Maturation of East Junggar oceanic arc related to supracrustal recycling driven by arc–arc collision: perspectives from zircon Hf–O isotopes

Abstract

<p>Oceanic arcs are crucial sites for producing new continental crust. However, how the continental crust has acquired its bulk “andesitic to dacitic” compositions is not well-understood. To address this issue, we carry out an integrated study for granitoids from the East Junggar oceanic arc, Central Asian Orogenic Belt. All the granitoid samples with ages of 332–280 Ma have high SiO₂ but low MgO contents, indicating a dominant crustal source. Based on zircon O isotopes, these granitoids can be divided into three groups: Group I (5.0 ± 0.46‰, 2SD), Group II (8.6 ± 0.47–9.4 ± 0.52‰, 2SD) and Group III (6.8 ± 0.36–7.4 ± 0.48‰, 2SD) with mantle-like, elevated and intermediate zircon δ¹⁸O ratios, respectively. The formation of Group I granitoids can be ascribed to partial melting of juvenile mafic crust, while Group II and III granitoids were likely derived from a mixed source of juvenile mafic crust and supracrustal rocks in variable proportions. Combined with their depleted mantle-like zircon εHf(t) values (+ 11.6 to + 13.5), it is inferred that these supracrustal rocks were mainly isotopically unevolved, immature volcanogenic sediments. The zircon Hf–O isotope array is compatible with mixing between juvenile mafic crust and supracrustal volcanics (40–70% for Group II and 20–40% for Group III) in their magma sources. The incorporation of supracrustal rocks into such high-δ¹⁸O granitoids was likely associated with fore-/intra-arc basin closure triggered by arc–arc collision. Our results thus highlight the role of supracrustal recycling induced by collisional events in driving the compositional differentiation of oceanic arc crust from basaltic to felsic.<br></p>