Granitization and High-Temperature Metasomatism in Mafic Rocks: Comparison between Experimental Data and Natural Observations

Abstract

The paper reports newly obtained data that append older results of experimental modeling of granitization processes. The experiments were aimed at modeling high-temperature metasomatism of mafic rocks, a process that involves the transfer of major components at 750°C and 500 MPa at a pressure gradient. The source of the transported Si, Ca, and Mg in the experiments was garnet. The solution was pure H2O and 25 wt % NaCl aqueous solution. In the experiments, garnet was decomposed into pyroxenes, amphiboles, plagioclase, and minor amounts of melt, ilmenite, and iron oxides. The associated partial dissolution led to the transfer and redeposition of the dissolved components on the surface of a gabbroanorthosite underlay and to the development of mineral rims, which were analogous to those produced at garnet decomposition. The compositions of the newly formed minerals in the rims were identical to those produced at metamorphism of gabbroanorthosite at Т ≥ 750°C, P > 700 MPa. When the mineral rim was formed, some elements are removed, and this process was controlled by the composition of the fluid phase. The pure H2O fluid removed Fe, Ca, and Mg. The aqueous fluid containing NaCl (XNaCl ≈ 0.1) did not extract Ca from minerals. This indicates that no high NaCl concentrations are typical of fluid in processes that form basificates at granitization. The experiments have shown that H2O and H2O–NaCl fluids remove more Fe that other elements. Preferable Fe extraction from naturally occurring associations is evident from the elevated Fe mole fractions of the mafic minerals and from the fact that the basificates typically contain magnetite and hematite.