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 ɛHf(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.

西准噶尔地体（West Junggar terrane, WJT）通常被认为与中哈萨克斯坦和东哈萨克斯坦地体存在构造关联；然而，西准噶尔地体南部的中古生代岩浆作用鲜有报道，这使得中亚造山带（Central Asian Orogenic Belt, CAOB）西南部的构造演化与对比研究存在显著不确定性。本文首次在阿拉山口地区识别出锆石U-Pb年龄为390~394 Ma的埃达克质石英闪长岩（adakitic quartz diorites），证实了西准噶尔地体南部存在早-中泥盆世岩浆作用。该类岩石具有低Yb（1.00~1.41 ppm）、低Y（8.9~14.4 ppm），但高Sr（301.5~475.9 ppm）、高Sr/Y（23~41）以及高球粒陨石标准化La/Yb比值[(La/Yb)N=8.1~11]的特征，与现代埃达克岩的典型地球化学特征一致。其兼具较高的SiO₂（64.96~67.71 wt.%）、中等K₂O（1.53~3.19 wt.%），但低MgO（1.56~2.31 wt.%）、低TiO₂（0.28~0.42 wt.%），以及低Cr（≤13.7 ppm）、Co（2.7~9.9 ppm）、Ni（8.0~18.7 ppm）和Th（3.7~4.8 ppm）含量；同时具有高正值的锆石εHf(t)值（+5.6~+10.3）和低Nb/Ta比值，指示其可能由轻微加厚的下地壳部分熔融形成，而该下地壳主体为西准噶尔地体南部的埃迪卡拉纪-中古生代洋内弧岩石。结合区域地质数据，本文新获得的研究结果支持西准噶尔地体南部在中古生代时期可能是哈萨克斯坦早-中泥盆世外侧火山带的东延部分。早-中泥盆世的构造伸展及由此引发的下地壳熔融，可能与准噶尔-巴尔喀什洋的板片回撤作用相关。