EUGEOSYNCLINE ROCK ASSOCIATION OF YANBIAN GROUP IN WESTERN SICHUAN, CHINA
川西盐边群的优地槽岩石组合

In Yanbian Group of Late Proterozoic in Western Sichuan Province, there is a well preserved eugeosyncline rock association. The association is composed of ophiolite, preflysch and flysch series and overlain disconformably by molass of Lieguliu Formation, Sinian Series.In the lower part of Yanbian Group, there is a well preserved ophiolite sequence. It consists successively of cumulate complex, sheet dyke swarm and pillow lava intercalating chert. Metamorphic peridotite representig mantle rock exposes at Dacao being 20 km east of Yanbian and in Shimian.The cumulate complex is mainly composed of harzburgite, Iherzolite, plagioclase-bearing peridotite, troctolite, anorthosite, norite and gabbro. These rock types expose as concentric circle-like shape which has an ultramafic core and transforms gradually to mafic towards peripheral rim. The successive sequence implies the continuous crystalline fractionation. The petrochemistry and REE patterns also suggest such a succession.The earlier diabase dykes interpenetrate the layer gabbro in the top of cumulate. The dykes intersect obliquely the gabbro layers other than normally, implying that the layer gabbro had been undergone a folding deformation prior the emplacement of dyke swarm.Basalt in the base part of lava series exhibits massive structure with slight schistization. The middle and upper parts of lava series show pillow structure. The basalts have subjected spilitization, but its Al2O3/TiO2, CaO/TiO2, Y and Zr, and REE patterns are still comparable with those of modern ocean ridge, Troodos and Island Bay ophiolites. Most of interbedded chert have been recrystallized and its original textures have been modified. Their chemical compositions are different from those of Phanerozoic cherts, indicating the lower SiO2 and higher A12O3 and FeO contents. This maybe resulted from stronger submarine volcanism and hydrodynamics.Pref lysch consists of carbonaceous sillic and pelagic carbonate rocks. It can be suggested that the preflysch deposited at ocean floor, of which the depth may be sometimes less than carbonate compensation level.The flysch series in Yanbian Group has a thickness more than 5000 m which consists mainly of the turbidite with Bouma sequence. In the upper part of turbidite section, there is an olistostrome statum, which may be deposited in submarine canyon of continental margin. The chemical compositions of the turbidite sandstones indicate that they are the sediments deposited in an eugeosyncline environment at an Andes-type continental margin.The reconstruction of this eugeosyncline association evolution suggests that it had undergone three tectonic stages. In stage 1, the ophiolite formed at mid-ocean ridge. In stage 2, the ophiolite cold-descends to ocean floor where the preflysch was deposited. Finally, in stage 3, the oceanic crust represented by ophiolite and preflysch transported into trench at the Andes-type continental margin where the flysch was deposited.The Rb-Sr isochron age d