Multiple ridge subduction processes in the southern Altaids: Implications from clinopyroxene chemistry and Sr–Nd–Hf isotopes of late carboniferous Nb-enriched, magnesian diorite-andesites in West Junggar, NW China

Abstract

Magnesian (Mg# > 45) and/or Nb-rich (Nb > 6 ppm) dioritic-andesitic rocks have been identified in the Mayile area of southern West Junggar, Northwest China. A systematic petrological, geochronological, whole-rock geochemical, Sr–Nd–Hf isotope, and clinopyroxene chemistry study was conducted to constrain the petrogenesis, mantle source, and P–T conditions of magmas as well as the tectonomagmatic processes in the southern Paleo-Asian Ocean subduction zone. Mayile dioritic-andesitic rocks generated in the Late Carboniferous (306–312 Ma) have a high SiO2 content (52.3–63.2 wt%) and high Mg# (46–66) as well as sanukitic-like low LaN/YbN and Sr/Y ratios. They have high Nb contents (6.27–11.6 ppm) and Nb/U (9–18) ratios and are Nb-enriched sanukitic high-Mg andesites (HMAs). Clinopyroxene thermobarometry indicates that the parental magma of the Mayile HMAs experienced a high-temperature (1137–1199 °C) and shallow-level (0.8–0.17 GPa) partial melting in a mantle wedge. They have εNd(t) (+5.14 to +6.42), εHf(t) (+8.8 to +14.4), and zircon εHf(t) (+9.4 to +15.8) values and a slightly high (87Sr/86Sr)t ratio ranging from 0.703214 to 0.704140 relative to normal mid-ocean ridge basalt (N-MORB). The results suggest that West Junggar was dominated by continuous records of sanukitic rocks hinting at multiple ridge subduction processes in the southern Altaids. We reconstructed the subduction history of the Junggar Ocean in the Carboniferous-Permian from these particular observations in West Junggar, and gained insight into the gradual evolution of the tectonic setting, which transitioned from normal intraoceanic subduction to hot subduction of the young oceanic lithosphere from the Early to Late Carboniferous, with subsequent ridge subduction occurring mainly in the Late Carboniferous.