A Paleoproterozoic mantle source modified by subducted sediments under the North China craton

Abstract

Subduction is an effective mechanism for deep recycling of supracrustal rocks and mantle refertilization. However, the timing of the onset of modern-style deep subduction is debated. Here we report geochronogical and geochemical data for an intrusive suite of ultramafic, syenitic and carbonatitic rocks, a carbonatite-hosted peridotite xenolith, and granulites from the Paleoproterozoic Trans-North China orogen in the North China craton (NCC). The xenolith differs in composition from typical cratonic mantle peridotites. It equilibrated at T ≈ 1100 °C and P ≈ 29 kbar, i.e. under hotter conditions than cratonic mantle xenoliths worldwide. The carbonatites and associated alkaline rocks were emplaced at 1810 ± 3 Ma and are related to the Paleoproterozoic collision between the Western and Eastern segments of the NCC. These intrusions show Sr-Nd isotope compositions consistent with an enriched mantle source [(87Sr/86Sr)i = 0.7027–0.7039; εNd(t) = −4 to −6], and contain zircon characterized by unradiogenic Hf isotope compositions [εHf(t) = −5 to −9] and high δ18Osmow values (7.9–9.5‰). In contrast, basement granulites and wall-rock granites are enriched in radiogenic Nd and Hf isotopes, but low in 18O. These differences rule out the possibility of crustal contamination. Our results thus suggest that modern-style plate tectonics dates back at least to the Paleoproterozoic Era, and the post-collisional Paleoproterozoic alkaline magmatism in the NCC tapped mantle sources modified by deep sediment recycling.