This paper provides a constraint for the composition and genesis of phlogopites on the basis of mineralogical and mineral-chemical studies of titanphlogopites from the Cenozoic volcanic rock and its mantle xenoliths in Western Qinling, Gansu province, in combination with data from high pressure experiments of phlogopites and minettes. The results show that these phlogopites belong to a transitional group in chemical composition, being different from phlogopites from kimberlites, lamproites and minettes. The megacrystalline and macrocrystalline phlogopites from the Cenozoic volcanic rock of Western Qinling are of the early crystallization phase under the conditions of some 1300℃，30×10-8 Pa, and rich CO2 and H20 in the mantle. Microcrystalline phlogopites may be a product of rapid chilling near liquidus. The genesis of the phlogopites in peridotites and gluboles seems to be related to fluid metasomatism, and the appearance of titanphlogopites further indicates that the Cenozoic volcanic rock originated from the metasomatic mantle with metasomatic fluids rich in incompatible elements K, Na and Ti and volatiles C02 and H2O.