Metal fractionation by arc magma differentiation in the continental crust

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Mafic magmas at subduction settings stagnate at different levels during the ascent through the continental crust and the fractional crystallization, assimilation and mixing processes affect the composition of the magmas. The variation in density of the mafic to intermediate magmas probably cause the layering of the crust because mafic magmas typically stagnate in the lower crust. Sulphide saturation in the mafic magmas probably causes a depletion of chalcophile elements like Cu and Au in the upper continental crust. We suggest to study magmatic rocks and sulphides from two mafic-ultramafic intrusions in the Svecofennian Vammala Ni-Cu Belt of S Finland in order to define the formation of the cumulates from Proterozoic arc magmas and the associated sulphide deposits. The results will provide important insights into fractional crystallization and crustal assimilation processes related to the formation and segregation of sulphides and the fractionation of chalcophile elements. Because the Proterozoic oceans probably were anoxic and sulphidic, subduction-related magmas at that time may have been more reduced than those in the Phanerozoic. We will study the mineral compositions in the Vammala rocks and the potential effects of more reduced arc melts on magma evolution and transport of chalcophile elements by comparing the Vammala rocks to those from intrusions in young arcs.