Neoarchean recycling of 18O-enriched supracrustal materials into the lower crust: Zircon record from the North China Craton

• Existence of a highly heterogeneous Neoarchean lower crust beneath the NCC.
• Identification of considerable recycling of supracrustal materials into Neoarchean lower crust.
• Homogenization of zircon oxygen isotopes in lower crust indeed happened during the Pb loss.

Oxygen isotope analyses were carried out using SIMS technique (Cameca-1280) on zircons from well-studied lower crust xenoliths/xenocrysts from the North China Craton (NCC). The results show that magmatic zircons with Archean-Paleoproterozoic 207Pb/206Pb ages have an extremely large variation in δ18O values that range from less than normal mantle value (4.74 ± 0.28‰ (2σ)) to 18O-enriched supracrustal value (8.88 ± 0.40‰ (2σ)) and differ in localities of the NCC (δ18OQindao from 5.27 ± 0.45‰ to 8.88 ± 0.40‰; δ18OHannuoba from 6.78 ± 0.23‰ to 7.69 ± 0.41‰; δ18OXiuyan from 4.74 ± 0.28‰ to 6.57 ± 0.39‰). Such a variation demonstrates that the Neoarchean lower crust beneath the NCC was highly heterogeneous in oxygen isotope compositions, even in an individual region. The fact that the predominant zircons have high δ18O values suggests that the recycling of 18O-enriched supracrustal materials into the Neoarchean lower crust indeed happened during zircon crystallization. Some zircons with core–rim texture have nearly identical δ18O both in core and its rim. On the other hand, others have a distinctive oxygen isotope ratio between the core and its rim, possessing slightly higher δ18O in the rim than its core or vice versa. Such an oxygen isotope feature in zoned zircons suggests a complicated evolutional history of ancient lower crust in the NCC. This study provides convincing evidence that a large amounts of the supracrustal materials have been recycled into the Neoarchean lower crust beneath the NCC, a process resulting in the highly heterogeneity in oxygen isotopes.

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