1.88–1.87 Ga post-kinematic intrusions of the Central Finland Granitoid Complex: a shift from C-type to A-type magmatism during lithospheric convergence

Abstract

The evolution of the Svecofennian Orogen spans from 1.95 Ga to 1.80 Ga with the main crust-forming activity having taken place 1.90–1.87 Ga ago. Subsequent deformation was largely concentrated to southern Finland where transpressional structures indicate lithospheric convergence until 1.80 Ga. Much of the Svecofennian Orogen in Finland is occupied by the Central Finland Granitoid Complex (CFGC) that mainly consists of I-type granodiorite and granite. These 1.89–1.88 Ga intrusive rocks are typically foliated and considered synkinematic. A distinct suite of post-kinematic, undeformed or slightly foliated plutons crosscut the synkinematic rocks. They are predominantly quartz monzonites and monzogranites, and have been divided into three types according to their petrographic, mineral, and chemical characteristics. Compared to the calc-alkaline synkinematic granitoids, the post-kinematic suite generally has an alkaline affinity, with higher Fe, Ti, K, Ba, Zr and Nb and lower Mg, Ca and Sr at a given SiO 2 content. The Type 1 post-kinematic plutons are peraluminous and contained a prominent sedimentary component in their source. The Types 2 and 3 plutons are marginally metaluminous to peraluminous. The Type 2 granites, especially those in the western CFGC, approach the classic 1.65–1.54 Ga rapakivi granites of southern Finland in their petrographic characteristics, magmatic association (tholeiitic mafic rocks) and elevated contents of incompatible elements, and hence have A-type characteristics. The Type 3 plutons contain a pyroxene-bearing margin (Type 3a) or contain pyroxene throughout (Type 3b). The Type 3b rocks have C-type (Charnockite magma type) affinities, and some plutons of this subgroup are alkaline. The Type 3a plutons appear to be transitional between the Type 2 and Type 3b plutons. The synkinematic magmatic episode in the CFGC area probably involved partial melting of intermediate high-K rocks in the lower crust, with a magmatic addition from a mafic underplate. This magmatic episode left a hornblende- and biotite-poor granulitic residue in the lower crust. Heat from the mafic underplate triggered partial melting of the granulite to produce the post-kinematic magmatism, and also introduced mafic material into the post-kinematic magmas. The post-kinematic magmatism took place over quite a short time period (ca. 15 Ma) during overall lithospheric convergence. It extended beyond the CFGC area and registers a temporal shift from the northeast toward the west and a general change in the character of magmatism from C-type to A-type. The post-kinematic magmatism probably resulted from extensional or transtensional events modifying the tectonically thickened crust. Partial melting of the mafic lower crust produced C-type magmatism soon after the first melting episode. The crust was already rather rigid and allowed the mafic magmas to follow a tholeiitic trend when the bimodal A-type magmas were formed in the western CFGC.