Petrogenesis of Late Carboniferous Granitoids in the Dunhuang Region, Southern Central Asian Orogenic Belt: Implications for the Crustal Differentiation of Orogenic Belt

To investigate how the anatexis of accretionary complexes contributes to crustal differentiation in orogenic belts, we present an integrated analysis of whole-rock geochemistry, zircon U-Pb geochronology, and zircon Lu-Hf isotopes for Late Carboniferous granitoids from the Dunhuang orogenic belt, northwestern China. Zircon U-Pb dating indicates that the granitoids were formed between 321 and 316 Ma. Their zircon εHf(t) values (−26.9 to 10.7) are largely negative, suggesting derivation primarily from ancient crustal materials. Geochemical correlations between the granitoids and their wall rocks suggest that the magmatic sources were dominated by meta-sedimentary rocks with minor meta-mafic rocks. The systematic whole-rock Rb-Sr-Ba variations are attributed to water-fluxed melting of muscovite from a metasedimentary source. Given the continued activity of the Dunhuang subduction zone during the Late Carboniferous, the coupling of an elevated geothermal gradient with a large fluid flux in the deeper accretionary wedge plausibly triggered partial melting of the accreted materials generating granitic magmas. As the granitic magmas segregated, extracted, and migrated upward, they gradually cooled and crystallized to form batholiths or dikes that intrude the overlying rock layers. This process left a residue with a composition broadly similar to that of the lower continental crust. This process enhances vertical differentiation within the orogenic belt crust and likely plays an important role in transforming loose accreted materials into more mature continental crust.