Granitoid episodes and mafic-felsic magma interaction in the Svecofennian of the Fennoscandian Shield, with main emphasis on the ∼1.8 Ga plutonics

Abstract

Three major episodes of felsic plutonism can be identified in the Svecofennian of the Fennoscandian Shield. Each of these episodes is associated with coeval (but not comagmatic) mafic mantle-derived plutonism. The first episode comprises the early Svecofennian (∼1.90-1.86 Ga) granitoids, which formed the first continental crust, together with associated supracrustals, in most of the Svecofennian by a separation from the mantle at that time. They are dominated by tonalitic-granodioritic compositions and contain frequent examples of mafic-felsic magmatic interaction. The second episode (∼1.83-1.77 Ga), which is of main concern here, includes the Transscandinavian Igneous Belt (TIB), a huge N-S trending batholith of granitoids, mafics and some associated volcanics, separating the Svecofennian from the Southwest Scandinavian Gneiss Domain, and the smaller massifs and plutons of the Within Svecofennian Episode 2 (WSE2) granites. The TIB contains several examples of extensive homogenized hybrids of mafic-felsic magma-mixing, related to large aeromagnetic anomalies. The hybrids generally grade into the coarse porphyritic TIB-granites, and have intermediate mineralogy and chemistry. The geochemistry of the hybrid rocks is consistent with mixing trends in the range between ≈55% SiO 2 and 65–70% SiO 2 , which represent the approximate end members. Mixing and mingling on a local scale is also commonly observed. The WSE2-granites have a more limited felsic composition, higher contents of incompatible trace elements, and usually lack evidence of mafic-felsic interaction, consistent with a generation by a lower degree of partial melting of the same composite, early Svecofennian crust compared to the TIB-granites. The third episode corresponds to the rapakivi granites (∼1.65-1.56 Ga). These are intimately associated with coeval mafic magmatism. Interaction between magmas has been identified in composite dikes, but is also proposed to have formed the dark, olivine- and pyroxene-bearing, more mafic rapakivi varieties, which are very similar to the homogenized hybrids of the TIB.