Timing of granite emplacement and cooling in the Songpan-Garze Fold Belt (eastern Tibetan Plateau) with tectonic implications

摘要

New U-Pb and Rb-Sr geochronology on syn- and post-orogenic granites provide constraints on the timing of major tectonic events in the Songpan-Garze fold belt, west Sichuan, China. The Ma Nai granite was probably syn-kinematic with the main deformation and yields an age of 197 +- 6 Ma that is interpreted as an upper age limit of the Indosinian event. Zircons and apatites from the post-kinematic Rilonguan granite also yield Jurassic ages (195 +- 6 and 181 +- 4 Ma). The post-orogenic Markam massif gives two ages of 188 +- 1 and 153 +- 3 Ma. Both granites are undeformed and cut structures in the Triassic sedimentary rocks. These results demonstrate that the major deformation and decollement tectonics in the Songpan-Garze fold belt occurred prior to the Early Jurassic. The wide range of ages obtained for post-kinematic granites (from Early Jurassic to Late Jurassic) suggests that, locally, magmatic activity persisted for a long time (at least 50 Ma) after the Indosinian compressional tectonism. No Tertiary ages have been obtained, suggesting that these granites were not affected strongly by the India-Asia collision.