Late Triassic andesitic accrtionary arc in the central Songpan-Ganzi terrane and its tectonic significance

Detailed observations presented in this paper show that the andesite blocks in the Upper Triassic turbidites in the central Bayan Har-Songpan Ganzi terrane (Songpan-Ganzi terrane in short) in the northern Tibet Plateau were the components of an accretionary arc based on the accretionary wedge. The andesites are highly inhomogeneous in the phenocryst and matrix textures, geochemical and isotopic compositions (e.g. A/CNK=0.62 similar to 1.15, s=1.99 similar to 2.35, Mg#=38 similar to 46, epsilon(Nd)(t)=-6.32 similar to-2.98), which indicates that magma of the accretionary arc was derived from various sources. High affinities of Sr and Nd isotopes with Triassic submarine turbidites suggest that andesitic magma of the accretionary arc was predominantly derived from melting of the turbidite matrix of the accretionary wedge, and somewhat contaminated by the ultramifc/mafic components form the oceanic lithosphere. Zircon U-Pb analyses for the andesites, plus syschronous S-type granites (228 +/- 2Ma similar to 204 +/- 7Ma), indicate that the accretionary arc was occurred in the Norian of Late Triassic (226.8 +/- 1.9Ma similar to 213.7 +/- 0.9Ma). Previous working showed that the northestern part of the Songpan-Ganzi terrane was actually a peripheral foreland basin. Consequently, the Songpan-Ganzi terrane was one integrated by at least two tectosomes, evolved paleo-Tethys ocean and a peripheral foreland basin. The former is the main part of the Songpan-Ganzi terrane and composed of subduction-related accretionary complexes followed with the rapid shortening of the Songpan-Ganzi paleo-Tethys ocean in the Triassic, on which developed the andesitic accretioanry arc in the Norian. The latter was constructed by collision between the West Qingling arc (248 similar to 234Ma) and the Yangtze block in the Ladinian to Norian.