Zircon U–Pb age, element and C–O isotope geochemistry of post-collisional mafic-ultramafic rocks from the Dabie orogen in east-central China

Abstract

Origin and genesis of post-collisional mafic–ultramafic rocks in the Dabie orogen of China are controversial issues with respect to Mesozoic transport of mass and energy between mantle and crust. Zircon U–Pb age, whole-rock major and trace elements, mineral and whole-rock oxygen isotopes, and carbon concentrations and isotope compositions of apatite and whole-rock were determined for these igneous rocks in order to find a resolution. The results show that the Dabie mafic–ultramafic rocks are characterized by strong LREE enrichment and negative HFSE (Nb, Zr, Ti and P) anomalies but positive anomalies of Pb and Ba. Zircon U–Pb dating yields consistent ages of 122–128 Ma for magma crystallization and 105–116 Ma for post-magmatic hydrothermal alteration. There is a large variation in oxygen isotope ratios of minerals and their host rocks, with whole-rock δ 18O values of 1.1–6.6‰, clinopyroxene 3.85–5.7‰, plagioclase 2.8–7.3‰, and zircon 3.85–6.04‰. Most of the zircons have δ 18O values different from the normal mantle values. Equilibrium oxygen isotope fractionations between minerals have been preserved for some of the samples, but the others show significant disequilibrium indicating post-magmatic subsolidus alteration. Carbon concentrations and isotope compositions vary widely from 0.03 to 0.18% and from −27.0 to −5.8‰, respectively. Most of the samples are characterized by the low δ 13C values typical of organic carbon. A comparison of the element and isotope data for the Cretaceous mafic–ultramafic rocks with the Triassic metamorphosed eclogites from the Dabie–Sulu orogenic belt shows that they share many geochemical and isotopic features in common, pointing to a genetic relationship in protolith origin between them. No signature of depleted mantle has been detected from Nd and Sr isotopes in the post-collisional igneous rocks. Protoliths of the Cretaceous mafic–ultramafic rocks and the Triassic eclogites are thus considered the mid-Neoproterozoic mafic–ultramafic rocks that were the products of rift magmatism along the northern margin of the Yangtze plate. Partial melting of the subducted lithosphere itself is proposed to produce the post-collisional igneous rocks. The Early Cretaceous superplume event is hypothesized to provide a heat source to trigger the melting of a large scale and a short period beneath the collisionally thickened orogen.