Dr E. J. Cobbing commented that the Thai Malaysia belt of per-aluminous tin granites presented a total contrast with the calc-alkaline granitoids of the Andes. Mr Beckinsale considered that a plate tectonic mechanism of continental collision could have provided the necessary conditions for granites of this type. It was observed that in areas of known continental collision such as the Alps and the Himalayas, the volume of associated granitoids is actually very small, and only in the Himalayas are they of per-aluminous character. In contrast, the granites in the Malaysia Thai tin belt and also in the European Hercynides are overwhelmingly abundant, and this fact alone would appear to suggest a different tectonic situation. In the case of the Hercynides the geological features differ in practically every respect to those of the Alpine belt to the S. It was urged that a reinterpretation of the data provided by these areas but divorced from the inhibitions of the plate tectonic model could be of value in reaching an understanding of the tectonic regime which favoured the generation of large volumes of per-aluminous granites.

Dr M. F. Ridd recalled that since radiometric ages began to be obtained from SE Asia, geologists there seemed to be beguiled by them, often accepting them in the face of contradictory field evidence. It is therefore gratifying to see that Mr Beckinsale and his co-workers are now drawing attention to the young K-Ar ages as indicating not the time of intrusion but a subsequent post-intrusion event.


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