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6th International Archean Symposium
6th International Archean Symposium

Late Archean accretion of continental crust through oceanic subduction in the North China Craton

Oral

Talk Description

The North China Craton (NCC) contains at least three continental nuclei, namely Jiaoliao, Xuhuai and eastern Alxa, and two of them include ancient rocks as old as 3.8 Ga. It is distinguished from the majority of Archean cratons by its extensive records of 2.6-2.5 Ga mafic magmatism but rare records of 2.8-2.7 Ga magmatism. It is enigmatic which role this late-formed craton had played in the Archean global tectonics. We present an integrated study of 845 suites of whole-rock major-trace element data and their associated zircon Hf and whole-rock Nd isotope compositions for three episodes of basaltic-andesitic magmatism at ca. 2.88-2.73 Ga, 2.65-2.60 Ga and 2.56-2.50 Ga, respectively, from the whole NCC. The first two episodes of rocks are mostly distributed close to the three continental nuclei, whereas the 2.56-2.50 Ga rocks are widespread in the craton. These rocks were divided into LREE-depleted tholeiitic basalt, LREE-enriched tholeiitic basalt, tholeiitic andesite, calc-alkaline andesite and sanukitoids mainly based on their SiO2 contents, (La/Sm)N ratios, La/Yb and Sr/Y ratios, and Sr-Ba concentrations. These five groups generally have arc-like trace element distribution patterns, with variable enrichments in fluid-mobile incompatible trace elements, which are consistent with their water contents calculated by their Mg-Fe variations. Such coupling features suggest that their mantle sources were enriched by subducting oceanic slab-derived fluids in the forms of aqueous solutions and hydrous melts. Compared to the other Archean cratons elsewhere on Earth, the NCC is significantly deficient in komatiites and komatiite-related rocks, but contains much higher proportions of the LREE-enriched basalts and calc-alkaline andesites. This difference suggests that plate tectonics gradually replaced mantle plume in dominating the continental growth during late Archean. Such cognition was further strengthened by the formation of 2.75 Ga metavolcanic rocks from the Alxa block in the westernmost NCC. The felsic volcanics show TTG-like geochemical compositions, with variable zircon Hf-O isotope compositions that can be categorized into three groups. Group1 has high εHf(t) values of 8.5 ± 0.3 at t = 2750 Ma and low δ18O values of 4.3-5.6‰, group 2 exhibits εHf(t) values of 1.8 ± 0.3 and elevated δ18O values of 5.7-7.0 ‰, and group 3 shows moderately high εHf(t) values of 3.9 ± 0.4 but the highest δ18O values of 5.6-7.9 ‰. Their magma sources would be the overthickened oceanic crust that was generated through seafloor spreading at different ages and underwent seawater hydrothermal alteration at different temperatures. Several failed subductions caused these oceanic crusts to collide with the continental nucleus, which was finally replaced by a successful subduction leading to both partial melting of the thickened crusts and the metasomatized mantle wedge.

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