Ordovician backarc setting in the Tongbai orogen, central China: Geochronological and geochemical constraints from the Erlangping metabasites

Abstract Uncertainty as to the tectonic setting of the Erlangping Group in the Tongbai orogen of central China has hindered reconstruction of the early Paleozoic arc-trench system, and limited the associated understanding of the evolution of the northern branch of the Proto-Tethys Ocean. We present new petrologic, geochronological, elemental, and Sr-Nd-Pb-Hf isotopic data for the Erlangping metabasites from the Tongbai orogen. The results indicate that the Erlangping metabasites formed between 478 Ma and 460 Ma. They are tholeiitic in composition with SiO2 = 47.44–57.31 wt%, Al2O3 = 14.15–19.71 wt%, and MgO = 3.44–11.03 wt%, geochemically subdivided into mid-ocean-ridge basalt (MORB)–like metabasites and high-MgO (>6.0 wt%) and low-MgO (<6.0 wt%) arc-like metabasites. High-MgO arc-like metabasites have lower TiO2, FeOt, Al2O3, Nb/La (mostly <0.20), and εNd(t) values (−0.9 to +3.2) relative to those of low-MgO arc-like rocks. However, low-MgO and high-MgO arc-like rocks have similar positive zircon in situ εHf values and whole-rock Pb isotopic compositions (with 206Pb/204Pb = 17.19–18.74, 207Pb/204Pb = 15.52–15.77, and 208Pb/204Pb = 36.51–38.89), suggestive of the input of recycled sediment-derived components in the depleted mantle. In comparison with the arc-like metabasites, the MORB-like metabasites have higher εNd(t) values (+5.5 to +7.4), and originated from a depleted mantle source newly modified by slab-released fluid. The synthesis of all available data has suggested that the Erlangping Group in the Tongbai orogen developed in a continental backarc basin setting. The Qinling-Tongbai orogen was formed through the northward subduction of the Shangdan Ocean during the Late Ordovician to Early Silurian and is a part of the Proto-Tethys northern branch.