Sediment recycling and adakite petrogenesis: Constraints from the late Ordovician tonalite in the North Qilian suture zone

Deciphering the slab melt contribution to arc igneous rocks in orogenic belts is of great importance for unraveling the fate of subducted slabs, adakite petrogenesis and geochemical recycling process. The fact of subducted slabs, particularly how different sections of the oceanic crust including its overlying sediment react during slab melting, remains poorly understood. Here we present a detailed study of the Leigongshan (LGS) tonalite pluton in the eastern section of the North Qilian suture zone (NQSZ). The tonalites are geochemically similar to low-Mg adakitic rocks characterized by high Sr/Y and (La/Yb)N ratios with low MgO, Cr, and Ni concentrations, indicating limited/no mantle interaction. They exhibit moderately radiogenic Sr, Nd, and Pb isotope compositions with initial 87Sr/86Sr ratio of 0.706216–0.708267, εNd(t) value of −0.7 to −2.1 and (206Pb/204Pb)t of 18.52–18.90, most likely a mixed melt derived from the mafic oceanic crust plus ca. 10–25% overlying sediments. Morphology, internal structure, and UPb dating of zircon crystals indicate a common preservation of old cores aged at ca. 470–2733 Ma. The sharp truncation of core zoning and embayed boundaries with the rims further indicate that the cores are probably clastic, abraded materials inherited from a (meta)sedimentary precursor from which the tonalitic magma was derived. The younger mean ages of ca. 453–450 Ma are interpreted as the emplacement age of the pluton. They have εHf (t) values of +0.5–7.0 and δ18OZrc values of 4.9‰ to 8.7‰, which somewhat deviate from those of the parent mafic oceanic crust, indicating the involvement of subducted sediments as well. The LGS adakitic tonalite represents mixed melts of the subducted slab including the overlying sediment that accounts for the enriched Sr–Nd–Pb isotope compositions. Future detailed research on the zircons of adakite/adakitic rocks would provide new important clues on their petrogenesis.