The late Yanshanian magmatite in the Youjiang Fold Belt of western China is represented by basic-ultrabasic dyke swarms and ultra-acid rock veins. The 40Ar/39Ar dating of muscovite phenocryst from quartz porphyry veins, which are located in the Bama-Fengshan-Linyun area of northwestern Guangxi, yields a plateau age of 95.59 ± 0.68 Ma and an isochron age of 95.0 ± 1.0 Ma for a NE-striking quartz porphyry vein in Longhuang of Fengshan county, and a plateau age of 96.54 ± 0.70 Ma and an isochron age of 95.91 ±1.1 Ma for a NW-striking quartz porphyry vein in Bama county. These ages are interpreted as an emplacement age for quartz porphyry veins. The new data support that the Yanshanian magmatic activity concentrated on 80-100 Ma in the Youjiang Fold Belt and the surrounding area, indicating large scale extension and lithospheric thinning in western China during the late Cretaceous. Since the granitic magmatic activity of late Cretaceous was related to a large scale Sn-(Cu-Pb-Zn) mineralization in western China, it is worth considering the prospecting potential for Sn-(Cu-Pb-Zn) in the area where quartz porphyry occurs within the Youjiang Fold Belt. We further discuss whether Carlin-type gold deposits and other epithermal ore deposits are related to this episode of magmatic activity within the Youjiang Fold Belt.%华南西部右江褶皱带内部燕山晚期岩浆岩主要为成群分布的基性-超基性岩墙群和超酸性岩脉带.本文对桂西北巴马—凤山—凌云一带的石英斑岩脉进行了斑晶白云母40Ar/39Ar法测年,获得凤山弄黄北东向岩脉的40Ar/39Ar坪年龄为(95.59±0.68) Ma,相应的等时线年龄为(95.0±1.0) Ma;巴马北西向岩脉的坪年龄为(96.54±0.70)Ma,相应的等时线年龄为( 95.91±1.1) Ma.上述年龄代表了岩脉的侵位年龄.这些年龄数据进一步证明右江褶皱带及其周缘燕山期岩浆活动集中于80～100 Ma之间,暗示该区晚白垩世发生了大规模的岩石圈伸展减薄事件.华南西部晚白垩世花岗质岩浆活动与大规模的锡多金属成矿有关,因此桂西北地区石英斑岩集中出露的地方是否具有寻找深部锡多金属矿的前景值得关注.同时,该期岩浆活动是否与以卡林型金矿为代表的低温热液矿床有成因上的联系尚需更多矿床地质和年代学资料的证实.
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