Abstract

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 People's 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 < 20 ppm, TiO2 < 0.60 wt%, Th/Yb > 0.60) in the north and rocks formed during backarc spreading (Y > 20 ppm, TiO2 > 1.0 wt%, Th/Yb < 0.30) in the south. These changes reflect the changing processes of melt generation associated with an evolving backarc basin such as the Ordovician backarc basin that is preserved in the Northern Qilian Mountains. The rift axis captured the arc magmatic flux in the early island-arc rifting stage so that the lavas that erupted from this part of the rift cannot be compositionally distinguished from arc lavas. As backarc extension continued and the backarc basin widened, the rift axis moved progressively away from the arc magmatic flux. The distance between the axis of extension and the zone of subarc mantle downwelling was eventually sufficient to allow mantle upwelling to become established under the axis of extension and to generate a mid-ocean ridge–like decompression melting system to support backarc-basin spreading.

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