Recycling of terrigenous sediments recorded in late Permian-early Triassic Fe-rich intrusions from Yunkai Massif, South China

Abstract

The origin of Fe-rich intrusion remains debatable regarding the respective role of magmatic evolution and Fe enrichment in the source. Here, we report detailed mineral and bulk-rock geochemistry of Permian-Triassic (251–250 Ma) mafic intrusions from the Tengxian area in Yunkai Massif, South China. These rocks consist of hornblende norites, show Fe-rich affinities with high FeO*(FeO* = FeOt / (FeOt + MgO) in weight ratio, >0.8, based on total iron oxide in the rock), and exhibit significant enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but depletion in Sr and high field strength elements (HFSEs), resembling subduction-related magmas. In addition, they show remarkable features that include very high δ¹⁸O values in zircon (δ¹⁸O = 8.8–10.6 ‰) and apatite (δ¹⁸O = 8.7–10.8 ‰), and extremely enriched SrPb [e.g. ⁸⁷Sr/⁸⁶Sr(i) = 0.7181–0.7196, ²⁰⁷Pb/²⁰⁴Pb(i) = 15.77–15.78, ²⁰⁸Pb/²⁰⁴Pb(i) = 38.95–39.05] and nonradiogenic NdHf isotopic compositions [e.g. ε_Nd(t) = −11.1 ∼ − 10.1, ε_Hf(t) = −8.8 ∼ −7.3] in bulk rocks. These characteristics distinguish the Tengxian norites from modern arc basalts and subduction-related magmas in the Paleo-Pacific Tectonic Domain. Instead, they are isotopically similar to Cenozoic Tibetan and Mediterranean potassic to ultrapotassic rocks in the Tethys Tectonic Domain. Regardless of variable influence by orthopyroxene and plagioclase accumulation, the intrinsically low SiO₂ and high FeOt and Fe/Mn ratios in these rocks were likely attributed to significant contribution of a Fe-rich mantle component such as Si-poor pyroxenite, which might have formed through crystal accumulation of mafic magmas at mantle conditions. The highly evolved Sr-Nd-Pb-Hf isotopic signatures and very high δ¹⁸O values in zircon and apatite required substantial (10–20 %) involvement of recycled crust in the mantle source. The combined mineral and bulk-rock geochemical data suggest that the parental magmas for the Tengxian norites originated from a metasomatized mantle wedge through addition of terrigenous sediment-derived melt following the subduction of Paleo-Tethys Ocean beneath the Yunkai Massif.