Geodynamic control on the geochemistry of kimberlitic carbonatites (Polar Siberia)

Rass I.T.

 Institute of geology of ore deposits, petrography, mineralogy & geochemistry, Russian Academy of Sciences, Moscow, Russia

rass@igem.ru

 

Geophysical evidence indicates that the Moho surface beneath the northern Siberian Platform composes crests (or ranges) up to 14 km high above deeper areas and 50-80 to 150 km wide (Chernyshov and Bokaya, 1983). These ranges at the Moho likely mark ancient rift zones with a thinner crust. More than 70% kimberlites in structures surrounding the Anabar Shield occur along these Moho crests (Kravchenko et al., 1997).

Carbonate melt separates either from an alkaline magma (corresponding to ijolite or nepheline syenite) in ring alkaline-ultrabasic-carbonatite complexes, at a high differentiation degree, or directly from alkaline-ultramafic magma producing carbonatite-kimberlite sills, and the respective carbonatites are referred to as kimberlitic carbonatites (Lapin & Marshintsev, 1984) or silicocarbonatites (Mitchell, 2005). Such carbonate-rich rocks (CO₂ > 29 wt.%) compose pipes, along with kimberlitic pipes, in kimberlitic fields. They abound in kimberlitic fields of both Paleozoic and Mesozoic age southeast and east of the Anabar Shield.

The average liquidus temperatures of related kimberlites, determined based on their major-component chemistries (1020 analyses), are 1429-1441^оC and 1349-1518^оC, respectively (Perchuk and Vaganov, 1980). Recent estimates of liquidus temperatures of unaltered kimberlites (Fedorchuk & Canil, 2004) and some experimental data (Sparks et al., 2009) have shown that the temperature values obtained by Perchuk & Vaganov (1980) may have been overestimated by about 250-300^o.

The geochemistry of kimberlitic carbonatites from both Paleozoic and Mesozoic kimberlitic fields exhibits a dependence on the position relative to the axes of Moho crests, and the temperatures of the accompanying kimberlites, from the Chomurdakh to the Alakite and from the Kuoika to the Lower Kuonamka fields respectively – from ~ 40 km to ~50 km.

1.Away from the maximum heights of the Moho surface, which correspond ancient rifts in the northern part of the Siberian Platform, the decrease of the liquidus temperatures of kimberlites is accompanied by systematic changes in the geochemistry of associated carbonatites. The Mz rocks become relatively enriched in Nb, Sr, REE and P, and depleted in (Ti), Cr, Zr and Ba, whereas the Pz rocks lose Ba and Zr and gain Ti, Cr and Nb.

Compared to the carbonatites of ring complexes, kimberlitic carbonatites are characterized by the lowest relative concentrations of P and Sr, slightly lower REE, and high contents of Cr, Ti, and Zr (Rass, 1998). The depths of the Moho surface beneath carbonatites in Mesozoic ring structures of the Odikhincha, Guli, Magan and Essei complexes in the Maymecha-Kotui alkaline-ultrabasic-carbonatite province west of the Anabar Shield and in Maldzhangarka complex south of the Shield are 36, <42, 42-46, and 50 km, respectively. Their geochemical characteristics show analogous zoning relative to the axial zones of the Moho crests.

2.If carbonatites inherited their trace element signatures from the parent alkaline-ultramafic magmas, which evolve compositionally in the mantle, we should expect that carbonatite geochemistry may vary from the Paleozoic to Mesozoic rocks. This is why the concentrations of P, Zr and Ba are lower, and Sr and REE are higher in Mz rocks compared with Pz ones (at similar positions relative to the axial zones of Moho crests).

3. The geochemical features of kimberlitic carbonatites provide information on the composition of parental melts and the degree of their differentiation at the moment of carbonatite melt separation. The Paleozoic rocks are rich in Cr and have high ratios Cr/P, Cr/Nb in kimberlitic fields above both the axes and slopes of Moho crests, whereas the Mesozoic carbonate rocks separated from weekly differentiated parental melts only in the axial parts of Mz rifts.

 

References

 

Chernyshov N.V., Bokaya L.I. Element morphostructures of the Siberian Platform crust // Structural elements of the Earth’s crust and their evolution. Novosibirsk: Nauka, 1983: 144-150 (in Russian)

Fedortchouk Y., Canil D. Intensive variables in kimberlite magmas, Lac de GRAS, Canada, and implications for diamond survival // J. Petrol. 2004. Vol.45. P. 1725-1745.

Kravchenko S.M., Schakhotko L.I., Rass I.T. Moho discontinuity relief and the distribution of kimberlites and carbonatites in the northern Siberian Platform // Global Tectonics and Metallogeny. 1997. Vol. 6. No.2. P. 137-140.

Lapin A.V., Marshintsev V.K. Carbonatites and kimberlitic carbonatites // Geology of Ore Deposits. 1984. No.3 P. 28-42.

Mitchell R.H. Carbonatites and carbonatites and carbonatites // Canadian Mineral. 2005. Vol. 43. P. 2049-2068.

Perchuk L.L., Vaganov V.I. Petrochemical and thermodynamical evidence on the origin of kimberlites // Contrib.Mineral.Petrol. 1980. Vol.72. P. 219-228.

Rass I.T. Geochemical features of carbonatite indicative of the composition, tvolution, and differentiation of their mantle magmas // Geochem. Int. 1998. Vol. 36. No. 2. P. 107-116.

Rass I.T., Ilupin I.P., Marchenko T.M., Schakhotko L.I.  Carbonatites of ring complexes and kimberlitic carbonatites of the Anabar Shield setting // The 2nd Russ. Conf. Transactions, Syctyvkar, 2000. Vol. 4. P. 307-308 (in Russian).

Sparks R.S.J., Brooker R.A., Field M., Kavanagh J. Schumacher J.C., Walter M.J., White J. The nature of erupting kimberlite melt // Lithos. 2009. Vol. 112S. P.429-438.