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40 Ar/ 39 Ar ages were determined for 6 phlogopite samples from a kamafugite pipe occurring in western Qinling, yielding a time span of 22 to 23 Ma for kamafugite eruption. Together with known geochronological data for the Qinghai-Tibet Plateau (QTP) and its peripheral regions, it is concluded that these Cenozoic potassic-ultrapotassic volcanic rocks are products of post-collisional volcanism. The age of the Cenozoic volcanic rocks in western Qinling partly overlaps that of potassic volcanics from the Gandise belt, reflecting the northward migration of the post-collisional volcanism in QTP. The age data not only confirm the temporal and spatial migration of post-collisional volcanism in QTP, but also provide geochronological constraints on the geodynamic setting at depth and the regimes of the tectonic evolution since Neogene in western Qinling regions.A suite of Cenozoic ultra-potassic volcanic rocks and carbonatites are widespread along the Lixian-Dangchang areas of western Qinling in Gansu Province. There are abundant mantle-derived xenoliths within these ultra-potassic volcanic rocks. Systemmatic petrological, petrochemical and geochemical studies were conducted [1][2][3][4][5] , resulting in the recognitions that the Cenozoic ultra-potassic volcanic rocks are an important part of the Cenozoic potassic volcanic belt in the Qinghai-Tibet Plateau (QTP), and the origin and geodynamic setting of which are related to subduction-collision process between the Indian-Eurasian continents. However, it is short of geochronological constraints on the timing of ultra-potassic volcanic eruption in western Qinling. In previous literature, the age of these volcanic rocks was mainly inferred on the basis of cross-cutting relationships between the volcanic rocks and their surrounding strata. It was postulated that the volcanic rocks are products of Paleogene-Neogene volcanism [6] . 13 samples of volcanic rocks from 7 volcanic pipes were dated by whole-rock K-Ar method to yield an age range of 18.9 to 7.1 Ma [2] . The scatter in the whole-rock K-Ar ages requires a more accurate dating for the Cenozoic kamafugite. In this paper, we report 40 Ar/ 39 Ar dating for phlogopite from 6 different kamafugite pipes. Geological setting and feature of samplesWestern Qinling orogenic belt is situated in the central part of China continent, which is the tectonic intersection, conjunction and transformation of all the tectonic units from south and north, east and west. Zhang et al. [7] named it western Qinling-Songpan continental syntaxis which is a gigantic tectonic unit formed by the jointing and convergence of several major orogenic belts and blocks. Due to the concerted interaction during the major piecing process of the 3 huge tectono-dynamic systems, namely, Tethyan-Himalayan, Pacific and Paleo-Asian. The Cenozoic volcanic rocks are mainly distributed in the vicinity of Lixian and Dangchang counties, among them the kamafugite occurs as pipes and (rarely) dykes. The rock is porphyritic, phenocrysts are olivine and clinopyroxene, and part...

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