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GEODYNAMICS AND ENERGY ASPECT OF FENNOSCANDINAVIAN SHIELD BIMODAL MAGMATISM Institute of Geology, Karelian Research Centre RAS, Petrozavodsk, Russia, sv@krc.karelia.ru
Bimodal mantle-crust magmatism of the Fennoscandinavian shield southern marginal part being typical for the Riphean is associated with geodynamics of Polkanov's marginal radial flexure (Fig. 1), along which a linear belt of mantle-crust polycyclic diapirs (Salmi, Vyborg, Aland) is developed. They are characterized by comparable succession of endogenic mantle-crust processes, but differed by their intensity and time of completion (Sviridenko, 2002). Two main types, i.e. the Early Proterozoic (Svecofennian) and Riphean represented by mantle and mantle-crust volcano-plutonism, including gabbro-anorthosite-rapakivi granite one. Dynamics of magmatism and regime of tectonic movements in the shield marginal flexure indicate duration of its functioning.
1 – border of platform cover; 2 – boundaries of the Caledonian covers; 3 – axial line of Polkanov's marginal radial flexure; 4 – FSS marginal limitations (Kolsko-Kaninskaya syncline in north-east and line – Tornquist-Teisser's line in south-west); 5 – axial line system of FSS standing waves of stresses; 6 – suture zones of geoblocks; 7 – geoblocks: I – Kolsko-Mezenskiy, II – Belomorskiy, III – Karelian, IV – Svecofennian, V – Dalslandskiy; 8 – intergeoblock faults; 9 – zones of shift dislocations; 10 – horst-graben systems: 1 – Viking, 2 – Central, 3 – Khorn, 4 – Oslo, 5 – Wettern, 6 – Finskiy bay, 7 – Ladoga, 8 – Kandalakshskaya, 9 – Onezhsko-Kuloiskaya; 11 – volcano-tectonic and tectono-magmatic structures: 10 – Siljan, 11 – Pechengskaya, 12 – Khibinsko-Lovozerskaya, 13 – Kontozerskaya caldera. 12 – arched dome and depression structures: I – Telemark, II – Bergslagen, III – Vyborg, IV – Ladoga, V – Onezhskaya. 13 – Norway deep water trench. Triangle denotes position of Valaam island in Ladoga lake.
Rather complete geochronologicaal knowledge of geological events succession that are associated with development of diapers allows to calculate time of some cycle completion. The end of Svecofennian cycle within the Salmi diapir is evaluated as 1856 Ma, that of Vyborg one – 1840-1820 Ma and Aland – 1810-1790 Ma. Local small depth granulite metamorphism occurred in central zone of all diapirs in the final period of this cycle. It was likely to contribute to warming-up the Earth's crust and its subsequent partial melting at development of bimodal gabbro-anorthosite-rapakivi granite magmatism. The time of rapakivi granite magmatism occurrence for Salmi diapir equals 1547-1530 Ma (Larin, 2008), Vyborg – 1667-1617 Ma and Aland – 1584-1556 Ma (Vaasjoki, 1977). The Riphean magmatism of the Salmi diaper completed by formation of Valaam sill (1457-1459 Ma) (Ramo et al, 2001), and that of Aland diapir - by formation of Middle Riphean olivine diabases near 1260 Ma. Four main features of polycyclic mantle diaper activity can be distinguished: 1) total structural and tectonic localization in marginal part of the Fennoscandinavian shield; 2) autonomousness of development in space and time; 3) cyclic recurrence of eruptive process dynamics; 4) local high-temperature and dehydrational melting in the above hearth zones. Gabbro-anorthosite-rapakivi granite volcano-plutonism is structurally closely associated with Polkanov's marginal radial flexure and horst-graben system being perpendicular to it. Igneous rocks of gabbro-anorthosite series formed as layered bodies in interblock spaces of flexure axial zone. Rapakivi granites in their turn have layered character of introductions and total blocking the enclosing rock structure. Geodynamics and energy aspect of Riphean bimodal magmatism of the Salmi polycyclic diaper can be considered on example of basic and acid magmatism ratio of the Salmi pluton and Valaam sill. The latter has a comparable composition of acid and basic rocks with the respective rocks of the Salmi massif. The main rocks of the Salmi massif and Valaam sill are characterized by higher contents of iron, titanium, potassium and phosphorus. High K and low Ca composition and higher fluidization are general peculiarity of granite composition in the Salmi massif and veined granites of the Valaam sill. Monzonites, quartz monzonites and syenites being associated with them are products of gabbro and fluidized granite melt interaction. The principal difference in occurrence of bimodal Early- and Middle Riphean magmatism of the Fennoscandinavian shield southern marginal part in Karelia is in correlation of mantle basite and crustal acid magmatism volumes that largely determines dynamics of these complexes formation. An alternation of granite and gabbroid layered bodies representing the many storey telescoped intrusions is observed by geophysical data in flat-dipping plutons that host rapakivi granites. The Valaam sill about 200 m thick occupies an area of about 16000 km2. In vertical section, the Valaam sill represents multistory, chamber-injection intrusion, where granites form thin layered bodies and also thin veins observed through the whole section of the sill and characterized by multiact introduction. Five times introduction of granite melt is substantiated for the Salmi massif. The duration of each introduction crystallization did not exceed 1-2 mln. years by A.M.Larin's estimation (Larin, 2008). Basalt melt is characterized by low viscosity that is explained by not high content of SiO2 and presence of Fe. Therefore, it rapidly passes into intermediate hearth of the lower crust on border with mantle, where crust partial melting occurs with formation of high potassium granite melt. It initiates the mechanism of mantle and crustal matter local movement and flow of fluid melt-solution. The study of the overhearth zones of mantle diapers allowed to establish that their development coincides with periods of asthenosphere excitation, mass formation of fluidized melts and intermediate and periphery hearths. The feeding magma conductive system reflects cellular organization of mantle playing a role of conductors (percolation) of fluidized mantle melts. The telescoped fluidodynamics determines stable continuous-discontinuous generation of melts and growth of diaper as the concentrated flow. Intensive monzonitization of the Salminskiy massif gabbro-anorthosites and Valaam sill gabbro-dolerites under effect of fluidized granite melt realized in temperature interval, beginning from the late magmatic stage up to hydrothermal (900-300oC). It serves an evidence of high energetic activity of melt. Wide occurrence of diapirism in the Pre-Cambrian of the Fennoscandinavian shield is likely to be caused by fluid saturation of mantle and its plasticity. References Larin A.M. (2008) Rapakivi granite-containing magmatic associations: geological position, age, sources. Avtoreferat dokt. dissertazii. M. 47p. Sviridenko L.P. (2002) Ultrametamorphism and granite formation of the Proterozoic overhearth zones in the southern margin of the Fennoscandinavian shield //Mantle plumes and metallogeny. Petrozavodsk – Moscow. P.204-206. Ramo O.T., Manttari I., Vaasjoki M., Upton B.G.J., Sviridenko L.P. (2001) Age and significance of Mesoproterozoic CFB magmatism, lake Ladoga region, NW Russia. Geological Society of America. Abstracts with Programs. V.33. N6. Boston. Vaasjoki M. (1977) Rapakivi granites and other postorogenic rocks in Finland: their age and lead isotope composition of certain associated mineralizations. Geol. Surv. Finland. Bull. v.294. 64p.
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