Post-collisional plutons in the Balikun area, East Chinese Tianshan: Evolving magmatism in response to extension and slab break-off

Abstract

The Late Carboniferous to Permian is a critical period for final amalgamation of the Central Asian Orogenic Belt (CAOB). Gabbroic and granitic intrusions formed in this period at Balikun, eastern Tianshan, and therefore provide important clues for understanding the tectonic events that took place at this time. The Shiquanzi gabbro was formed at 301 ± 6 Ma and consists of low-Si (LS) (SiO 2 < 47 wt.%) and high-Si (HS) (SiO 2 > 48 wt.%) members, and both are characterized by high Nb (> 9 ppm), high εNd (t) (+ 7.12–+ 8.71) and low initial 87Sr/ 86Sr ratios (0.7030–0.7041). The HS gabbros have HFSE (Nb = 17.5–24.2 ppm) contents, and Nb/Y (0.56–0.72) and Ta/Yb (0.52–0.60) ratios, higher than those of the LS samples and exhibit E-MORB-like characteristics, while the LS samples have higher Ba/Th ratios (150–236) and more prominent subduction-related signatures than those of the HS samples (Ba/Th = 10–91). The HS rocks probably resulted from decompressional melting of asthenospheric mantle, whereas the LS rocks originated from a depleted mantle source metasomatised by slab-derived fluid. The Shiquanzi gabbro therefore reflects an extensional environment, in which asthenospheric mantle upwelled and triggered partial melting of metasomatised depleted mantle at a shallower level. Granitic intrusions at Balikun (Daliugou Pluton, Dajiashan Pluton and Barkol Tagh Batholith) formed in the Early Permian (284–288 Ma), and generally have alkali-calcic compositions and exhibit characteristics of A2-type granite. Their variable Nd (εNd (t) = + 1.92–+ 6.03) and Sr ( 87Sr/ 86Sr (i) = 0.7040–0.7152) isotope compositions and young T DM model ages (0.56–0.75 Ga) suggest that they were derived from juvenile crustal sources, probably earlier arc intrusions. From the Late Carboniferous to Early Permian, magmatism at Balikun evolved from gabbroic bodies to large A2-type granitic intrusions, implying a gradually increasing impact of mantle-derived magma on the crust. A slab break-off regime, following accretion of the Harlik Arc onto the Angara continent, may explain the post-collisional magmatism and geodynamic transition of the eastern Tianshan.