松潘甘孜地块与秦岭褶皱带、扬子地台的关系及其发展史

THE SONGPAN-GARZE MASSIF: ITS RELATIONSHIP WITH THE QINLING FOLD BELT AND YANGTZE PLATFORM AND DEVELOPMENT

松潘甘孜地块存在与否是个有争议的问题。本文论证了地块的古老基底为前震旦纪花岗质岩系。地块与南秦岭褶皱带可能都是古扬子地台的一部分。由于早古生代末的加里东运动造成的裂陷,使地块从扬子地台的西北部解体出来,但这次解体并不彻底,裂陷槽后来逐渐被弥合填平。茅口期开始的峨眉地裂运动使地块再次从扬子地台解体出来,裂陷线大致沿木里、平武一线。这次裂陷是扬子地台周缘及内部同期张裂的一部分,反映扬子地台迅速北移造成的后缘、侧缘及内部的拉张作用,使地块连同南秦岭地区一起与邻区割裂开来。拉丁期时,松潘甘孜地块与南秦岭区整体强烈下陷,直至晚三叠世末晚印支运动将其海槽关闭,全部褶皱隆起成山。

The validity of the Songpan-Garze Massif, a long-term issue of argument, is now strongly supported by discoveries of pre-Sinian granitic and volcanic pebbles and clasts derived from the old massif and redeposited in conglomerates and elastics of the Sinian Baiyigou Group, the Silurian Bailongjiang Group and the Early Carboniferous Zasilang Formation, all distributed along its northern margin, as well as in the Sinian conglomerates of the Upper Bikou Group on the northeast side of the massif. This demonstrates that the massif has a pre-Sinian granitic basement. The Songpan-Garze Massif and South Qinling may both be parts of the old Yangtze Platform. Due to the Caledonian rifting in the late Early Palaeozoic, the massif was disintegrated from the northwest Yangtze Platform. This disintegration, however, did not lead to thorough separation, and the rift trough was later gradually closed and filled up. From Carboniferous to Anisian a carbonate shallow sea covered the Songpan-Garze Massif, South Qinling fold belt and Yangtze Platform, yielding uniform faunas in all the three regions. On the other hand, beginning from the Maokouian of the Early Permian, the Emei taphrogeny invoked a second disintegration of this massif from the Yangtze Platform. The rift goes approximately along the Muli-Pingwu line, and has a branch along the Upper Mingjiang River in its. northern section. The rifting weakens northwards. Roughly synchronous rifting occurred in an east-west direction along the Lingtan-Sanyang Fault-the demarcation line between South and Central Qinling, weakening eastwards. The peripheral rifting of the Yangtze Platform, including the above-mentioned two, and synchronous intrac- ratonic extensional rifting within the platform reflected the posterior, lateral and interior tensional taphrogeny caused by rapid northward shift of the Yangtze Platform as depicted by palaeomagnetic data, and led to the dismembering of this massif and the South Qinling fold belt from adjacent areas. During the Ladinian, North China and the Qilian Mts. were elevated, Central and North Qinling folded up to form mountains, and the Yangtze Plat form was also uplifted and became land; while the Songpan-Garze Massif and South Qinling experienced strong subsidence. The geosyncline was finally folded up during the Late Indosinian Orogeny.