Sr and Nd systematics of the Tajno alkaline-ultramafic complex, NE Poland: Identification of depleted and enriched components in the subcontinental lithospheric mantle

Krzemiński L.*, Krzemińska E.*, Anczkiewicz R.**, Pécskay Z.***

* Polish Geological Institute – National Research Institute, Warszawa, Poland; ** Institute of Geological Sciences, Polish Academy of Sciences, Kraków, Poland; *** Institute of Nuclear Research, Hungarian Academy of Sciences (ATOMKI), Debrecen, Hungary.

leszek.krzeminski@pgi.gov.pl

 

The Tajno alkaline-ultramafic complex and alkaline lamprophyre dykes intruded the Proterozoic crystalline basement that belongs to the Fennoscandian block within the East European Craton. The K-Ar dating of mineral fractions and whole rock sample yielded variable cooling ages for alkaline lamprophyres: 1152.7±32.6 and 973.5±29.3 Ma from Wigry (Meso-Neoproterozoic), and 301.4 to 278.9±10.6 Ma from Bargłów (Late Carboniferous/Early Permian). Alkaline lamprophyres have camptonite affinity with tephrite/basanite composition on the TAS classification scheme. REE and multi-element spidergrams of all lamprophyric dykes show uniform pattern and simultaneously strong variability of LREE enrichment (La_N/Yb_N from 19 to 110). Such characteristics are typical for high-K anorogenic rocks from continental within-plate environment.

The Tajno complex is a polygenic volcano-plutonic structure with an area of ca. 5 sq. km on the pre-Triassic erosional surface occurring under the Mesozoic-Cenozoic sedimentary cover. It is essentially consist of clinopyroxene cumulates and intrusive breccia composed of shattered pyroxenite fragments cemented with malignites and nepheline syenites. A special variety is the pyroxenite breccia cemented with phonolite. The plutonic rocks are cut with numerous carbonatite and silicate veins. The latter constitute a continuous series from olivine melanephelinite, through nephelinite and tephriphonolite, up to peralkaline phonolites and trachytes. A volcanic pipe, filled with a lapilli-ash tuffs and chimney breccia, is located in the central part of a complex. The K-Ar age obtained on the kaersutite from the malignite (346.5±10.6 Ma) is in good agreement with the independently known Early Carboniferous alkaline magmatic events in the Polish part of the EEC (U-Pb age of 345.5–348 Ma; Krzemińska et al. 2006).

Most rocks from the Tajno complex plot in the depleted mantle quadrant of the Nd–Sr correlation diagram with fairly low ⁸⁷Sr/⁸⁶Sr(t) ratios from 0.7033 to 0.7042 and positive εNd(t) values from +3.6 to +1.4 (Fig. 1). These results indicate derivation from a mantle source that has a higher time integrated Sm/Nd ratio than CHUR and mostly a lower Rb/Sr ratio than Bulk Silicate Earth. Alkaline lamprophyres from Bargłów and Tajno fall within enriched mantle field with lower εNd(t) from –0.3 to –1.5. The parent magma of one sample of camptonite, exceptionally rich in radiogenic Sr, most probably have undergone a weak crustal contamination which disturbed only Rb-Sr isotopic system.

Two contrasting mantle components are required to produce the isotopic variations observed in the Tajno complex: isotopically depleted “MORB-like” mantle component, and enriched “EM1-like” end-member, in proportions of ca. 96–98% DMM and 2–4% EM1 (Fig. 2). Plutonic rocks and carbonatites have low ²⁰⁶Pb/²⁰⁴Pb(t) ratios ranging from 17.33 to 18.05 (Brassinnes, 2006), so the contribution of the HIMU-like component with highly radiogenic Pb within their mantle sources can be excluded (Fig. 2).

  

Fig. 1. εNd(t) vs. 87Sr/86Sr variation diagram for samples from the Tajno alkaline-ultramafic complex. Mixing lines between DMM, HIMU, EM 1 and EM 2, with tick marks indicating mixing proportions, are calculated using the parameters from Zindler and Hart (1986), Workman and Hart (2005), Ben Othman et al. (1989) and Beard et al. (2007).

Fig. 2. εNd(t) vs. 206Pb/204Pb variation diagram for the Tajno complex. Pb isotope ratios for pyroxenites, malignites and carbonatites are from Brassinnes (2006). FOZO composition is from Stracke et al. (2005). Symbols as for Fig. 1.

 

In the proposed model, depleted end-member is interpreted as a residual sub-continental lithospheric mantle developed in the result of basaltic melt extraction from an originally near-chondritic mantle in the Palaeoproterozoic (ca. 1800 Ma). Model age of this depletion correspond to the age of major crust-forming events in the province. Previously depleted lithosphere subsequently underwent metasomatic enrichment in incompatible elements in Neoproterozoic (ca. 850 Ma). Enrichment appears to have occurred by an infiltration of asthenosphere-derived silicate partial melts.

Age-corrected Sr and Nd isotope compositions demonstrate that the alkaline-ultramafic carbonatite Tajno complex have close affinity with two other rift-related magmatic provinces within Fennoscandia, 10–30 Ma older than Tajno: Kola Alkaline Carbonatite Province and diatreme fields on the northern flank of the Pripyat-Dnieper-Donets paleorift.

 

This study was financially supported by PGI-NRI grant no. 61.2901.0603.00.0.

 

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