Early-Middle Devonian adakitic magmatism generated by slab retreat in southern West Junggar, NW China: implications for tectonic correlation with central and East Kazakhstan

The West Junggar terrane (WJT) is known to tectonically correlate with the Central and East Kazakhstan terrane; however, the Middle Palaeozoic magmatism has rarely been reported in the southern WJT, leading to significant uncertainties on the tectonic evolution and correlation of the southwestern Central Asian Orogenic Belt (CAOB). Here, we present the adakitic quartz diorites with zircon U-Pb ages of 390-394 Ma in the Alashankou region to demonstrate the presence of Early-Middle Devonian magmatism in the southern WJT for the first time. They are characterized by low Yb (1.00-1.41 ppm) and Y (8.9-14.4 ppm) but high Sr (301.5-475.9 ppm), Sr/Y (23-41), and (La/Yb)(N) (8.1-11) values, typical of modern adakites. As suggested by the relatively high SiO2 (64.96-67.71 wt.%), moderate K2O (1.53-3.19 wt.%), but low MgO (1.56-2.31 wt.%), TiO2 (0.28-0.42 wt.%), Cr (<= 13.7 ppm), Co (2.7-9.9 ppm), Ni (8.0-18.7 ppm), and Th (3.7-4.8 ppm) concentrations as well as the high positive e(Hf)(t) values (+5.6 to +10.3) and low Nb/Ta ratios, the adakitic quartz diorites were probably generated by the partial melting of a slightly thickened lower crust, which consisted mainly of the Ediacaran-Middle Palaeozoic intra-oceanic arc rocks in the southern WJT. Combined with regional data, our new results support that the southern WJT probably evolved as the eastern extension of the Kazakhstan Early-Middle Devonian outer volcanic belt during the Middle Palaeozoic. The Early-Middle Devonian tectonic extension and resultant lower crust melting may thus be related to the slab retreat of the Junggar-Balkhash Ocean.