Geochemistry of ophiolite cherts from the Qinling orogenic belt and implications for their tectonic settings

Paleozoic cherts from the Mianlue and the Erlangping ophiolite zones of the Qinling orogenic belt are characterized by low Si/Al ratios (52.14-683.52 in the Mianlue cherts, 12.29-58.62 in the Erlangping cherts), Fe2O3 (0.01-0.35 and 0.02-1.24) and high Al2O3/(Al2O3+Fe2O3) ratios (0.82-0.99 and 0.83-0.99). The negative correlation between Si2O and Al2O3 in the cherts reflects the important role of terrigenous components. The Erlangping cherts have La-n/Ce-n = 0.9-1.15 and Ce/Ce = 0.95-1.15 with low contents of V, Ni and Cu, consistent with those of cherts forming on the continental margin. In contrast, the Ce/Ce ratios of the Mianlue cherts range from 0.71 to 1.18 and La-n/Ce-n from 0.88 to 1.43 with slightly high V, Ni and Cu, which are similar to cherts found in the mid-ocean ridges and pelagic basins. Combined with the features of basic lavas associated with the cherts, it is suggested that during the Paleozoic, when the back-arc basin represented by the Erlangping ophiolite commenced shrinking in size in the mid-Ordovician, the southern Qinling was still in an extensional regime and finally grew into a new limited oceanic basin in the early Carboniferous.