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

P.K. Skufin

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 four 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 ; εNd(): from 3.39 to 4.19) with varying T(DM) in the range of 2844-2878 Ma). Subalkalic rocks of the overlying II VS have 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) and tuffaceous suites (TS) (from bottom to top): Kalevian time Kallojarvi TS and Bragino TVS; Vepsian time Menel VS, Kaplja VS and Kassesjoki TS. Picrites and picrobasalts of the Menel VS have the attributes of the partially enriched mantle ( = 1960 ; 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 ; 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 ; εNd(T) = 5.06; T(DM) = 2519 Ma).

Taking 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 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 (189440 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.

Sample

Rock

Contents, ppm

Isotopic ratio

T, Ma

εNd(T)

Sm

Nd

147Sm/144Nd

143Nd/144Nd

89-76

Picrite

3.31

10.97

0.181832

0.51247310

1850

0.37

89-81

Picrite

22.37

126.21

0.107142

0.51154510

1850

0.43

89-88

Picrite

2.24

7.63

0.177815

0.51243015

1850

0.51

89-95

Picrite

3.31

12.82

0.156087

0.5121576

1850

0.47

89-62

Mg-basalt

3.24

12.47

0.157121

0.5121645

1850

0.35

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 188418 Ma (Bibikova et al., 1993), and with the U-Pb age of the syenites from the Soustov and Gremjacha-Virmes massifs (18728 Ma and 18846 Ma, respectively) (Bea et al., 2001).

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

Sample

Rock

Suite

Contents, ppm

Isotopic ratio

87Rb

86Sr

87Rb/86Sr

87Sr/86Sr

89/78

Picrite

Menel

0.34

37.40

0.0273

0.7044118

89/77

Picrite

Menel

62.70

0.70

0.0322

0.7046522

89/88

Picrite

Menel

2.70

97.00

0.0827

0.7049420

YP-1

Kahusite

Menel

2.70

9.10

0.3726

0.7107221

Y

Kahusite

Menel

0.70

4.00

0.5194

0.7127419

88/130

Andesite

Kaplja

140.00

563.00

0.7399

0.7174115

 

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

 

The received Rb-Sr isochrone of the Southern Pechenga volcanites (126073 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 Kostomuksha Area lamproites (12305 Ma) (Marchenko, 1997).

 

References:

Balashov Yu.A. Geochronology of the Early Proterozoic rocks of the Pechenga-Imandra Varzuga Area // Petrology. 1996. Vol. 4, 1. P. 3-25. (in Russian).

Bea F., Arzamastsev A., Montero P., Arzamastseva L. Anomalous alkaline rocks of Soustov, Kola: evidence of mantle-derived metasomatic fluids affecting crustal materials // Contrib. Mineral. Petrol. V. 140. 2001. P. 554-566.

Bibikova Ye.V., Melnikov V.F., Avakjan K.H. Lapland granulites: petrology, geochemistry and isotopic age // Petrology. 1993. Vol. 1, 2. P. 215-234. (in Russian).

Marchenko A.G. Geochemical data of the Kostomuksha lamproites in Karelia. Precambrian of Northern Europe. St.-Petersburg. 1997. P. 59-60. (in Russian).

Skufin P., Bayanova T. The Early Proterozoic volcano of the central type in the Pechenga Area and its connection with the ore gabbro-wehrlitic complex (Kola Peninsula). // Petrology. 2006. Vol. 14, 6. P. 649-669. (in Russian).


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