Age of Palaeozoic granites and metamorphism in the Tuvino-Mongolian Massifof the Central Asian Mobile Belt: loss of a Precambrian microcontinent

The Tuvino-Mongolian Massif (TMM) was previously interpreted as a Precambri an block within the Central Asian Mobile Belt. According to this idea, it c onsists of tectonic slices composed of metamorphic rocks of pre-Mesoprotero zoic basement that experienced two episodes of regional metamorphism, and M esoproterozoic 'cover rocks' that were reworked together with the basement during high-grade metamorphism. Zircon U-Pb dating of granitoids from all m etamorphic complexes demonstrates that the earliest metamorphic event occur red at 536 +/-6 Ma, significantly later than the deposition of the cover ro cks. Regional upper amphibolite-facies metamorphism, which affected all met asedimentary units of the TMM, occurred in the short time interval of 497 /-4 to 489 +/-3 Ma. We propose that there is no simple basement-cover relat ionship in the Tuvino-Mongolian Massif. Instead, the massif consists of tec tonic domains, composed of heterogeneous metasedimentary successions with d istinct pre-metamorphic and pre-tectonic histories that were juxtaposed by thrusting prior to 497 Ma and then metamorphosed under upper amphibolite fa cies conditions. We suggest that this thrusting event was associated with e arly Palaeozoic collisional processes related to amalgamation of the Centra l Asian Mobile Belt. Our new model for the evolution of the TMM implies der ivation of Neoproterozoic terrigeneous metasediments from similar to 800-90 0 Ma granitoids formed in an Andean-type active continental margin setting. (C) 2001 Elsevier Science B.V. All rights reserved.