Slab-derived fluid contributes to subducted material recycling and crust growth: Insights from the arc volcanics in East Junggar (NW China)

Abstract

<p>Arc volcanic rocks provide important insights into material recycling and crustal growth in subduction zones; however, the recycling mechanism (i.e., slab-derived fluid versus mélange diapirs) is poorly constrained. Here, we focus on Devonian basalts and andesites identified recently in the Shaerbulake area, in the northern part of the East Junggar (EJ) terrane. Zircon U-Pb ages indicate that the samples were formed at ∼390 Ma, and the zircon ε_Hf(t) values range from +13 to +14.2. The investigated samples yield variable SiO₂ (48.4–62.9 wt%) and MgO (1.6–6.9 wt%) contents and similar subduction-related trace-element patterns, with enrichment in Ba, Pb, and Sr and depletion in Nb, Ta, and Ti. Whole-rock Sr-Nd isotopic compositions and geochemical discrimination diagrams (Ba/Th versus La/Sm and Th/Nb), along with quantitative modelling, suggest low-degree (2 %–5 %) partial melting of a sub-arc mantle source that had been metasomatized by ∼10 % slab-derived fluid, which consisted of 95 % altered oceanic crust-derived fluid and 5 % subducted sediment melt. Our whole-rock SiO₂, MgO, trace-element, and Sr-Nd isotopic data and zircon Hf isotopic compositions, together with previously published data from across the EJ terrane, argue for a considerable contribution from slab-derived fluid to the recycling of subducted slab material and crustal growth during the Devonian in the EJ terrane. This implies that slab-derived fluid might not only be metasomatic agents responsible for material recycling in subduction zones, but might also contribute to arc magmatism and crustal growth. <br></p>