METASOMATIC ORIGIN FOR PALEOPROTEROZOIC ALKALI-FELDSPAR SYENITES, SUOMENNIEMI RAPAKIVI GRANITE COMPLEX, SE FINLAND

The 1640-Ma Suomenniemi rapakivi granite complex consists of metaluminous granites that host numerous dike-like bodies of mildly peralkaline alkali-feldspar syenite. These syenitic rocks consist mainly of hypersolvus or subsolvus alkali feldspar, coarse grains of perthite, and aegirine-augite. Euhedral zircon and apatite, anhedral to subhedral quartz, amphibole, titanite, epidote, melanite garnet, and magnetite are accessory. The alkali-feldspar syenites and granites of the complex have indistinguishable in situ zircon ²⁰⁷Pb/²⁰⁶Pb ages (ca. 1640 Ma), initial whole-rock εNd values (ca. -1.5 at 1640 Ma), and zircon δ¹⁸O_VSMOW values (ca. +8 ‰). The alkali-feldspar syenites are enriched in Na₂O, Fe₂O₃ and Al₂O₃ and depleted in SiO₂, FeO and F relative to the granites. Limited redistribution of Rb, Sr, Ba, Nb, Zr, the REE, and Y is also evident. These observations and petrographic features suggest that the alkali-feldspar syenites formed via pervasive alkali metasomatism and dequartzification of the subalkaline granites. Brecciation, limited mobilization, and near-complete overprinting of the original mineralogy by an oxidized, sodic assemblage attest to the metasomatic origin of these rocks. The process mirrors contact fenitization around alkaline intrusions and may imply a hidden alkaline body at depth. In anorogenic (A-type) granite complexes, aluminosity and modal mineralogy of granitic and syenitic rocks may be modified by post-magmatic processes. Formation of fenitic dikes probably requires focused (e.g. fault-controlled) flow of high-temperature quartz-undersaturated alkaline fluids. Less oxidized peralkaline overprint, devoid of desilication, may form in more confined systems such as granite cupolas.