Metal fractionation by arc magma differentiation in the continental crust

俯冲背景下的镁铁质岩浆在穿过大陆地壳上升过程中不同程度地停滞，分离结晶、同化和混合过程影响岩浆的成分。镁铁质岩浆到中质岩浆密度的变化可能导致地壳分层，因为镁铁质岩浆通常停滞在下地壳中。镁铁质岩浆中硫化物的饱和可能导致上大陆地壳中亲铜、亲金等元素的枯竭。我们建议对芬兰S镍铜带两个镁铁质-超镁铁质侵入体中的岩浆岩和硫化物进行研究，以确定元古代弧状岩浆和伴生硫化物矿床的堆积物的形成。这些结果将对与硫化物的形成和分离以及亲硫元素的分馏有关的分离结晶和地壳同化过程提供重要的见解。由于元古界海洋可能是缺氧的和硫酸盐的，当时与俯冲有关的岩浆可能比显生宙的岩浆减少得更多。我们将通过将Vammara岩与年轻弧中的侵入岩进行比较，研究Vammara岩中的矿物组成以及更多还原弧熔体对岩浆演化和亲硫元素迁移的潜在影响。

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.