Magmagenesis in the Ordovician backarc basins of the Northern Qilian Mountains, China
This paper reports the results of a petrologic and geochemical study of lavas from early Paleozoic backarc basins in the Northern Qilian Mountains in the People9s Republic of China. Petrographic and geochemical characteristics of lavas reflect the changing styles of extension: typical basic lavas due to backarc seafloor spreading are found in the north, but to the south, the lava compositions become increasingly similar to arc lavas. Glassy (devitrified now), sparsely phyric basic lavas characterize the regions of seafloor spreading. Felsic lavas and porphyritic basic lavas occur in the southern, island-arc rifting part of the study area. Geochemical compositions distinguish between mature arc rocks (Y 2 0.60) in the north and rocks formed during backarc spreading (Y > 20 ppm, TiO 2 > 1.0 wt%, Th/Yb