The peculiarities of plume-tectonic processes in the Early Proterozoic Pechenga Area from the isotope Sm-Nd and Rb-Srš data.

P.K. Skuf’in

Geological Institute of the Kola Science Centre, RAS, Apatity, Russia

The Early Proterozoic Pechenga Area is formed by volcanic and sedimentary rocks of the Northern Pechenga and Southern Pechenga Complexes. The first one is generated by šfour volcanic suites (VS), at which basis sedimentary suites (SS) occur. Sedimentation and volcanism occurred in this region during the Sarioli (I SS and I VS), Jatuli (II SS and II VS) and Ludikovi (III SS, III VS, IV SS and IV VS) times. Andesite-basalts and andesites of the most ancient I VS have the characteristics of the enriched mantle (× = 2324 øÒ; εNd(×): from –3.39 to –4.19) with varying T_(DM) in the range of 2844-2878 Ma). šSubalkalic rocks of the overlying II VS šhave the characteristics of the partially enriched mantle (× = 2214 Ma, εNd(T): from –1.56 to –2.83; T_(DM) = 2590-2789 Ma). Younger tholeiitic basalts of the III VS are representatives of the depleted mantle (× = 2114 Ma, εNd(T): from +1.99 to +2.90; T_(DM) = 2161-2273 Ma). Basalts and ferropicrites of the youngest IV VS have the characteristics of the partially enriched mantle (× = 1980-1956 Ma; εNd(T): from +1.43 to –0.48) with varying T_(DM) in the range of 2142-2470 Ma). The young South Pechenga complex is generated by volcanic suites (VS), tuff-volcanic suites (TVS)š and tuffaceous suites (TS) (from bottom to top): Kalevian time – Kallojarvi TS and Bragino TVS; Vepsian time – Menel VS, Kaplja VS and Kassesjoki TS. šPicrites and picrobasalts of the Menel VS have the attributes of the partially enriched mantle (× = 1960 øÒ; εNd(T): from +0.18 to +0.62; T_(DM) = 2317-2762 Ma), while andesites and dacites of the Kaplja VS are typical products of the enriched mantle (× = 1950 øÒ; εNd(T): from –3.32 to –6.07; T_(DM) = 2539-2574 Ma), as well as granites of the Shuoni-javr massif from the southern frame of the Pechenga Area (× = 1940 øÒ; εNd(T) = –5.06; T_(DM) = 2519 Ma).

šTaking into account volcanic isotope data, and also modern ideas about consistency of plume-tectonic activity, we offer the following scenario of evolution of plume-tectonic processes in this region. Sarioli is the time of display of the shallow super-plume of the first generation (effusion of the Mg-rich “boninitic” andesites, enriched in the lithophile elements –š K, Rb, Ba, Th, Sr). Jatuli is the time of the life termination of this super-plume (effusion of the alkalic Ti-Fe-rich trachybasalts and trachyandesites, enriched in K, Rb, Ba, Th, Sr, P, Nb, Zr). Ludikovi, Kalevi and Vepsi are the time, accordingly, of the beginning and termination of activity of the deep super-plume of the second generation (effusion of the Ti-Fe-rich tholeiites of III and IV VS, and later effusion of the bimodal picrite-andesite volcanites of the Menel and Kaplja Suites).

The certain composition inheritance is typical for the Southern Pechenga magmatism, that is manifested in the inheritance petrochemical signs in Kalevian basalts (Bragino Suite) as compared with the Northern Pechenga basalts. The received Sm-Nd isochrone of the Southern Pechenga picrites (1894â40 Ma) dates the beginning of the Vepsian time in the Southern Pechenga zone (Fig. 1 and Table 1).

Table 1. Isotopic Sm-Nd data for the Menel Suite volcanites.

Fig. 1. Sm-Nd isochrone for Menel Suite volcanites.

This isochrone confirmes the earlier Rb-Sr isochrone age of Vepsian events in the Pechenga Area (Balashov, 1996). The received dates correlate with granite events in the Lapland Belt from the southern frame of the Pechenga Area – 1884â18 Ma (Bibikova et al., 1993), and with the U-Pb age of the syenites from the Soustov and Gremjacha-Virmes massifs (1872â8 Ma and 1884â6 Ma, respectively) (Bea et al., 2001).

Table 2. Isotopic Rb-Sr data for the Menel and Kaplja Suites volcanites.

Fig. 2. Rb-Sr isochronÈ for Menel and Kaplja Suites volcanites.

The received Rb-Sr isochrone of the Southern Pechenga volcanites (1260â73 Ma) show the age of the Middle Riphean thermal events in this area (Fig. 2 and Table 2). The received date correlates with the age of šKostomuksha Area lamproites (1230â5 Ma) (Marchenko, 1997).

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