Anomalous alkaline rocks of Soustov, Kola: evidence of mantle-derived metasomatic fluids affecting crustal materials

The intrusive complexes of Gremiakha-Vyrmes and Soustov represent the two e xtremes of the Early Proterozoic alkaline plutons of Kola, predominantly co mposed of feldespathoidal syenites. Gremiakha-Vyrmes rocks (zircon age: 1,8 84+/-6 Ma) have trace-element and isotope signatures (Sr-87/Sr-86(t) approx imate to 0.704, epsilon Nd-t approximate to -3-1.3) compatible with an ulti mate mantle origin. Soustov syenites (zircon age: 1,872+/-8 Ma) are totally different and show an acute crustal imprint. They have sodaline and analci te instead of nepheline, contain a plethora of REE-HFSE-rich accessories, a nd are characterised by elevated contents of F, Cl, REE, Y, Th, U, Zr, Hf, Nb, Ta, Sn, Be, Li, Rb, Tl, Pb and Cs, negative Eu anomalies, K/Rb approxim ate to 190-160, Nd/Th approximate to 3, and Nb/Ta approximate to 12, with e xtremely high Sr-87/Sr-86(t) (> 0.720) and, at the same time, relatively hi gh epsilon Nd-t (approximate to -1.6-1.7). In this paper, we explore the id ea that the anomalous features of Soustov syenites can be explained if we a ssume they are derived from a metasomatic agent, initially an H2O-CO2 super critical fluid released by alkaline mafic magmas, that was profoundly conta minated during percolation through crustal materials. As percolation advanc ed, the bulk composition of the fluid solute changed from alkali halides an d carbonates to a silica-undersaturated alkaline melt. When the fluid coole d to a temperature of similar to 550-600 degreesC, it reached the point at which vapor and melt were no longer miscible and split into two components, a vapour phase and a Cl- and F-rich silica-undersaturated silicate melt th at crystallised to produce Soustov syenites. To study this process, we have developed a numerical method for modelling the solute composition of the f luid during the infiltration metasomatism. Our results, using the LREE abun dances and the Sr and Nd isotope composition of a Gremiakha-Vyrmes pegmatit e as the starting solute composition of the fluid, and the mode and mineral trace-element and isotope composition of a common Kola gneiss as represent ative of percolated materials, indicate that the fluid would have acquired a signature closely matching Soustov's, even in the case of Nd isotopes, if the gneiss age is 2.9 Ga, near its real age. This model is still a mere wo rking hypothesis that needs further refinements, but may represent a reason able explanation of the genesis of anomalous alkaline rocks with high (8)7S r/Sr-86(t) and epsilon Nd-t greater than or equal to 0, either saturated or undersaturated, which are difficult to understand in terms of magmatic fra ctionation/contamination.