Early Cretaceous continental arc magmatism in the Wakhan Corridor, South Pamir: Mantle evolution and geodynamic processes during flat subduction of the Neo-Tethyan oceanic slab

The petrogenesis of continental arc magmas provide critical insights into thermal evolution and geodynamics of the continental lithosphere and crust-mantle interaction and deep dynamic processes. In this study, we report new zircon U-Pb ages along with isotopic and elemental whole-rock geochemistry, mineral chemistry and Hf-O isotope data, for Kalaqigu diorites and monzogranites in the Chinese Wakhan Corridor, South Pamir. Zircon U-Pb dating indicates that the Kalaqigu pluton was emplaced in the Early Cretaceous (ca. 108–106 Ma). The diorites are geochemically characterized by low SiO₂ (51.9–54.5 wt.%) and CaO (7.7–9.4 wt.%) contents, but high MgO (5.3–8.3 wt.%), Al₂O₃ (12.8–16.8 wt.%) and TiO₂ (0.6–1.1 wt.%) contents as well as high Mg^# (56–65) values, and so are similar to high-Mg diorites. They are enriched in large ion lithophile elements (LILEs, e.g., K, Sr and Ba) and light rare earth elements (LREEs), while depleted in high field strength elements (HFSEs, i.e., Nb, Ta, Zr and Hf). Combined with negative ε_Nd(t) (-6.9 to -14.0), ε_Hf(t) (-9.9 to -12.2) and high (⁸⁷Sr/⁸⁶Sr)i (0.7075–0.7086) ratios, these observations indicate they originated from an enriched lithospheric mantle source. High δ¹⁸O_zrn (7.49–9.01‰) values in conjunction with relatively high ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios suggest that the source was modified by subducted sediment-derived melts. Variable Cr contents (54–117 ppm) are likely controlled by minor fractionation of olivine and orthopyroxene. The monzogranites show high SiO₂ contents (69.2–72.0 wt.%), low Rb/Sr (0.4–0.6), (K₂O+Na₂O)/CaO (2.6–4.8) and FeO^T/MgO ratios (2.6–3.2). They contain diagnostic cordierite and show strongly-peraluminous characteristics (A/CNK > 1.1) with high δ¹⁸O_zrn (7.82–8.85‰) values, compatible with typical S-type granites. Their abundant inherited zircons, with age populations similar to those of detrital zircons from regional Early Paleozoic metasedimentary rocks, indicate they were derived from partial melting of ancient metasedimentary rocks. Phase equilibrium modelling is consistent with biotite-dehydration melting of metagreywacke, probably at ~750 ℃ and ~6.0 kbar indicated by the biotite chemistry. A south-to-north magmatic migration based on regional geochronology suggests that northward flat-slab subduction of the Neo-Tethys oceanic slab played an important role in the generation of these widespread Early Cretaceous continental arc magmatic rocks. However, the granitoids were generated earlier than the mantle-derived mafic rocks, which suggests that crustal melting occurred during the early stage of subduction. The subsequent flat subduction resulted in continuous metasomatism by subducted sediments. Contemporaneous regional compression primarily occurred far north of the subduction zone (i.e., North and Central Pamir), inducing deformation as well as crustal shortening. With the flare-up of continental arc magmatism in South Pamir, crustal shortening moved southward. These processes, combined with addition of voluminous mantle-derived magmas, played an important role in crustal thickening in the Pamir during the Early Cretaceous.