U-Pb dating on single detrital zircon grains from the Triassic Songpan-Ganze flysch (Central China): provenance and tectonic correlations

The Songpan-Ganze flysch belt (Central China) covers a huge triangular area of more than 200,000 km(2) and is bounded by the continental blocks of South Chins North China and the Tibetan plateau. Detrital zircons extracted from three flysch samples collected in the central part of the belt were analyzed grain by grain using the U-Pb method. Two samples of Middle Triassic sandstones, collected at different locations in the belt, provide identical results, which suggests similar source regions, The detrital zircons yield a wide range of ages and indicate their principal derivation from Mid-Proterozoic (1.8-2.0 Ga) source rocks with minor contribution from late Archean (ca. 2.5-2.6 Ga) material. The discordance and Pb loss patterns from low-U zircons indicate disturbances during a subsequent event which may be of Caledonian age, as suggested by concordant zircon grains at ca, 420 and 450 Ma. One sample collected within the Palang Shan Pass zone provides concordant zircon grains at around 230 Ma (231 +/- 1 Ma and 233 +/- 1 Ma). These Triassic ages are synchronous to flysch deposition and suggest intense geological activity (calc-alkaline volcanism?) at that time in the area close to the basin. The data support an origin of the elastic material mainly from a northeastern landmass, corresponding to the southern margin of the Sino-Korean craton, To a lesser degree, inputs from the Yangtze craton and possibly from the northern margin of the basin (Kunlun are) are also detected. The age spectrum from the Upper Triassic sandstone is significantly different and shows predominance of Sinian (ca. 760 Ma) grains, probably derived from the Yangtze craton. This change in the source region is interpreted as reflecting the tectonic evolution of this area and in particular as being linked to the late Triassic collision between South China and North China. In the Middle Triassic, while subduction of the Songpan sea northward beneath the North China plate was still taking place, continental subduction of South China in the Dabie region was responsible for uplift of the overriding plate (i.e. the Sino-Korean craton) which supplied large volumes of sediments. During the Late Triassic, clockwise rotation of the South China block uplifted the Indo-Sinian part of the Qinling belt and closed the basin. As the accretionary wedge was thickening along the southern margin of North China, detritus derived from this source region were unable to reach the flysch basin. The age spectrum presented by detrital zircons indicates predominance of Sinian material derived from source area located on the northern margin of the Yangtze craton; a source region which was until this period swamped by Luliang material from the Sino-Korean craton. (C) 1997 Elsevier Science B.V.