Arzamastsev A.A.*, Arzamastseva L.V.*, Shanina S.N.**, Zaraisky G.P.***
*Geological Institute of the Kola Science
Centre RAS, Apatity,
The Paleozoic Khibina and Lovozero massifs are mostly composed of
agpaitic syenites with minor foidolites, ultrabasic alkaline rocks and
carbonatites, which occupy the area of ca. 2000 km2. The host rocks
exposed in the contact zones are represented by the Late
Archean tonalite-trondhjemite-granodiorite gneisses. Metabasalts and green schists of the
Early Proterozoic Pechenga-Imandra-Varzuga paleoriftogenic
complex occur only in the southern contact of the Khibina massif. Geological
observations indicate that the thickness of the fenites varies from few meters
The hallmark of the agpaitic syenites is their immense abundances of REE, Y, Sr, Zr, Hf, Nb, Ta, and Th. These are either concentrated in apatite, titanite, perovskite, and other accessories or, given their concentrations in melts were high enough, form their own discrete minerals, such as loparite, pyrochlore, and eudialyte, which economic deposits provide the basis for the region’s mining industry.
In order to decipher the behavior of gases and trace elements in the fenitization process, two profiles across the contacts of the Khibina and Lovozero massifs have been investigated. 78 samples from AR gneisses, fenites, alkaline syenite veins, zircon and ilmenite bearing albite-microcline veins have been analysed for major and trace elements (ICP-MS for V, Cr, Co, Ni, Ga, Rb, Sr, Y, Zr, Nb, Ba, Hf, Ta, Pb, Th, U). The microprobe data have been used for the mineralogical characteristics of the fenites. All samples have been analysed for gases (F, Cl, CO2, H2O, H2S).
The endocontact zones. The data obtained so far unequivocally
give evidence for different agpaitic magma - gneiss interaction in the Khibina
and Lovozero endocontacts. In the Khibina, in the
proximal vicinity of the contact the alkaline rocks are represented by syenites,
which are suggested to be formed as a result of contamination of agpaitic
syenites by gneisses. The endocontact extends up to
The exocontact zones. Two types of fenite can be distinguished in the exocontacts of the Khibina and Lovozero massifs.
The first type fenite (fenite I) form the narrow (<
The fenites II are closely related to the veins of alkaline rocks and
pegmatoids that spread within the
Since the volatiles and most of the trace elements exhibit distinct signs of mobility in the metasomatic process, we have attempted to assess the role of the H2O, F, CO2 in the migration of LILE, REE and HFSE. Additionally, we have performed a stepwise heating for extraction of H2, N2, CH4, C2H4, C2H6 within the range of 20-400, 400-500, 500-600, 600-700 and 700-800oC in the set of the Khibina samples. From the correlation analysis between the above volatiles and trace elements, the following conclusions can be drawn.
(1) The strong positive correlation with H2O appears to suggest that hydrothermal fluids, which followed alkaline pegmatoid veins were responsible for the transport of the REE, Zr, Hf, Nb, Ta. Rb and Sr into the host gneisses. š
(2) As the temperature of the fluid increases to >400oC, the positive correlation between REE and CH4 decreases. For example, the KLa gradually changes from 0.74 to 0.56. By contrast, correlation between REE and C2H6 during heating increases. Zr, Hf, Nb, Ta, Rb show the same behavior during the step-wise heating.š
In general, the now available information on the contact zone of the Khibina and Lovozero intrusions appears to suggest that the alkaline magma-host rock interaction has occurred via porous media flow (in the immediate contact) or by the discrete channels formed during marginal fracturing of the calderas. The obtained U-Pb age of single zircons (359 ±5 Ma, SHRIMP II data) shows that formation of albite-microcline veins followed the main Lovozero and Khibina intrusions, which occurred at ca 370 Ma (Kramm and Kogarko, 1994). Numerous concentric fissures were filled by post-magmatic hydrothermal fluids, which are suggested to be the main agent of fenitization of Archean gneisses.
This study was financially supported by the RFBR grant 09-05-00224.
Zaraisky G.P., Shamatrina, A.M., Arzamastsev, A.A. Experimental modelling
of alkaline metasomatic reactions in the contact zone of the Lovozero massif.
// Geochemistry of igneous rocks. Proceedings of the XXV
International seminar. School: "Alkaline magmatism of the