Relict basin closure of the Paleo-Asian Ocean: New insights from geochronological and geochemical analysis of the Yangjiagou Formation, NE China

The Yangjiagou Formation is distributed along the Changchun-Yanji suture zone in NE China and records the closure of the Paleo-Asian Ocean. The age of the Yangjiagou Formation remains uncertain. Here, we present new data from petrology, whole-rock geochemistry, zircon U-Pb geochronology and in situ zircon Hf isotope analysis of sedimentary and volcanic rocks from the Yangjiagou Formation to shed new light on the tectonic evolution of the eastern Paleo-Asian Ocean. Laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb age dating indicated that newly recognized volcanic interlayers in the Yangjiagou Formation formed during the Early Triassic (similar to 246 Ma). This, combined with previously published age data from the Yangjiagou Formation, indi-cated deposition during the Late Permian to Early Triassic. Early Triassic volcanic rocks (andesites) exhibit high Sr/Y and (La/Yb)N ratios but low Y contents and YbN values, showing adakitic affinity. Geochemical data suggest that adakitic magmatism was generated by the partial melting of an oceanic slab and overlying sediments during slab detachment. Geochemical data suggest that the parental magma may have been modified by the melt-peridotite reaction during ascent through the mantle wedge. Geochemical data for sedimentary rocks from the Yangjiagou Formation reveal a low degree of weathering, compositional immaturity, and poor sorting. Detrital zircon U-Pb geochronology and Lu-Hf isotope analysis for two graywackes from the Yangjiagou Formation showed mixed provenance of the sedimentary rocks. The main source was determined to be felsic igneous rocks related to Paleozoic subduction and accretion along the Changchun-Yanji suture zone. The sedimentary rocks also originate from Late Mesoproterozoic-Neoproterozoic materials from the Songliao-Jiamusi blocks, as well as Neoarchaean-Early Mesoproterozoic materials from the North China Craton. Considering our new data alongside previously published data for the Changchun-Yanji suture zone, we suggest that the Yangjiagou Formation formed in a syn-collisional setting. The existence of marine bivalves and crinoid stems in the Yangjiagou For-mation and the identification of coeval thick marine carbonate deposits in the Kaiyuan region imply that a shrunken remnant ocean basin remained along the Changchun-Yanji suture zone during the Early Triassic.