南盘江盆地中三叠统碎屑锆石地质年代学:物源及其地质意义

南盘江盆地的物源及其构造性质至今尚未取得较为一致的认识。目前主要的观点认为其物源主要来自盆地东南面的云开大山地区,盆地的性质为古太平洋板块俯冲所形成的前陆盆地。本文采用碎屑锆石年代学方法,首次报道该盆地中三叠统碎屑锆石年龄组成,分析了该盆地的沉积物源,并基于物源分析探讨了成盆机制和板块间动力学关系。该盆地中三叠统碎屑锆石年龄组成最主要的特征性的年龄分布在320～250Ma之间,此区间内存在280Ma和250Ma两个峰值。通过与相邻各块体锆石年龄谱对比,本研究认为:(1)南盘江盆地的主要物源区应为古特提斯北缘(华南一侧)的石炭-二叠纪岩浆弧,SongMa构造带可能是三叠纪的缝合线,而不是因Sibumasu板块增生到印支板块而导致的板内活化带;(2)该盆地三叠纪时为古特提斯闭合在华南形成的前陆盆地,与古特提斯沿SongMa带向北俯冲闭合有关,前人认为的南盘江盆地形成于古太平洋板块向西北俯冲作用的观点值得商榷。

Geochronology of Detrital Zircons from the Middle Triassic Sedimentary Rocks in the Nanpanjiang Basin:Provenance and its Geological Significance

The nature and provenance of the Nanpanjiang basin,Southwest China,are highly controversial.It is at present widely suggested that it is a foreland-basin related to the flat subduction of the Paleo-Pacific plate beneath the Southeast China block during Permo-Triassic,and the Yunkaidashan area was the major source area of the basin.The present paper reports detrital zircon U-Pb ages carried out by LA-ICP-MS from the Middle Triassic sandstones of the Nanpanjiang basin,aiming to constrain the provenance of the Nanpanjiang basin,and to discuss its geological implications.Detrital zircon U-Pb ages of the Nanpanjiang basin were characterized by the age cluster range from 330 to 250 Ma with a peak of ca.250 Ma,and a subordinate ca.280 Ma,which do not match with any ages known from South China.To compare with zircon peaks from potential source areas,it is suggested that(1) the main source area of the Nanpanjiang basin was a magmatic arc formed during Permian on the northern border of Tethys,the southwest margin of South China.(2) The Nanpanjiang basin is a foreland-basin associated with the closure of the Paleo-Tethys ocean along the Song Ma zone(in Vietnam),and the Song Ma zone is a suture,rather than an intraplate reactivated zone between South China and Indochina block.Previous views like(1) the Nanpanjiang was a foreland-basin formed by flat subduction of the Paleo-Pacific plate;and(2) the Indosinian orogeny of South China is originated from Paleo-Pacific plate need to be reevaluated.