Recently, the Altyn strike|slip fault in western China has become a hot topic to the research on continental dynamics of Tibetan Plateau. The chronological research is very important to constrain the age of tectono|thermal event within Altyn fault belt. Many isotopic dating researches, related to the ophiolites, high|pressure metamorphic rocks and some granitic rocks, have been done by Chinese and foreign geologists in the area. There, however, are only few isotopic dating researches on the syntectonic|growing minerals within Altyn fault. We collected a sample of Caledonian mylonitized granite (At3a) in the north of Dangjin pass and two samples of Jurassic meta|sedimentary rocks in Qaidam gate fault|valley (At37c) and Geshi fault|valley (At30d). All the samples contain the syntectonic|growing minerals such as white mica, chlorite, sericite and biotite etc. By dating these minerals, we can constrain the time of the tectonic events occurred in Altyn fault belt.Sample At3a, mylonitized granite, has been strongly deformed with undulating extinction of quartz. The plagioclase and quartz were elongated and surrounded by fine|grained white micas and chlorites with strain shadow texture. These suggest that the sample has been suffered ductile deformation. The estimation of p|T condition is 350℃, 180MPa. The isochronal age of (89.2±1.6)Ma has been obtained by laser probe 40 Ar/ 39 Ar dating analyses of six white mica grains. Recently, the Altyn strike | slip fault in western China has become a hot topic to the research on continental dynamics of Tibetan Plateau. The chronological research is very important to constrain the age of tectono | thermal event within Altyn fault belt. Many isotopic dating researches, related to the ophiolites, high | pressure metamorphic rocks and some granitic rocks, have been done by Chinese and foreign geologists in the area. There, however, are only few isotopic dating researches on the syntectonic | growing minerals within Altyn fault. We collected a sample of Caledonian mylonitized granite (At3a) in the north of Dangjin pass and two samples of Jurassic meta | sedimentary rocks in Qaidam gate fault | valley (At37c) and Geshi fault | the valley (At30d). All the samples contain the syntectonic | growing minerals such as white mica, chlorite, sericite and biotite etc. By dating these minerals, we can constrain the time of the tectonic the tectonic events occurred in Altyn fault belt.Sample At3a , mylonitized granite, has been specifically deformed with undulating extinction of quartz. The plagioclase and quartz were elongated and surrounded by fine | grained white micas and chlorites with strain shadow texture. These suggest that the sample has been suffered ductile deformation. The isochron age of (89.2 ± 1.6) Ma has been obtained by laser probe 40 Ar / 39 Ar dating analyzes of six white mica grains.