萨迦穹隆北侧苦堆淡色花岗岩的形成机制及对喜马拉雅造山带演化的约束  

Formation Mechanism of the Kuday Leucogranite from the Sakya Dome and Constraints on the Evolution of the Himalayan Orogen

作　　者：王宏立 范云松 张进江[1] 古大祥 林超 王晓先 WANG HongLi;FAN YunSong;ZHANG JinJiang;GU DaXiang;LIN Chao;WANG XiaoXian(MOE Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Sciences,Peking University,Beijing 100871,China;Zijin Mining Group Company Limited,Xiamen 361000,China;National Institute of Natural Disaster Prevention,Ministry of Emergency Management,Beijing 100085,China)

机构地区：[1]北京大学地球与空间科学学院,造山带与地壳演化教育部重点实验室,北京100871 [2]紫金矿业集团股份有限公司,厦门361000 [3]应急管理部国家自然灾害防治研究院,北京100085

出　　处：《岩石学报》2024年第5期1647-1668,共22页Acta Petrologica Sinica

基　　金：第二次青藏高原综合科学考察研究(2019QZKK0703);中国地质调查局地质调查项目(DD20190001)联合资助.

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SiO_(2)(73.26%~74.87%)、Al_(2)O_(3)(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87 Sr/86 Sr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�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 SiO_(2)(73.26%~74.87%),Al_(2)O_(3)(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

关 键 词：苦堆岩体 淡色花岗岩 藏南拆离系 地球化学 喜马拉雅造山带 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]