Kimberlites and basalts with differing age of the Nyurbinskaya pipe (Nakyn field, Yakutia):

a portrayal of regional upper mantle evolution from the Riphean to the Carboniferous time,

and its geodynamic relationships

Sablukov S.M.* , Sablukova L.I.* , Stegnitsky Yu.B.** , Karpenko M.A.***, Neustroev R.G.****

^* “RUSGEO” Limited, Moscow, Russia;^** NIGP ALROSA Co. Ltd., Mirny, Russia;^*** Nyurbinskaya Mine, ALROSA Co. Ltd., Mirny, Russia;^**** ALROSA-Nyurba Co. Ltd., Nyurba, Russia.

sablukoff@rambler.ru

 

Nakyn field kimberlites differ drastically from kimberlites of other Yakutian kimberlite fields (Tomshin et al., 1998), primarily, by being free of picroilmenite and extremely geochemically depleted, with peculiarly low concentration of Ti, Fe and all incompatible elements except for K, Rb and P.  These sharp distinctions may be related to different intrusion age of kimberlites in different fields reflecting the particular condition of the mantle characteristic of its different evolution stages. 

A series of Sm-Nd, Rb-Sr and K-Ar isotope studies revealed a significantly older kimberlite intrusion age (D₁em, 3994.6 Ma, Sablukov et al, 2007) and an older model age of mantle source (T_Nd(DM) = 1100 Ma, eNd = +1.0, eSr = +25.2) for Nyurbinskaya pipe as compared to other diamondiferous kimberlites of the Yakutian province (D₃–C₁ , 370-320 Ma, Brakhfogel, 1995; T_Nd(DM) = 0.6-0.7 Ga, Bogatikov et al., 2004).

A peculiar feature of the Nyurbinskaya pipe consists in the joint presence of a pre-kimberlite dolerite intrusion (xenoliths of which occur in pipe kimberlites) and a post-kimberlite dolerite intrusion (penetrating the kimberlites) in the Nyurbinsky volcanic complex. 

Our investigation revealed that petrographically similar but obviously nonsynchronous mafic volcanic rock bodies (separated from each other by the kimberlite intrusion stage) show not simply dissimilar but radically differing geochemical characteristics.  Only SiO₂, Na₂O, K₂O and Co contents of pre-kimberlite and post-kimberlite dolerites are somewhat similar.  Pre-kimberlite dolerites are 1.5 times higher in Al₂O₃ and MgO, 2 to 3 times higher in  Ba and Pb,  5 times higher in Ni and 15(!) times higher in Cr as compared to post-kimberlitic dolerites.  Along with this, post-kimberlitic dolerites are richer than the pre-kimberlitic ones in the overwhelming majority of elements: FeOtot, CaO, Be, V, Ga, Rb, Cs, Sr, (by a factor of 1.5-2.5), TiO₂, MnO, P₂O₅, Cu, Zn, Y, Zr, Hf, REE, Th, U, (by a factor of 3 to 6), and more than ten times richer in Nb and Ta !  Normalized REE distribution for volcanic rocks (Boynton, 1984) show Еu-maximum for the pre-kimberlite basites and Еu-minimum for the post-kimberlite basites.

The most prominent geochemical features of the pre-kimberlitic (“old”) mafic rocks of the Nyurbinskaya pipe are their significantly lower Ti and Fe contents and higher Al content as compared to those of the post-kimberlitic (“young”) mafic rocks of this pipe.

Geochemically, the pre- kimberlite dolerite of this pipe is close to high-Al calc-alkali basalt rocks of volcanic arc in subduction zones, the post-kimberlite dolerite is similar to sharply geochemically enriched, high-Fe within-plate tholeitic basalt rocks, and kimberlites correspond to within-plate alkaline basalt rocks (Wood, 1980). 

According to K-Ar and Rb-Sr dating, the pre-kimberlite dolerite is Late Riphean (700 Ma), and the post-kimberlite dolerite is Middle Carboniferous (330 Ma).  As regards the age and type of mantle source, the pre-kimberlite dolerite appears to be related to old, enriched lithospheric mantle, with probable participation of some old lower crust matter (EMII, eNd = -12.2, eSr = +54.6; T_Nd(DM) = 2450 Ma), whereas the post-kimberlite dolerite would be related to depleted mantle with probable participation of young upper crust matter (eNd = +4.7, eSr = +43.7; T_Nd(DM) = 770 Ma).

The different volcanic rocks with differing age incorporated in the Nyurbinskaya pipe represent a The different volcanic rocks with differing age incorporated in the Nyurbinskaya pipe represent a “record” of temporal evolution for mantle sources of mafic and ultramafic magmatic melts, from the Late Riphean to the Middle Carboniferous.

Time of formation pre-kimberlite dolerite (Late Riphean) is a time of cardinal tectonic reorganization of Northern Asia, when approximately on a boundary 700 Ma under influence of riftogenes processes there was a split of the supercontinent Rodinija including as a component Siberian kraton (Yarmolyuk et al., 2003).

The prominent geochemical distinctions between the “old” and “young” dolerites in the Nyurbinskaya pipe, concurrent with the well defined geochemical dissimilarity between the relatively old Nyurbinskaya pipe kimberlites and relatively young kimberlites from other Yakutian kimberlite fields (high Fe and Ti contents and similarity of composition and model age of mantle sources for the relatively young magmatic rocks) might be caused by some geological process of common type.  The role of such a global geological process, which could sharply change (at least) the geochemical characteristics of the upper mantle and give rise to intense Fe-Ti mantle metasomatism, could be played by a Late Devonian asthenospheric diapir (plume).  The relatively old mafic rocks and pre-plume kimberlites might reflect the composition of old, geochemically depleted lithospheric mantle, whereas the relatively young mafic rocks and post-plume kimberlites appear to reflect the composition of the heterogeneously metasomatized geochemically enriched (owing to the plume effect) upper mantle.

 

References

Bogatikov O.A., Kononova V.A., Golubeva J.J. etc. Petrogeochemic and isotope variations of composition kimberlites from Yakutia and their reason. // Geochemistry. 2004, Vol. 9. P. 915-939 (in Russian).

Sablukov S.M., Banzeruk V.I., Sablukova L.I., Stegnitskij Ju.B., Bogomolov E.S., Lebedev V.A. Ancient age of the Nakyn field kimberlites (Yakutia) - one of the reasons of an originality of their material structure // VIII International conference «New ideas in sciences about the Earth». Moscow. 2007. Reports. Vol. 5, P. 209-212 (in Russian).

Tomshin M.D., Fomin A.S., Kornilova V.P., Cherny S.D. and Yanygin Ju.T. Feature of magmatic formations Nakyn kimberlite fields of the Yakutia province // Geology and Geophysic. 1998. Vol.39, 12. P.1693-1703 (in Russian).

Yarmolyuk V.V., and Kovalenko V.I.  Mantle plumes in phanerozoic history of the Northern Asia// Plumes and problems of deep sources of alkaline magmatism. Proceedings of the III Interntional Conference. Khabarovsk. Publishing House of the Irkutsk State Technical University. 2003. P. 5-35 (in Russian).