Caledonian subduction of Proto-Tethyan Ocean beneath the Yunkai Massif: Petrological and geochemical evidence from Late Ordovician mafic intrusions

This paper presents a comprehensive study including petrography, zircon U-Pb dating and in-situ Hf-O isotope compositions, whole-rock major and trace element and Sr-Nd-Pb-Hf isotope compositions on two mafic intrusions from the Gaozhou region in Yunkai Massif, South China, with aims to understand their petrogenesis and tectonic setting. Zircon U-Pb dating reveals an age range of 447 451Ma, confirming their emplacement in the Late Ordovician. Samples from the Chenliukeng Village contain a large amount of amphibole and can be regarded as appinites, whereas those from the Youweiyong Village are Ca-Al-rich and Mg-rich gabbros with the occurrence of calcic plagioclase (An >95). Both types of mineral assemblage indicate that the parental magmas were hydrous subalkaline basalts probably formed at a subduction zone. The two mafic intrusions are characterized by enrichment of large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but depletion in high field strength elements (HFSEs), analogues to modern arc mafic magmas. They show highly variable and enriched zircon Hf-O isotopic compositions and bulk-rock Sr-Nd-Pb-Hf isotopic features, while correlations between the bulk-rock Nb/La and Sr-Nd-Hf isotopes argue against a significant role of crustal assimilation and/or contamination. We thus conclude that the large Sr-Nd-Pb-Hf-O isotopic variations reflect the different proportional addition of recycled crustal components in the mantle sources. Further zircon Hf-O isotope modelling results show that the melting source for the appinites likely contained 20% 50% of the subducted terrigenous sediment, while the proportion of this recycled sediment component in the source of gabbros was 35% 45%. Based on the paleogeographic reconstruction that the Yunkai Massif was ever located between the Proto-Tethys Ocean and the East Gondwana continent during the Late Ordovician, we consider the subduction of Proto-Tethys Ocean as a likely mechanism to interpret the petrogenesis of contemporaneous arc mafic magmatism in the study region. Following the subduction, the terrigenous sediments had been dehydrated and melted to enrich the overlying mantle wedge, which experienced hydrous melting to form the water-rich subalkaline basaltic magmas. The subsequent fractional crystallization and/or accumulation generated the appinites and Ca-Al-rich and Mg-rich gabbros. Our results therefore provide key geochronological and petrological evidence for the subduction of Proto-Tethys Ocean beneath the Yunkai Massif during Early Paleozoic.