藏北蚕眉山地区火山岩和夷平面的时代

Ages of volcanic rocks and planation surface in the Canmei Mountain area, northern Tibet

藏北蚕眉山地区发育2个时代的新生代火山岩①上新世的花岗斑岩和流纹斑岩,白云母K-Ar同位素年龄为(3.65±0.31)MaBP.;②中新世的粗面英安岩、粗安岩和玄武粗安岩等火山熔岩,全岩K-Ar同位素年龄为(12.81±0.4)MaBP、(12.85±0.56)MaBP和(14.51±0.23)MaBP。由中新世各火山熔岩体周边的下伏地表高程和非火山岩构成的局部区域最高山岭的顶部高程恢复出火山喷发时的古地表形态,完全可与现代夷平面形态特征相对比。结合火山熔岩下伏地面表层古风化壳的发育,认为蚕眉山地区火山熔岩的下伏地面为古夷平面,本区为古夷平面残留区。

There occur Cenozoic volcanic rocks of two ages in the Canmei Mountain area, northern Tibet: Pliocene granite-porphyry or rhyolitic-porphyry with a muscovite K-Ar age of 3.65±0.31 Ma and Miocene trachydacite, trachyandesite and basaltic trachyandesite with whole-rock K-Ar ages of 12.81±0.4, 12.85±0.56 and 14.51±0.23 Ma respectively. The Miocene surface morphology during volcanic eruption is recovered according to the altitude of the overlying surface around the various volcanic lavas and the altitude of the top of the highest local mountain made up of non-volcanic rocks. It may be completely correlated with the modern planation surface morphology. On that basis, combined with the development of the paleo-crust of weathering below the volcanic lavas, it is thought that the surface below the volcanic lavas in the Canmei Mountain area is the paleo-planation surface and that the study area is a remnant area of paleo-planation surface.