～9.44%）.The trace elements of all the three types of rocks show similar characteristics with arc-type magmas,with enrichment in light rare earth elements,depletion in heavy rare earth elements,enrichment in large ion lithophile elements（LILEs,such as Th,U,K,Pb,Sr）,and depletion in high field strength elements（HFSEs,such as Nb,Ta,Ti）.Both gabbro and acidic dykes have depleted zircon Hf isotopes(ε_Hf（t）=+7.8～+11.4)and young crustal model ages（<506Ma）.Combined with the characteristics of Nb/La,Nb/U and Ce/Pb of gabbro and basic dykes,it indicates that these gabbro and basic dykes were derived from the sub-arc mantle wedge metasomatized by the subducted slab sediments.On the contrary,the acidic dykes with high Sr/Y and（La/Yb）_Nratio,showing adakitic rock affinity,were derived from a thickened juvenile mafic lower crust.On the basis of previous results and our study,the widely distributed magmatism with ages ranging from 95Ma to 85Ma in Gangdese batholith,at least in the central section,the Neo-TethysOcean might have undergone slab roll-back in early Late Cretaceous.更多还原