GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 19-8
Presentation Time: 10:30 AM


SUIKKANEN, Einari and RÄMÖ, O. Tapani, Department of Geosciences and Geography, P.O. Box 64, University of Helsinki, 00014, 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 δ18OVSMOW 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.