LATE TRIASSIC MELANGE SERIES IN WESTERN SICHUAN

Melanges are widespread in western Sichuan and may be found along the Jinshajiang fracture zone,the Delei-Dingqu fracture zone,the Dege-xiangcheng fracture zone, the Garze-Litang fracture zone, the Xianshuihe fracture zone and the Minjiang fracture zone as well as the peripheries of the Zoige median massif.As fossils of various ages occur in them and different scholars assign them into different ages, confusion has been brought about in the stratigraphic division and structural analysis.After an overview of relevant data, the authors have found that the mélanges are a multi-constituent fragment and block accumulation,which makes up a complete series and,like enlargea breccia,consists of "matrix"and"angular fragments".What is different from breccia is that their matrix embraces a stratigraphic unit formed in a rather long geological period of time.Their time interval may be an age or epoch.The angular fragments are composed of exotic masses ot different ages and lithologies and highly varied sizes,some of which may be of very large size.Their accumulation is distributed in a particular horizon and may extend as far as several hundreds of kilometers.The age of the mélange series can be only determined by the fossils in the matrix.The matrix of the melange series in western sichuan is mainly derived from the Xikang or Yidun Group and is Late Triassic in age.The series is of tectonic-sedimentary origin.It is of special singificance for indicating the facies occurring during the period of transformation from the stable to active or from the active to more active in the geological development.