Abstract: The Tethys Himalaya is a newly-recognized rare metal metallogenic belt(RMMB). It mainly produces rare metal such as beryllium, tungsten, tin, niobium and tantalum. Cuonadong Be-rare polymetallic metal deposit is located in the Cuonadong Gneiss Dome(CGD) with three types of mineralized bodies, consisting of pegmatitic type, skarn type and hydrothermal vein type. Among them, the skarn type is mainly occurred in the deformed zone around the mantle area of the CGD. Field and microscopic features show that Ar-Ar age of muscovite can represent the hydrothermal activity time of skarn. The Ar-Ar isotope of muscovite from the skarnized marble in the Cuonadong dome was dated, yielded a Ar-Ar plateau age of 14.21±0.22Ma, corresponding to a ⁴⁰Ar/³⁶Ar-³⁹Ar/³⁶Ar isochron age of 14.21±0.27Ma. This age consistented with emplacement activity time (~14Ma) of the highly fractional muscovite granite within the CGD, and it was confirmed that the skarn-type rare metal mineralization in this period indeed result from the muscovite granite from chronological perspect.