Neoarchean leucogranitoids of the Kianta Complex, Karelian Province, Finland: Source characteristics and processes responsible for the observed heterogeneity

Abstract

In this study we describe the geochemistry and discuss the genesis of Neoarchean (ca. 2.7Ga) leucogranitoids of the Kianta Complex within the Karelian Province of the Fennoscandian Shield. The leucogranitoids occur extensively as dykes and intrusions of varying size and represent substantial recycling of older crust, occurring as the last magmatic event before final cratonization of the Karelian Province. Geochemically the leucogranitoids resemble the leucosomes of migmatized TTGs. Both groups are compositionally heterogenous, but are characterized by high silica contents (mean SiO2=73.4wt.%) and leucocratic appearance, reflecting a paucity of mafic minerals (mean MgO=0.37wt.%, Mg#=32.6). The majority of samples analyzed are weakly peraluminous (mean A/CNK=1.05). Mineralogically the leucogranitoids form a series grading from trondhjemite to leucogranite, which is observable as highly variable CaO (0.07–3.84wt.%, mean=1.47), K2O (0.30–8.76wt.%, mean=3.53) and Na2O (2.22–7.23wt.%, mean=4.49). We consider the local TTGs sensu lato which comprise both true TTGs (K2O/Na2O<0.5) and transitional TTGs (K2O/Na2O>0.5), to be the predominant source material for the leucogranitoids. However, some samples overlap compositionally with local TTGs and could represent either partial melting of garnet amphibolites or alternatively products of nearly complete fusion of TTGs. Leucogranitoids and transitional TTGs can be differentiated to some extent on the basis of geochemistry, but definitive discrimination requires demonstration of field relationships and/or isotope dating. The observed compositional scatter is interpreted to be a result of source heterogeneity, retention of variable accessory minerals and accumulation processes. Melting is attributed to thickening of the crust in response to collisional tectonics and occurred dominantly under water fluxed conditions. Possible fluid sources are the sanukitoid and quartz diorite suites plus dehydration reactions and partial melting of the lower crust.