Early Paleozoic tectonic transition from oceanic to continental subduction in the North Qaidam tectonic belt: Constraints from geochronology and geochemistry of syncollisional magmatic rocks

Abstract

The North Qaidam tectonic belt (NQTB) records the tectonic evolution of the South Qilian Ocean from subduction to closure followed by continental subduction. However, the timing of closure of the South Qilian Ocean and the nature of the tectonic transition from oceanic to continental subduction are poorly constrained. In this contribution, an integrated study of petrology, geochemistry, geochronology and Sr–Nd–Hf isotopes is performed on two groups of intermediate–mafic rocks from the Chahanhe intrusive complex in the NQTB. Group 1 intermediate–mafic rocks (441–428 Ma) have arc-like geochemical compositions, high (87Sr/86Sr)i ratios (0.711171 to 0.713181), negative εNd(t) values (−7.22 to −5.49) and low zircon εHf(t) values (−6.3 to +0.5), suggesting that they are derived from partial melting of enriched subcontinental lithospheric mantle. The enriched lithospheric mantle was produced by interaction of the subcontinental mantle wedge peridotite with subducting slab-derived melt and fluid during previous subduction of the South Qilian Ocean. Group 2 mafic rocks (441–439 Ma) exhibit E-MORB-like geochemical compositions with positive εNd(t) values (+2.44 to +3.31) and high zircon εHf(t) values (+1.3 to +6.9), but with high (87Sr/86Sr)i ratios (0.706775 to 0.708661); these features indicate derivation from partial melting of asthenospheric mantle with the involvement of aqueous fluid and minor melt that were probably derived from subducted oceanic crust. We propose that rollback of subducted South Qilian oceanic slab during the initial stage of continental subduction triggered decompression melting of asthenospheric mantle to produce E-MORB-like mafic rocks; convecting and upwelling asthenosphere also provided heat that induced partial melting of the pre-existing enriched lithospheric mantle and generated mafic magmas with arc-like geochemical signatures. Our results improve the understanding of the tectonic transition process from oceanic subduction to continental subduction in the NQTB, the closure of the Proto-Tethyan Ocean, and provide new insights into the importance of crustal growth during initial continental collision.