Abstract:

Two types of Cenozoic potassic volcanic rocks, kamafugite and potassic trachybasalt, were recognized in Lixian and Dangchang counties in West Qinling, Gansu Province (N104°20′N104°50′, E33°30′E34°10′). The first type, kamafugite, is plagioclasefree, consisted of Tirich phlogopite, melilite, leucite and nephelite. They have lower contents of SiO2 and Al2O3, but have high TiO2, CaO, MgO contents and K/Na and Mg# ratio whereas the second type, potassic trachybasalt, is abundant in plagioclase and  contains no phlogopite, melilite, leucite and nephelite. They have higher SiO2, Al2O3, (Na2O+K2O), and lower TiO2, CaO, MgO, and K/Na and Mg#, in comparision to the first type of rocks. The whole rock K/Ar and phlogopite 39Ar/40Ar ages of kamafugite range from 71 to 23 Ma, and the potassic trachybasalt yields the whole rock 39Ar/40Ar age of ~9 Ma. They are both Miocene magmatism. Both kamafugite and potassic trachybasalt are enriched in incompatible trace elements and LREE. The kamafugite (87Sr/86Sr=0704 030707 49, 143Nd/144Nd=0512 740512 94, εNd=1125.95) and the potassic trachybasalt  (87Sr/86Sr=0.704 120.705 22, 143Nd/144Nd=0.512 650.512 76, εNd=1.125.95) have similar Pb isotopic compositions (206Pb/204Pb=8374 618.998 6, 207Pb/204Pb=15.5295.669 3, 208Pb/204Pb=38.497 139.414) and both show plumerelated OIBlike features, suggesting a possible mixture of the geochemical endmembers of EM1, DMM and HIMU. Combining the available regional geological and geophysical research, the petrogenesis of the magmatism was studied. As a response and absorption to the IndiaAsia collision, the northeastward flow of the asthenosphere beneath the Tibetan Plateau along the 400 km depth interface was blocked by the North China craton, and caused a systemic sequences in West Qinling Region, including the strikeslip fault systems and pullapart basins in the shallow crust and the origin of the twotype magmatism in this work in deep lithosphere. This explanation is consistent with the lithology and plumelike geochemical features of the kamafugite and potassic trachybasalt, which are significantly different from the potassic magmatism within the plateau and its adjacent regions.

Key words: West Qinling; Cenozoic; potassic volcanic rocks; magmatic source region; dynamics