Protracted accretion and amalgamation in the Altyn orogen: Insights from field study, detrital zircon ages, and U-Pb-Hf isotopes of the North Altyn accretionary complex, Northwest China

The Altyn orogen records the evolution of the northern branch of the Proto-Tethys Ocean. However, the timing and process of accretion and amalgamation remain debated. In this study, we investigate the well-exposed North Altyn accretionary complex of Northwest China, with particular emphasis on sedimentary rocks that have been largely overlooked in previous studies but provide critical constraints on the accretionary and collisional evolution. Field study indicates that the North Altyn accretionary complex is composed of ophiolites, magmatic rocks, high-pressure/low-temperature metamorphic rocks, Neoproterozoic tectonic fragments, intraoceanic arc relics, and sedimentary sequences. Sedimentary samples collected from different units are dominated by volcanic detritus and display low compositional maturity, consistent with deposition in forearc to backarc basin settings. Detrital zircon U-Pb geochronology yields maximum depositional ages ranging from ca. 505 Ma to ca. 402 Ma and reveals an overall northward-younging trend across the accretionary complex. Variations in detrital zircon age spectra and Hf isotopic compositions indicate distinct provenance signatures and temporal changes in sediment supply. These data demonstrate that subduction of the North Altyn Ocean initiated before ca. 540 Ma and persisted with a southern-dipping polarity until at least ca. 419 Ma, as indicated by the predominance of detritus from a southern sediment source (Central Altyn Terrane). A significant provenance shift to bidirectional sediment input occurred between ca. 419 Ma and ca. 402 Ma, recording the onset of amalgamation between the North and Central Altyn terranes. Furthermore, our new provenance data reveal the presence of an Ediacaran intraoceanic arc and provide the first sedimentary evidence linking it to the South Altyn orogenesis, thereby constraining the timing of amalgamation and exhumation across the entire Altyn orogen. The diverse geological record preserved in the North Altyn accretionary complex highlights its dominant role in unraveling the subduction evolution of the Proto-Tethys Ocean and the construction of ancient orogenic systems.