摘要
近年来研究发现,青藏高原南部的喜马拉雅造山带和拉萨地体新生代岩浆活动伴随着广泛的稀有金属成矿作用。然而,与喜马拉雅地区淡色花岗岩相关的稀有金属成矿作用研究相比,拉萨地体的研究程度还明显偏低。本文对拉萨地体中新世淡色花岗岩和相关的伟晶岩型稀有金属矿化开展了研究。本次研究选取的拉萨地体二云母花岗岩-花岗伟晶岩-细晶岩出露于隆格尔裂谷的肩部,呈南北向带状展布,与裂谷内的拆离断层活动相关。二云母花岗岩以岩席状产出,并具有一定的弱变形,而花岗伟晶岩-细晶岩呈岩脉方式侵入到二云母花岗岩和早期的花岗岩中,局部偶见绿柱石,变形明显,反映经受了伸展拆离作用的影响。LA-ICP-MS独居石和锆石U-(Th)-Pb定年显示,隆格尔裂谷东、西两侧发育的二云母花岗岩-花岗伟晶岩-细晶岩的成岩年龄分别为14.3~12.9Ma和16.8~16.0Ma。二云母花岗岩-花岗伟晶岩-细晶岩均具有高硅(72.11%~82.42%)、过铝质(A/CNK=1.01~1.65)的特征,并且它们都富集Li、Rb、Cs、K、Th,而相对亏损Ba、Sr、P、Ti。花岗伟晶岩-细晶岩具有明显的Eu负异常(Eu/Eu*=0.01~0.38),而二云母花岗岩Eu负异常(Eu/Eu^(*)=0.33~0.39)相对较弱。结合二云母花岗岩低的Zr饱和温度(684~762℃)以及富集的Sr-Nd-Hf同位素(^(87)Sr/^(86)Sr(i)=0.7082~0.7102;ε_(Nd)(t)=-7.6~-7.2;锆石ε_(Hf)(t)=-5.3~-0.2)特征,我们认为它们起源于拉萨中下地壳和印度下地壳共同参与的水致熔融,并经历了高程度的分离结晶作用。而含绿柱石花岗伟晶岩-细晶岩的Zr/Hf、Nb/Ta和Y/Ho比值呈现出非球粒陨石异常,并且它们还具有稀土元素四分组效应和与二云母花岗岩类似的Sr-Nd-Hf同位素组成,表明它们是二云母花岗岩进一步结晶分异作用的结果。青藏高原南部发育的中新世淡色花岗岩大部分沿着东西向或南北向伸展构造分布,很可能与连续发生的俯冲印度板块断离和撕裂有关,是软流圈地幔物质上涌诱发青藏高原南部的中下地壳发生脱水/水致熔融,产生的熔体迅速上升至中上地壳水平,然后沿着构造薄弱带迁移并经历了强烈结晶分异作用的产物。通过拉萨地体中、新生代岩浆岩的对比研究发现,俯冲的印度大陆地壳物质参与熔融和花岗质岩浆的高分异演化过程是拉萨地体中新世稀有金属成矿作用的两个关键因素,二者缺一不可。
In recent years,Cenozoic magmatic activity in the Himalaya orogen and Lhasa terrane of southern Tibet is believed to have been accompanied by extensive rare-metal mineralization.However,in contrast to the well-studied Himalayan leucogranite and associated rare-metal mineralization,relatively little work has been completed in the Lhasa terrane.The present work conducts a study on the Miocene leucogranite and related pegmatite-type rare-metal mineralization developed in the Lhasa terrane.In this study,we identified a N-S-trending felsic magmatic belt along the shoulder of the Lunggar rift,whose formation is associated with the activity of detachment faults within the rift.The felsic magmatic rocks include two-mica granites,granitic pegmatites,and aplites.The sheet-like two-mica granites are weakly deformed,while the strongly deformed granitic pegmatite and aplite dikes intrude into the two-mica granites and early granites with occasional occurrences of beryl,reflecting the impact of extensional detachment faults on their formation.LA-ICP-MS monazite and zircon U-(Th)-Pb dating suggest that the intrusive ages of the two-mica granites-granitic pegmatites-aplites in east and west sides of the Lungger rift are 14.3~12.9Ma and 16.8~16.0Ma,respectively.These granitic rocks are characterized by high SiO_(2)(72.11%~82.42%),peraluminous(A/CNK=1.01~1.65),enrichment in Li,Rb,Cs,K,and Th,and depletion in Ba,Sr,P,and Ti.Granitic pegmatites-aplites have strongly negative Eu anomalies(Eu/Eu^(*)=0.01~0.38),while relatively weak negative Eu anomalies(Eu/Eu*=0.33~0.39)were found in two-mica granites.Combined with the low Zr saturation temperatures(684~762℃)and enriched Sr-Nd-Hf isotopic compositions(^(87)Sr/^(86)Sr(i)=0.7082~0.7102;ε_(Nd)(t)=-7.6~-7.2;zirconε_(Hf)(t)=-5.3~-0.2)of the two-mica granites,we suggest that they originated from water-induced melting of Lhasa and Indian middle-lower crust,followed by high-degree fractional crystallization.Beryl-bearing granitic pegmatites and aplites show non-chondritic anomalies in Zr/Hf,Nb/Ta and Y/Ho ratios,and they also display REE tetrad effects and similar Sr-Nd-Hf isotopic compositions to the two-mica granites,suggesting that they are the produces of further fractional crystallization of the two-mica granites.The majority of southern Tibetan Miocene leucogranites distributed along east-west or north-south extensional structures may be associated with the continuous break-off and tearing of the subducted Indian continental slab.The upwelling of asthenosphere may induce melting of the southern Tibetan Plateau s middle-lower crust.The resulting melts rapidly rise to the middle-upper crustal level,then migrate along the tectonically weak zone and undergo high-degree fractional crystallization.Given a comparative study of the Mesozoic to Cenozoic magmatic rocks in the Lhasa terrane,we suggest that the involvement of subducting Indian continental crust and highly differentiated evolution of granitic magmas are two key factors for the Miocene rare-metal mineralization of the Lhasa terrane,and both are indispensable.
作者
李家亮
纪伟强
胡方泱
徐强
LI JiaLiang;JI WeiQiang;HU FangYang;XU Qiang(State Key Laboratory of Lithospheric and Environmental Coevolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China)
出处
《岩石学报》
北大核心
2025年第3期859-883,共25页
Acta Petrologica Sinica
基金
第二次青藏高原综合科学考察研究项目(2022QZKK0203)
国家自然科学基金项目(42488201、42403045)联合资助.
