Petrogenesis of Cenozoic potassic volcanic rocks in the Nangqen basin

The Nangqen basin is one of the Tertiary pull-apart basins situated in the east of the Qiangtang block. Similar to the adjacent Dengqen basin and Baxo i basin, there occurred a series of potassic volcanic and sub-volcanic rock s, ranging from basic, intermediate to intermediate-acid in lithology. Based on the study of petrology, mineralogy and geochemistry, including REE s, trace elements, isotopic elements and chronology, the authors concluded that the Cenozoic potassic volcanic rocks in tile Nangqen basin were formed in the post-collisional intraplate tectonic settings. The relations betwee n the basic, intermediate and intermediate-acid rocks are neither different iation nor evolution, but instead the geochemical variability is mainly att ributable to the different partial melting degrees of the mantle sources fo rmed at depths of 50-80 km. The sources of the potassic rocks are enriched metasomatic mantle that has experienced multiple mixing of components mainly derived from the crust. Th e recycling model can be described as follows: after they had subducted to the mantle wedge, the crust-derived rocks were metasomatized with the mantl e materials. In view of the fact that the ratio of crust-derived rocks incr eases by the age of volcanism, it can be concluded that the sources of the potassic rocks moved upwards progressively with time. The underplating of small scattered magmas upwelling from the asthenosphere may have induced partial melting of the sources of the volcanic rocks in s ome pull-apart basins in the Hengduanshan area and the intense tectonic mov ements of large-scale strike-slip belts provided conduits for the ascending melts.