Ridge subduction and its role in crustal generation in the accretionary belts: Evidence from Late Carboniferous to Early Permian felsic igneous rocks in the southeastern Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens on Earth and has voluminous crustal growth in the Phanerozoic [1]. However, the mechanism of the crustal growth remains controversial [1, 2]. In an attempt to address this dispute, we carry out petrological, geochemical, bulk Sr-Nd isotopic and zircon U-Pb and Hf isotopic analyses of the Late Paleozoic felsic igneous rocks from Xilinhot, southeastern CAOB. Zircon U-Pb data suggest precise emplacement ages of 319±2, 279±1, and 279±1 Ma, respectively, for granodiorite, alkaline granite and rhyolite. The granodiorites are characterized by enrichment in LILE and depletion in HFSE, which are consistent with those of typical subduction-related arc granitoids. These features, together with their positive ɛ_Nd(t) values (+7.5-+8.0) and zircon ɛ_Hf(t) values (+14.7-+16.1), suggest that the granodiorites were derived by melting of metasomatized mantle wedge, followed by fractional crystallization and assimilation of crust materials. By contrast, the alkaline granites/rhyolites show an A₂-type affinity with enrichment in LILE and LREE, depletion in Nb, Ta and Ti, and positive ɛ_Nd(t) values (+3.1-+6.4) and zircon ɛ_Hf(t) values (+7.6-+16.0), they are inferred to form by partial melting of juvenile lower crust via underplating of mantle-derived magmas through a "slab tear" [3]. Taking into account the coexistence of adakites and high-Mg andesites in Xilinhot [4], this may imply a dramatic change in composition from calc-alkaline arc magmatism to mantle-derived magmatism. This suggests that ridge subduction may have proceeded in this region during the Early Permian. Furthermore, large-volume magmatic flare-ups and depleted Nd-Hf isotope compositions suggest that over 90% of juvenile crust in the southeastern CAOB was generated in a relatively short interval (20-40 Ma) during each long-lived cycle (>140 Ma from subduction to collision) of magmatic activities. Ridge subduction may have played a significant role in the rhythmic juvenile crust generation in the CAOB during the Phanerozoic, which could be applied to other accretionary belts on Earth.

[1] Jahn et al. (2004) JAES 23, 599-603. [2] Sengör et al. (1993) Nature 364, 299-307. [3] Lee and Stephen. (2012) Nature 482, 314-315. [4] Wang et al. (2017) Acta Geol. Sinica 8, 1776-1795.