Major-Element Geochemistry, Redox Systematics, and Petrogenesis of the Estonian Rapakivi Intrusions, Finnish Wiborg Batholith, and Onas intrusion: A Revised Analysis of Fennoscandian AMCG Magmatism

Abstract. Rapakivi granitoids are a distinctive expression of Proterozoic intracontinental magmatism and provide constraints on crust–mantle interaction, lithospheric reworking, and the thermal evolution of stabilised continental crust. In Fennoscandia, the Wiborg suite represents the earliest major rapakivi–AMCG (anorthosite–mangerite–charnockite–granite) magmatic episode, emplaced at ∼1.65–1.62 Ga following the Svecofennian orogeny and comprising the Wiborg Batholith and broadly coeval intrusions in southern Finland and Estonia. Here, we integrate 186 whole-rock analyses from this older suite, including the Wiborg Batholith and the Taebla, Ereda, Neeme, Onas, Märjamaa, Kloostri, and Naissaare granites, together with the younger Riga Batholith (∼1.59 Ga) as a temporal and geochemical comparator. Major-element and iron-speciation data, CIPW normative mineralogy, and thermobarometric proxies are used to test whether the older Fennoscandian rapakivi–AMCG suite records a common regional evolution or distinct magma-reservoir histories. The granitoids are predominantly high-K, ferroan, alkali-calcic to alkaline, and metaluminous to marginally peraluminous, but their major-element systematics define distinct intrusion- and phase-specific evolutionary trends rather than a uniform regional differentiation path. This compositional heterogeneity is also captured by centred log-ratio principal component analysis (CLR-PCA), in which the first two components account for 74.01% of the total variance. PC1 primarily contrasts relative enrichment in P₂O₅, TiO₂, and MgO with relative enrichment in SiO₂, K₂O, Na₂O, and Al₂O₃. Multiphase intrusions record internally systematic but contrasting evolutionary trajectories along these compositional gradients, including shifts towards more silicic and alkali-rich compositions and variable depletion of components associated with calcic plagioclase, Mg-bearing silicates, Fe–Ti oxides, and apatite. Hierarchical clustering similarly separates several evolved phases from their earlier counterparts, reinforcing the interpretation of phase-specific differentiation. These compositional differences are consistent with variable fractional crystallisation, locally modified by crystal accumulation, magma recharge, volatile evolution, and wall-rock assimilation. Ferroan indices and Mg# primarily record Fe–Mg fractionation, whereas Fe³⁺# reveals non-monotonic, reservoir-dependent redox evolution. The resulting discordance between major-element affinity, normative oxide proportions, and ferric-iron speciation indicates that ferroan character and whole-rock oxidation state evolved only partly in tandem. Temperature proxies indicate crystallisation over approximately 620–900 °C, while most normative pressure estimates remain below ∼5 kbar, consistent with predominantly shallow- to mid-crustal emplacement. Collectively, the results indicate that these intrusions represent related but independently evolving A-type magma reservoirs within the Fennoscandian rapakivi–AMCG system, whose contrasting compositions record local differentiation and redox histories superimposed on a shared post-orogenic tectono-magmatic framework.