Genetic relation of charnockite to fluids derived from high-grade rocks in the Limpopo Belt

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On the basis of mineralogic, petrologic and chemical features of various high-grade metamorphic rocks in the Limpopo Belt, southern Africa, it is concluded that charnockite formation took place at P_<solid> (=P_s) < P_<total fluid> (=P_f) interacting with fluids derived from mainly sheered quartzofeldspathic gneisses in the process of crustal uplift or during retrograde metamorphism. Accordingly, in the case of charnockitization, source of CO_2 not necessarily originate in either mantle or underlain greenstone belt to cause decomposition of biotite into orthopyroxene. This report also thermodynamically examined how the oxygen fugacity and X (CO_2) vary in this process in terms of dehydration reactions observed in the rocks. In addition to this results, microthermometric data (homonization temperature, freezing temperature, and final melting temperature) of fluid inclusions in both charnockite and quartzofeldspathic gneiss represent that most of them are H_2O-rich in composition. On the … More other hand, in order to examine behavior of the fluids in the fracture zones, thermodynamic treatments for phase analysis and petrogenesis were summarized under conditions of P_s>P_f. The equilibria at P_s>P_f were applied to charnockitization of quartzofeldspathic gneisses and formation of the orthoamphibole isograd in the Limpopo belt. The fluids responsible for both charnockitization and formation of the orthoamphibole isograd are considered to have a wide range of composition and rather H_2O-rich at P_f<P_s, although they are CO_2-rich (X (H_2O) <0.2) at P_s=P_f. The calculation results at P_f<P_s may account various compositions of fluid inclusions in quartz in the charnockites. In the fracture system created during the crustal uplift, it is proposed that the "fluid path" with repetitive cycles of decreasing and increasing P_f has playd an important role for stabilization of orthopyroxenc. Generally migration (infiltration or influx) of fluids in the fracture zones gives rise to not only important dehydration and hydration reactions, but also may be related to formation of progressive, regionally metamorphosed rocks in the orogenic belt. Less