Recycling of subducted continent slab in an accretionary orogen: Insight from the Liangwan potassic granitoids in the Tongbai orogen, Central China

Abstract

Granitoids provide a vital window to probe the lithological configuration and chemical evolution of orogenic belts. In this contribution, we present an integrated study of mineral compositions, zircon UPb ages and HfO isotopes, titanite UPb ages and SmNd isotopes, as well as whole-rock element and Sr–Nd–Hf isotope compositions of the Liangwan granitic pluton in the northern Paleozoic accretionary orogenic unit of the Tongbai orogen, central China. The Liangwan pluton is composed mainly of monzogranites and contains abundant mafic microgranular enclaves (MMEs). The monzogranites have relatively high silica contents (SiO₂ = 67.40–69.91 wt%) and mainly exhibit shoshonite to high-K calc-alkaline and metaluminous features. The MMEs have similar mineral and geochemical compositions and zircon and titanite UPb ages of ca. 129 Ma to the monzogranites, suggesting that they are likely early-crystalized cumulates of the host monzogranites. The MMEs and monzogranites are both characterized by enrichment of large ion lithophile elements but depletion of high field-strength elements. The monzogranites have whole-rock initial Sr (I_sr) of 0.7069–0.7074, ε_Nd(t) of −14.2 to −13.7, and ε_Hf(t) of −17.5 to −17.2, zircon ε_Hf(t) of −17.8 to −17.0 and δ¹⁸O values of 5.27–5.98 ‰, and titanite ε_Nd(t) of −14.50 to −13.78. These features are distinct from the rocks in the northern Tongbai accretionary orogenic unit but similar to those of the post-collisional granitoids in the southern Tongbai collisional orogenic unit. The Liangwan pluton was likely stemmed from the subducted South China Block (SCB) underneath the northern segment of the Tongbai orogen. Our results show that the subducted continental mass can act as an alternative source for the magmatic rocks occurred in the accretionary unit.