Magmagenesis of Ordovician back-arc basins in the Northern Qilian Mountains

Abstract: This paper reports the results of a petrogeochemical study of lavas in lower Paleozoic back-arc basins in the Northern Qilian Mountains. The distribution of samples links regions of back-arc seafloor spreading to the north with regions farther south where a progression of island arc rifting styles occurred in the earliest stage of back-arc basin extension. Petrographic and geochemical characteristics of lavas reflect the change in style of extension, with typical back-arc basin basic lavas in the north passing southward to arc lavas. Glassy (devitrified now), minor-porphyritic basic lavas characterize regions of seafloor spreading. Felsic lavas and porphyritic basic lavas occur in the southern, island-arc rifting regions. Geochemically, mature arc portions (Y<20×10-6, TiO2<0.60%, Th/Yb>0.60) are distinguished from regions of back-arc spreading (Y>20×10-6, TiO2>1.0, Th/Yb<0.60). Samples show progressive marked changes in geochemical compositions, from back-arc basin basic lavas formed by seafloor spreading southward through lavas that increasingly resemble arc lavas to arc lavas that are indistinguishable from arc lavas encountered in the southernmost portion of the rift. These changes reflect the changes in the mechanism of mantle convection and processes of melt generation beneath the evolving back-arc basin in the Northern Qilian Mountains: from mantle downwelling beneath incipient island arc rifts caused by slab subduction to mantle upwelling beneath zones of back-arc seafloor spreading. The rift axis captured the arc magmatic flux in the early island arc rift stage, so that the lavas erupting from this part of the rift cannot be compositionally distinguished from arc lavas. As back-arc extension continued and the back-arc basin widened, the arc magmatic flux moved progressively away from the rift axis, finally generating a mid-ocean ridge-like decompression melting system-back-arc basin magmatic system.