Slab roll-back and crustal growth in the Eastern Junggar terrane, NW China: evidence from Carboniferous A-type granitoids and adakitic rocks

Revealing the formation mechanism of granitic plutons is vital for understanding crustal growth of Earth. Here, we present an integrated study of geochronological, elemental, whole rock Nd and zircon Hf isotopic data for a granitic pluton system that formed in the Eastern Junggar, NW China. Zircon U–Pb dating suggests the seven granitic plutons were emplaced at 356–312 Ma. The monzogranites have high alkaline and Zr contents, and high Ga/Al and TFe₂O₃/MgO ratios, which are geochemically similar to A-type granitoids. They display positive ɛ_Nd(t) (+4.3 to +5.4) and ɛ_Hf(t) (+7.6 to +12.4) values, and young Nd and Hf model ages, suggesting a lower arc crust source. The quartz monzodiorites have high Sr, low Y and Yb contents, yielding relatively high Sr/Y ratios that are similar to modern adakites. These rocks also have high positive ɛ_Nd(t) (+7.6 to +8.0) and ɛ_Hf(t) (+13.9 to +15.5) values, and young Nd and Hf model ages, indicating a subducted oceanic crustal source. In combination with data from other plutons in this area, we suggest that they were likely generated by roll-back of the subducted Junggar oceanic slab, which resulted in back-arc extension, asthenosphere upwelling, and partial melting of oceanic crust and lower arc crust.