Correlation between modern heterogeneous lithosphere and phanerozoic alkaline magmatism

Balashov Yu.A.

Geological Institute of the Kola Science Centre of Russian Akademy of Sciences, Apatity, Russia

 

Variations in the contents of rare earth elements, Sm/Nd and εNd(0) values in peridotite xenoliths have been used to reveal the isotope-geochemical heterogeneity in the vertical section of the mantle continental lithosphere. The section covers all the changes in the mineral composition of peridotites and their clinopyroxenes from the upper plagioclase facies (Pl) to lower high temperature garnet peridotite (Gt2) facies in the range of ≈ 15-250 km.The uppermost (Pl-Sp) and the lowermost high temperature garnet (Gt2) peridotite facies were established to be homogeneous in relation to the middle part of the lithosphere section represented by spinel (Sp), spinel-garnet (Sp-Gt) and low temperature garnet (Gt1) peridotite facies (Fig.1).

Fig.1. The changes in the Sm/Nd ratio and εNd(0) values in peridotites from various depths correlate well reflecting the successive transition from the primary oceanic (OM) value - Sm/Nd > 0.3247 (average for the primitive mantle) and εNd(0) >> CHUR=0 to the values reflecting the effect of the mantle metasomatism and other secondary factors which is registered by Sm/Nd << 0.3247 and appearance of εNd(0) < CHUR negative values.The comparison of Sm/Nd and εNd(0) values of peridotites and alkaline magmas of various composition and age (Fig.2) showed a sheer coincidence of these parameters for Phanerozoic alkaline rocks and carbonatites: (1)- (0.1-130 Ma) and (2-) (239-404 Ma) and a deviation of the εNd(0) values to distinctly negative values up to -30 and -45 for Precambrian alkaline series, which reflects a dependence on the age of formation of these alkaline rocks in Precambrian: (3)- alkaline intrusions (1160-1884Ma), (4)- carbonatites (1299-1872Ma), (5)- volcanites of greenstoun belts (4) (2575-2681Ma) and (6)-carbonatites (2661-2677 Ma). [7- ocean mantle; 8- primitive mantle; HARZ LHER harzburgites and lherzolites].

Fig.2.

 

The maximum range of variations in the REE distribution in the lithosphere peridotites in relation to the influence intensity of the mantle metasomatism on them was revealed from variations of the light and heavy REE (with La and Yb as an example Fig. 3 and 4). The optimal La enrichment in harzburgites and lherzolites exceeds the average mantle level by a factor of 15-30.This is equal to La concentrations in pyroxenites early mantle melts of lithosphere. If we compare this with the oceanic mantle, the difference reaches two orders.. Thus, the role of the processes of the light lanthanoid secondary supply into continental peridotite can be considered to be a real factor of formation of the anriched mantle zones of the lithosphere that are sufficiently enriched in these elements for considering them to be potential sources for generation of alkaline magmas with sharply increased concentrations of mobile rare elements, that is incompatible with Yb and other heavy REE.

 

 

Differentiation of La (Fig. 3) and Yb (Fig. 4) in mantle harzburgites (1) , lherzolites (2), primitive (3) and oceanic (4) mantle and pyroxenite xenolites (5) of lithosphere.

References (1-83) may to see in the paper Development of a Heterogeneity in the Lithosphere: Geochemical Evidence (Balashov Yu. A. Petrology, 2009, Vol. 17, No. 1, pp. 90100).

This study was financially supported by RFBR grants 07-05-00572-a, 07-05-00759-a.


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