The Veikkola granite area in southern Finland: Emplacement of a 1.83-1.82 Ga plutonic sequence in an extensional regime

Abstract

Four granite intrusions and coeval small enderbite intrusions, belonging to the lateorogenic (1850 - 1820 Ma) magmatism of the Svecofennian orogeny, are described in the Veikkola area of southern Finland. The granites are leucocratic, heterogeneous and characterized by abundant garnet. The oldest granite is layered whereas the younger ones are more homogeneous. Geochemically the granites are peraluminous and have high contents of SiO2 (71 - 76 wt.%). Their major and minor element composition varies significantly but the REE patterns are fairly similar, Garnet mainly crystallized from melt but some garnet in the granites may be restitic. Garnet composition is relatively constant implying slow equilibration with the melt. In addition to garnet-bearing felsic layers in the oldest granite, the granites are crosscut by garnet-bearing dikes and veins. The layers, dikes and veins are higher in REE and Fe/Mg than the host granites and are considered partial melts from the granites. The abundant garnet in the felsic layers, dikes and veins crystallized from melt. Thermobarometry calculations indicate 3.5 - 5 kbar and ∼650-700°C estimates for crystallization of the felsic layers in the oldest granite whereas the host granite yields ∼1.5 kbar higher pressure. The granites were probably derived by dehydration melting of layered, metagreywacke-dominated rocks. Melting was fluid-absent both during generation of the granites and during formation of the felsic layers, dikes and veins. The oldest granite (1850 Ma) was emplaced at mid-crustal depth before extensional deformation. Layering in the oldest granite is regarded as the result of deformation-induced remelting of the host granite at a higher crustal level, during protracted extension and progressive regional metamorphism. The other three granites and the enderbite were emplaced at a late stage of extension, at or close to the peak of metamorphism (1.83-1.82 Ga).