Petrological Study on Continental Lower Crustal Materials.

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Aims of this study are to understand petrological and chemical features of lower parts of continental crust and to evaluate chemical changes in the lower crust associated with granulite facies metamorphism and partial melting processes. This study particularly focuses on the processes related to formation of granulite facies metasedimentary rocks under lower crustal conditions. The main results of this study can be summarized as follows ;1. Metamorphic conditions, age, spatial distribution are determined for granulite facies metasedimentary rocks of lower crustal origin occurring in Labrador, Canada. Peak metamorphic condition of 1000^ﾟC and 10 kbar are estimated on the basis of mineral parageneses of the rocks. Osumilite, sapphirine, and other metamorphic minerals were formed in metasedimentary rocks by contact metamorphism associated with an emplacement of anorthositic magma under lower crust conditions. This is fifth metamorphic osumilite occurrence ever reported. Geochemical characteristics of the metasedimentary rocks suggest that partial melting and consequent melt segragation are important processes resulting in chemical modification of the lower crustal matireals.2. High pressure and temperature experiments were done to determine the stability relationship of osumilite in pelitic compositions under lower crustal conditions. The results suggest that osumilite is stable lower crustal phase as high as 12 kbar at 1000^ﾟC.3. This study clearly demonstrates that granulite facies metamorphism and crustal anatexis are closely related processes. Extremely low water activity in granulites would be caused by partial melting and melt segregation processes. It is most likely that parts of the continental lower crust is restite origin which is probably related to granitic magma generation.