Juvenile source of the North Tianshan turbidites and implication for continental growth of the Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB), also known as the Altaids, is one of the world's largest accretionary orogen and it is estimated that ca. 50% of the present crust in Central Asia is juvenile. However, some researchers argued that the amount of continental growth in the CAOB was overestimated. One evidence is that many intra-arc sediments and accretionary wedges in the CAOB contain heterogeneous sources (large proportion of detritus from basement rocks), and no examples from the CAOB where the sediment mainly derived from erosion of juvenile crust has been reported. Here, we conducted geochemistry and Nd isotope study on the turbidites from the North Tianshan Accretionary Complex (NTAC) in the Chinese West Tianshan orogen, which might be an good example of sediment derived from juvenile materials. The turbiditess in the NTAC are mainly composed of fine-grained sandstone, siltstone and argillaceous siliceous rocks. In the southern part near the North Tianshan Fault, the turbidites were deformed and metamorphosed into slate. Geochemically, all the collected rocks (sandstoen/siltstone and slates) have relatively low CIA (Chemical Index of Alteration) values (35 to 63) and PIA (plagioclase index of alteration ) values (34 to 68), indicating that their source rocks experienced relatively weak weathering before erosion and deposition. Both the sandstone/siltstone and slate samples display high ICV (Index of Compositional Variability) values of 0.89 to 1.50 and 0.89 to 0.93, higher than the PAAS, suggesting a relatively immature source. Based on geochemical data, it is suggested that the sandstone/siltstone were mainly derived from intermediate and felsic igneous rocks, while the slates were mainly derived from felsic igneous rocks, and their source rocks were most likely formed in oceanic/continental arc settings. Most of the samples from the NTAC display high positive ε_Nd(t) values (+5.5 to +7.9) with only one exception of +0.8, and the Nd model ages (cluster between 672 Ma and 522 Ma, with one exception of 1.1 Ga) are only slightly older than their depositional age (Carboniferous). Our previous study has revealed that the detrital zircons from the turbidites display unimodal age patterns with peaks at 320 to 310 Ma, and have high positive ε_Hf(t) values (+2.9 to +15.8, mostly greater than +10). These results indicate that the turbidites in the NTAC were mainly derived from intermediate to felsic igneous rocks with juvenile arc signature. The northern Chinese West Tianshan is a typical site with significant Phanerozoic continental growth, and the mechanism needs further study.