The majority of the Songpan-Ganzi Triassic flysch sequence is believed to have derived from denudation of the Dabie and Sulu ultrahigh-pressure (UHP) metamorphic belt in eastern China (e.g., Nie et al., 1994; Zhou and Graham, 1996); however, intense debate still exists on the sources of these rocks. Many authors have suggested various sources, including the Kunlun magmatic arc (e.g., Gu, 1994; Zhang, 2002), the North and South China blocks (e.g., Bruguier et al., 1997), or the Central Qiangtang UHP metamorphic belt (Zhang et al., 2006). Recently, Weislogel et al. (2006) presented 870 single grain U-Pb detrital-zircon ages coupled with 250 paleocurrent measurements to shed new light on the origin of the Song-pan-Ganzi complex and its link to Qinling-Dabie orogen unroofing. They suggested that a southern Songpan-Ganzi deposystem initially was sourced solely by erosion of the Qinling-Dabie orogen during early Late Triassic time, then, during middle to late Late Triassic time, by the Qinling-Da-bie orogen, the North China block, and the South China block. A northern Songpan-Ganzi system was sourced by erosion of the Qinling-Dabie orogen and the North China block throughout its deposition. These separate deposystems were later tectonically amalgamated to form one complex.
However, the background ages of the North China block, the South China block, and the Qinling Orogen that Weislogel et al.'s interpretations were based on are incomplete, which may have somewhat undermined their conclusions about the accumulation of the Songpan-Ganzi turbidites and the tectonic evolution of the basin. Clearly, the two oldest populations, 2.4–2.5 Ga and 1.85–1.95 Ga, exist not only in the North China block (Weislogel et al., 2006, and references therein) but also in the South China block. For example, Qiu et al. (2000) reported >3.2 Ga and ~1.9 Ga zircons in the Kongling metamorphics in the northernmost South China margin near the Qinling Mountains. Importantly, there are two clusters of ~1.9 Ga and 2.4–2.5 Ga U-Pb SHRIMP-dated zircons in the Kangding Metamorphic Complex, which is located to the east of the Songpan-Ganzi basin (Chen et al., 2005). Even in the southern margin of the North China block, there are ~820 Ma zircon ages (Li et al., 2005), and in the North Qinling orogen there is a cluster of ~950 Ma zircons (e.g., Chen et al., 2004; Wang et al., 2005). Therefore, these zircon background ages should be included in the database when modeling the accumulation of the Song-pan-Ganzi turbidites and the tectonic evolution of the basin.
This research was supported by the Hundred Talents Project of the Chinese Academy of Science and the Natural Science Foundation of China (grant 40572173). We are grateful to Tai-Ping Zhao for helpful discussion, and Bradley Ritts for constructive and thoughtful review comments, which significantly improved the manuscript.