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 blo cks of South Chins North China and the Tibetan plateau. Detrital zirco ns extracted from three flysch samples collected in the central part o f the belt were analyzed grain by grain using the U-Pb method. Two sam ples of Middle Triassic sandstones, collected at different locations i n the belt, provide identical results, which suggests similar source r egions, The detrital zircons yield a wide range of ages and indicate t heir principal derivation from Mid-Proterozoic (1.8-2.0 Ga) source roc ks with minor contribution from late Archean (ca. 2.5-2.6 Ga) material . The discordance and Pb loss patterns from low-U zircons indicate dis turbances during a subsequent event which may be of Caledonian age, as suggested by concordant zircon grains at ca, 420 and 450 Ma. One samp le collected within the Palang Shan Pass zone provides concordant zirc on grains at around 230 Ma (231 +/- 1 Ma and 233 +/- 1 Ma). These Tria ssic ages are synchronous to flysch deposition and suggest intense geo logical activity (calc-alkaline volcanism?) at that time in the area c lose to the basin. The data support an origin of the elastic material mainly from a northeastern landmass, corresponding to the southern mar gin of the Sino-Korean craton, To a lesser degree, inputs from the Yan gtze craton and possibly from the northern margin of the basin (Kunlun are) are also detected. The age spectrum from the Upper Triassic sand stone 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 evolut ion of this area and in particular as being linked to the late Triassi c collision between South China and North China. In the Middle Triassi c, while subduction of the Songpan sea northward beneath the North Chi na plate was still taking place, continental subduction of South China in the Dabie region was responsible for uplift of the overriding plat e (i.e. the Sino-Korean craton) which supplied large volumes of sedime nts. During the Late Triassic, clockwise rotation of the South China b lock uplifted the Indo-Sinian part of the Qinling belt and closed the basin. As the accretionary wedge was thickening along the southern mar gin of North China, detritus derived from this source region were unab le to reach the flysch basin. The age spectrum presented by detrital z ircons indicates predominance of Sinian material derived from source a rea located on the northern margin of the Yangtze craton; a source reg ion which was until this period swamped by Luliang material from the S ino-Korean craton. (C) 1997 Elsevier Science B.V.