【摘要】 萨迦穹隆位于北喜马拉雅片麻岩穹隆带中部。野外构造解析表明，萨迦穹隆北侧剖面可划分为由浅变质特提斯喜马拉雅沉积岩系构成上构造层，糜棱状石榴石片岩构成中构造层，糜棱状的正片麻岩和副片麻岩复合体构成下构造层（穹隆核部）。从上构造层向中构造层变质变形程度连续增高，但没有明显的拆离断层分隔，中构造层与下构造层被拆离面分隔。不同构造层次都具有统一的近北倾的线理和面理，运动学特征都指示了上盘向北的伸展剪切，表明萨迦穹隆北侧剖面的剪切变形可能代表了藏南拆离系在北喜马拉雅穹隆的出露。苦堆白云母花岗岩和石榴石花岗岩以顺层（面理化）和切层形式侵位于下构造层片麻岩中，呈现明显的同构造变形特征。两类淡色花岗岩具有相似的全岩主量元素特征：高的SiO₂（73.26%～74.87%）、Al₂O₃（14.37%～15.03%）和CaO（1.51%～1.81%）,A/CNK指数在1.08～1.16之间。白云母花岗岩表现出较高的Ba(398×10^-6～739×10^-6)和Sr(135×10^-6～165×10^-6),低的Rb(180×10^-6～201×10^-6)和Rb/Sr比（1.22～1.34）,变化的Eu/Eu^*（0.76～1.47）,轻重稀土分异明显，球粒陨石标准化稀土元素配分图呈明显右倾趋势。石榴石花岗岩表现出低的Ba(204×10^-6～282×10^-6)和Sr(81×10^-6～103×10^-6),较高的Rb(243×10^-6～281×10^-6)和Rb/Sr比（2.37～3.22）,明显的Eu负异常（Eu/Eu^*=0.46～0.66）,轻重稀土分异不明显，球粒陨石标准化稀土元素配分图呈“海鸥型”。苦堆淡色花岗岩的初始⁸⁷Sr/⁸⁶Sr比为0.762732～0.766668,ε_Nd（t）为-13.1～-12.5,与大喜马拉雅结晶岩系一致。年代学结果表明苦堆白云母花岗岩形成于约27Ma,石榴石花岗岩形成于27.5～28.1Ma。根据岩相学、年代学、运动学、地球化学等分析，认为苦堆白云母花岗岩起源于大喜马拉雅结晶岩系白云母水致熔融，源岩为大喜马拉雅较深位置的变杂砂岩，石榴石花岗岩是白云母花岗岩进一步结晶分异的结果。同构造的苦堆淡色花岗岩反映了渐新世早期藏南拆离系已经启动，藏南拆离系的启动诱发了地壳深部的白云母水致熔融，形成淡色花岗岩，并沿着藏南拆离系侵位。喜马拉雅造山带在渐新世（集中于28～26Ma）已从碰撞增厚转换为代表松弛与折返的伸展构造背景。更多还原

【Abstract】 The Sakya Dome is located in the central part of the Northern Himalayan Gneiss Dome（NHGD） Belt. Field observations show that the northern section of the Sakya Dome can be divided into three structural levels: the upper level of shallowly metamorphosed Tethyan Himalayan Sequence, the middle level of mylonitic garnet gneisses, and the lower level of mylonitic orthogneiss and paragneiss of the dome core. There is a continuous increase in metamorphism and deformation from the upper to the middle level without an obvious discontinuity. But a detachment fault developed between the middle and the lower tectonic level. Uniform north-dipping foliation and north-plunging lineation were formed in the three levels, and all the kinematic features indicate a uniform top-down-to-north shearing, suggesting that the shear zone in the northern section of the Sakya Dome should be an outcrop of the Southern Tibetan Detachment System（STDS）. The Kuday leucogranites（muscovite and garnet leucogranites） intruded into the mylonitic gneiss of the lower level, in the forms of layer-parallel（foliated） and layer-cutting dykes, showing obvious syn-tectonic characteristics with the STDS. Zircon U-Pb dating results of muscovite leucogranites were about 27Ma, and the garnet leucogranites were formed at 27.5 ～ 28.1Ma. For whole-rock geochemistry, both muscovite leucogranites and garnet leucogranites are characterized by high SiO₂（73.26%～74.87%）, Al₂O₃（14.37%～15.03%） and CaO（1.51%～1.81%） with A/CNK indices between 1.08 and 1.16. The muscovite leucogranites are characterized by high Ba(398×10^-6～739×10^-6), Sr(135×10^-6～165×10^-6), low Rb(180×10^-6～201×10^-6), Rb/Sr ratio（1.22～1.34）, various Eu anomalies（0.76～1.47） and obvious differentiation of HREE and LREE. The chondrite-normalized REE distribution pattern shows an obvious right-leaning trend. The garnet leucogranites are characterized by low Ba(204×10^-6～282×10^-6), Sr(81×10^-6～103×10^-6), high Rb(243×10^-6～281×10^-6), Rb/Sr ratio（2.37～3.22） and obvious negative Eu anomalies（Eu/Eu^*=0.46～0.66）. The differentiation of HREE and LREE is not obvious. The chondrite-normalized REE distribution pattern is “seagull-type”. The initial ⁸⁷Sr/⁸⁶Sr ratio and ε_Nd（t） range from 0.762732 to 0.766668 and-13.1 to-12.5, respectively, which is consistent with the Greater Himalayan Crystalline（GHC）. Based on the analysis of petrography, chronology, kinematics and geochemistry, it is believed that the Kuday muscovite leucogranites originated from the water-present melting of muscovite in the GHC, the source rocks are the meta-graywacke in a deep position of the GHC, and the garnet leucogranites are the results of further crystallization differentiation of muscovite leucogranites. The syn-tectonic Kuday leucogranites indicate that the STDS has been active in the Early Oligocene, which induced the water-present melting of muscovite in the deep crust to form leucogranites and emplacement along the STDS. During the Oligocene（28～26Ma）, the Himalayan orogen has changed from a convergent thickening setting to an extensional tectonic background caused by the relaxation and exhumation of the orogen.更多还原