【摘要】 班公湖-怒江特提斯洋盆的闭合时限对理解班公湖-怒江结合带的构造演化意义重大。本文采用碎屑锆石年代学的方法,分析了班公湖-怒江结合带西段革吉地区沙木罗组下段与沙木罗组上段的物源特征及差异,为班公湖-怒江特提斯洋盆西段的闭合时间提供了新的证据。研究结果表明沙木罗组下段碎屑锆石年龄分布于2586±26Ma～130±1. 5Ma之间,显示出1个主峰年龄约165Ma,1个次峰年龄约240Ma。碎屑锆石年龄分6个组别,集中在2600～2400Ma、2000～1900Ma、1000～840Ma、500～400Ma、340～200Ma和200～130Ma,分别与新太古代克拉通汇聚事件、哥伦比亚超大陆相关的构造热事件、新元古代构造热事件、冈瓦纳大陆边缘安第斯型造山作用的岩浆活动事件、龙木错-双湖结合带岩浆活动事件和班公湖-怒江特提斯洋盆俯冲消减的岩浆活动事件一一对应。沙木罗组上段碎屑锆石年龄分布于2558±22Ma～101±1. 0Ma之间,显示出1个主峰年龄约107Ma,2个次峰年龄分别约226Ma、150Ma。碎屑锆石年龄集中分布于1个年龄组为122～101Ma,记录了班公湖-怒江特提斯洋盆俯冲消减的岩浆活动。通过与潜在源区的年龄谱峰进行对比,沙木罗组下段物源区与南羌塘地块有良好的亲缘性,表明其沉积物物源主要为南羌塘地块,其中南羌塘岩浆弧可能为其主要贡献者。沙木罗组上段物源区与南羌塘地块和北拉萨地块均具有良好的亲缘性,其沉积物物源具有双源性,分别来源于南羌塘岩浆弧和北拉萨岩浆弧。沙木罗组下段和上段沉积物物源的差异性暗示它们形成的沉积环境和物源区不同。对比分析沙木罗组下段和上段岩石学、碎屑锆石形态学等特征,也揭示了类似的差异性。结合其他相关地质资料,认为班公湖-怒江特提斯洋盆在革吉地区的主体闭合时间约115～110Ma,其代表性岩浆事件为南羌塘地块和北拉萨地块的早白垩世岩浆弧,沙木罗组为班公湖-怒江特提斯洋盆由俯冲消减到闭合过程中的沉积产物。更多还原

【Abstract】 The closure time of the Bangong Co-Nujiang Tethys Ocean is crucial to better understand the tectonic evolution of this suture zone. In this contribution,we carried out a systematic U-Pb analysis of detrital zircons to provide insights into the provenance of the lower and upper subunits of the Shamuluo Formation in Geji region of central Tibet,and the closure time of the Bangong Co-Nujiang Tethys Ocean. The dating results show that the concordant detrital zircon ages from the lower subunit of the Shamuluo Formation range from 2586 ± 26 Ma to 130 ± 1. 5 Ma with a main peak at about 165 Ma and a second peak at about 240 Ma. Detrital zircon ages show six age clusters at 2600 ～ 2400 Ma,2000 ～ 1900 Ma,1000 ～ 840 Ma,500 ～ 400 Ma,340 ～ 200 Ma and 200 ～ 130 Ma respectively,representing craton convergent events in Late Archean,tectonothermal event related to the Columbia Supercontinent,tectonothermal event in new Proterozoic,magmatic activity event of Andean-type orogenic activity on the margin of the Gondwana Supercontinent,magmatic activity event of the Longmu Co-Shuanghu suture and magmatic activity event of the Bangong Co-Nujiang Tethys Ocean subduction correspondently. However,the concordant detrital zircon ages from the upper subunit of the Shamuluo Formation range from 2558 ± 22 Ma to 101 ± 1. 0 Ma with a main peak at about 107 Ma and two secondary peaks at about 226 Ma and 150 Ma. Detrital zircon ages show one age cluster at 122 ～ 101 Ma which records the subduction of the Bangong Co-Nujiang Tethys Ocean. Compared with age spectrum from the adjacent regions,provenance of sediments from the lower subunit of the Shamuluo Formation shows close affinity to the South Qiangtang terrane,or more specifically the southern Qiangtang magmatic arc. Whereas those from the upper subunit of the Shamuluo Formation has a bidirectional provenance,showing close affinity to the southern Qiangtang magmatic arc and the North Lhasa magmatic arc respectively. The difference of their provenances indicates the different sedimentary environment and source area between the lower and upper subunit of the Shamuluo Formation. By comparison with results of sandstone petrology and detrital zircon morphology analysis,a similar difference between these two subunits has also been revealed. Combined with results from other studies,we propose that the closure time of the Bangong Co-Nujiang Tethys Ocean in Geji region is about 115 Ma to 110 Ma,during which extensive magmatism occurred in the southern Qiangtang magmatic arc and the North Lhasa magmatic arc,and as a result,the Shamuluo Formation is closely associated with the tectonic evolution of the Bangong Co-Nujiang Tethys Ocean from subduction to closure.更多还原