2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Precambrian high-alkali magmatism in Karelia

Dmitrieva А.V.

Institute of Geology, Karelian Research Centre, RAS

DmitrievaA-V@e-mail.ru

The moderate-alkaline series of igneous rocks comprises mafic monzogabbro andessexites (45≤SiO₂≤53, 3≤Na₂O+K₂O≤5), intermediate monzonites ans syenites (53≤SiO₂≤64, 5≤Na₂O+K₂O≤12), felsic granosyenites, moderately alkaline granites and leucogranites (64≤SiO₂≤78, 8≤Na₂O+K₂O). These rocks differ in mineral parageneses and are genetically complex. Associated with them are Ti, Fe, P deposits of rare and rare-earth elements (Nb, Ta, Zr, Hf, TR and Y).  

In Karelia, moderately alkaline rocks; occur at various time levels in the geological evolution of the region. They were formed in different geotectonic settings. As a result of a decrease in the Earth’s heat flow and a change in geodynamic regime in Late Archaean time, dominant tonalite-trondhjemite-granodiorite (TTG) magmatism was succeeded by typical island-arc (BADR) series. At the AR-PR boundary, the partial melting of a metasomatized mantle gave rise to high-Mg complexes common in all Precambrian terrains. In the Karelian granite-greenstone domain the Archaean stage in the evolution of the Earth’s crust was completed by the intrusion of late- and post-tectonic sanukitoids and moderately alkaline and alkaline granites. Associated with the Palaeoproterozoic stage in the evolution of riftogenic structures (2.5-1.75 Ga) is the manifestation of basic magmatism, whose extreme members give rise to high-alkali differentiates during long melt evolution (~2.0 Ga.) and the development of an alkaline-ultramafic-gabbro formation represented in North Karelia (~1.88 Ga). Bimodal mantle-crustal magmatism at the Palaeo-Mesoproterozoic boundary gave rise to a gabbroanorthosite-rapakivi granite formation.

Examples of Neoarchaean high-Mg massifs (sanukitoids) in Central Karelia (Table.) are West Khizhjarvi, Syargozero, Sharavalampi, Elmus, Hautavaara, Amindomaoya and Panozero pyroxenite-gabbro-monzonite-syenite complexes studied by V.V. Ivanikov, V.D. Slyusarev, G.М. Pavlov, S.B. Lobach-Zhuchenko, V.P. Chekulaev and N.S. Guseva. The basic geochemical characteristics of sanukitoid series at high mg# (0.5-0.75) is enrichment in K, Ba, Th, LREE, Sr and Р. The Panozero multi-phase complex, which comprises five magmatic phases formed during three discrete impulses with variations in rock composition from ultramafics to quartz monzonites, is most representative and well-studied (Lobach-Zhuchenko et al., 2007). The Amindomaoya ans Sharavalampi massifs represent 2-to 3-phase intrusions consisting of ultramafics (I), moderately alkaline gabbro and monzodiorites (II) and syenites (III). They contain (ppm): 4975-8975 Ti, 21-444 Cr, 20-193 Ni, 100-2144 Ba, 976-2749 Sr, 1.7-12.5 Th, 323-1195 LREE and 2635-9260 P. Ore association in them is represented by apatite, ilmenite and sphene; sulphides and minor noble-metal minerals appear at a postmagmatic stage (author’s data).

 A Palaeoproterozoic gabbro-dolerite (titaniferous) formation in South and Central Karelia is represented by ~2.0 Ga sheet bodies and dykes. The formation and intrusion of basic melts, rich in Fe, Ti and alkalies, took place in в a crustal extension regime. Pt mineralization was determined for the Pudozhgora and Koikary-Svyatnavolok intrusions located at the eastern and western flanks of the Onega structure with V-Ti-magnetite mineralization (Trofimov et al., 2008). Quartz diorites-monzodiorites were revealed in the upper horizons of the intrusions: SiO=55.4-59.9 %; ∑alkal. = 6.4-6.5 %, (ppm) V 50-64, Y 46-108, Zr 177-491 and Sr 177-194. The Kumsa sill is probably another example of this type: (ppm)  Ti 5183-11583, V 190-267, Mn 346-985, Y 22-35, Zr 183-283, Th 7,8-17,8, REE  82-250, P 943-1219, Sr 17-41.        

Belonging to a differentiated alkaline-ultramafic-gabbro formation in North Karelia are the Yeletozero and Tikshozero massifs produced by rifting at ~1.9-1.8 Ga (they were studied by А.I. Bogachev, V.D. Slyusarev and V.V. Shchiptsov). The massifs occur as multi-phase intrusive units with apatite-Ti-magnetite mineralization. The rock composition varies from ultramafics to nepheline syenites (SiO₂ 33.13-45.5 % to 54.23 %; ∑alkal. 2.23-4 %, in syenites up to 13.46 %) and accompanying carbonatites. The complexes are enriched in Fe 3-34 %, Ti, P, K and Na 0.26-4.78 %. Late differentiates contain high concentrations  (ppm) of Zr 202, Nb 930, Ta 35, REE 1258, Be, Br, Sr 78-4364.  

A gabbro-anorthosite-rapakivi granite formation (1.65-1.45 Ma) occurs in the southern Fennoscandian Shield and is associated with bimodal mantle-crustal magmatism caused by geodynamics at the boundary between the Fennoscandian Shield and the Russian Plate, along which a linear belt of mantle-crustal polycyclic diapirs extends (Salmi, Vyborg, etc.). In Karelia, plutons were studied in detail by L.P. Sviridenko (1980). The massifs are multi-phase intrusive units, in which 3-5 intrusion phases are distinguished. Differentiates are represented by gabbronorites, gabbro-anorthosites, monzonites, quartz monzonites and rapakivi granites. Associated with them is skarn mineralization in the Lake Ladoga area. Riphean magmatism was completed by the formation of the Valaam sill at 1457-1459 Ma. The mafic rocks of the Salmi massif and those of the Valaam sill typically contain elevated Fe, Ti, K and P concentrations. Associated with Salmi and Valaam granites are monzonites, quartz monzonites and syenites produced by interaction of gabbro and fluidized granitic melt (Sviridenko & Svetov, 2008).

Karelian moderately alkaline rocks are associated with various magma-generating processes. According to some authors, Archaean postcollisional sanukitoid complexes, having transitional characteristics, were formed by melting of mantle peridotite pre-fluidized by dehydration of a subducting slab. The rocks are rich in Mg, Ti, K, Ba, Sr, REE and P. Associated with Palaeoproterozoic rifting processes is gabbroid magmatism characterized by high Fe, Ti, V and PGE concentrations; monzonites are enriched in Na, Th and Zr. The formation of rift in Karelia at about 1.8 Ga gave rise to an alkaline-gabbroid formation with Fe, Ti, P, K and Na, Zr, Nb, Ta, REE and Sr.  

Table. Precambrian igneous high-alkaline complexes in Karelia

References

Lobach-Zhuchenko, S.B., Rollinson, H.,Chekulaev,V.P., Guseva, N.S., Arestova,N.А., Kovalenko, А.V. Geology and petrology of the Archaean high-K and high-Mg Panozero massif, Central Karelia // Petrology. 2007. V. 15. No.5. P. 493-523. 

Sviridenko, L.P. & Svetov, А.P. Valaam gabbro-dolerite sill and the geodynamics of the Ladoga Lake basin. Petrozavodsk. 2008. 122 p.

Trofimov, N.N. & Golubev, A.I. Pudozhgora noble-metal titanomagnetite deposit.

Petrozavodsk. KarRC, RAS. 2008. 123 p.