Recycling of Paleo-oceanic crust: Geochemical evidence from Early Paleozoic mafic igneous rocks in the Tongbai orogen, Central China

Abstract

Crustal subduction generally develops from oceanic to continental subduction in collisional orogens, with the disappearance of previously subducted paleo-oceanic crust. However, it is not straightforward to trace the fate of subducted paleo-oceanic crust due to modification by subsequent continental collision. In this study, we present a combined study of secondary ion mass spectrometry (SIMS) zircon UPb ages and O isotopes, laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) zircon LuHf isotopes, whole-rock major-trace elements and SrNd isotopes of Early Paleozoic mafic igneous rocks in the Tongbai orogen, central China. These geochemical data are interpreted and modelled in terms of crust-mantle interaction in a subduction channel, in order to account for geochemical transfer from the subducting crust to the mantle wedge. The results provide insight not only into paleo-oceanic crust recycling but also into the nature of orogenic lithospheric mantle. SIMS zircon UPb dating of the mafic igneous rocks yields Early Paleozoic ages of 436 ± 4 to 449 ± 4 Ma for magma crystallization. These mafic rocks have arc-type trace element distribution patterns, with enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) but depletion in high field strength elements (HFSE) relative to rare earth elements (REE) and LILE. They have depleted whole-rock SrNd isotopic compositions, with initial 87Sr/86Sr ratios of 0.7037 to 0.7047 and positive εNd(t) values of 2.7 to 5.0. The mafic igneous rocks also display positive zircon εHf(t) values of 8.9 to 14.8 and variable δ18O values of 4.4 ± 0.3 to 5.9 ± 0.3‰. These geochemical features suggest derivation from fertile mantle sources enriched in LILE and LREE but depleted in radiogenic isotopes. The mantle sources are suggested to be part of the orogenic lithospheric mantle, which was generated by metasomatic reaction of the overlying mantle wedge with previously subducted Erlangping oceanic crust-derived fluids. Some samples have high Ba/Th, Th/Nb and (La/Yb)N ratios, suggesting that the metasomatic agents include both basaltic oceanic crust-derived aqueous solutions and sediment-derived hydrous felsic melts. Model calculations confirm that the geochemical composition of the Tongbai mafic igneous rocks can be explained by reaction of depleted mid-oceanic ridge basalt (MORB) mantle peridotite with 1–10% of subducted Erlangping oceanic crust-derived aqueous solutions plus 0.05–0.1% of sediment-derived hydrous melts. This melt-peridotite reaction would generate a pyroxenite-rich lithology in the mantle source. During the tectonic transition in the Ordovician to Silurian, partial melting of such mantle sources gave rise to mafic magmatism due to back-arc extension. Thus, the Early Paleozoic mafic igneous rocks in the Tongbai orogen record the recycling of earlier subducted paleo-oceanic crust and the nature of the orogenic lithospheric mantle in the collisional orogen.