Abstract:
Enormous outpourings of granite magma and associated silicic volcanism were very important attributes of folded regions at the times when they were tectonically active. The silicic magmas melted out in these stages are highly alkalic and therefore resulted in specific granite associations. Petrogenetic analysis of such belts and the results of experiments on the melting of rocks and synthetic mixtures have led to the almost unanimous conclusion that the initial magmas were generated by melting of a sialic substrate in the crust. Quantitative calculation of the pressures at which magmas are generated is very difficult due to the limited availability of geobarometer minerals in silicic rocks. However, in aluminous granites and rhyolites from active zones one encounters a pyrope-enriched liquidus granite that crystallized at at depths greater than the normal thickness of the crust. Recently it has been suggested that such magma, which yields mainly A-type granite, was generated in the lower part of an abnormally thick crust. It is obvious that when such zones of much thicker crust become activated, their bottom layers find themselves under PT conditions equivalent to those in the hot mantle. What melts then is a crustal substrate that has already been largely dried out and depleted in the sialic component. One piece of direct evidence of high pressure and high-temperature melting of crustal material at great depth is the finding of metamorphic blocks with osumilite-sapphirine-quartz granulite.