Metasomatic Processes in the Contacts of the Agpaitic Massifs: Constraints for the Mobility of Trace Elements

Arzamastsev A. A., Arzamastseva L. V.

Geological Institute of the Kola Science Centre RAS, Apatity, Russia

arzamas@geoksc.apatity.ru

We present results of investigation of the contact zones of the Khibiny and Lovozero Paleozoic agpaitic plutons which are settled in the Archean basement in the central part of the Kola Peninsula, NW Russia. The host rocks exposed in the contact zones are represented by the Late Archean tonalite-trondhjemite-granodiorite gneisses. The description of the contacts is accessible from our previous presentation (Arzamastsev et al., 2009). Two sections (each 1.2 km long) were made across the northern contacts of the massifs. Investigations included Sr and Nd isotope studies, U-Pb zircon age determinations, microprobe analysis of mineral phases, ICP-MS determinations of the whole rock samples. Additionally, experimental modelling in order to examine trace element interaction between nepheline syenite, fluid and gneiss was performed (Zaraisky et al., 2008).

The contact zones of the massifs imprint the successive steps of formation of multiphase agpaitic massifs, which took place, beginning with a first manifestations of magma activity in calderas and ending with the developing of post-magmatic hydrothermal processes. Fenitization in gneisses exposed to the agpaitic magmas manifests itself as: (i) recrystallization of the protolith phases and appearing of the newly formed alkaline minerals; (ii) latent changes in the chemical composition of phases (e.g. amphiboles, pyroxenes, Cl- to F-apatite); (iii) changes in fluid composition of the rocks due to input of F, SO₄^2-, CO₂, CH₄; (iv) isotope composition of both exocontact and endocontact rocks.

Two main stages of fenitization were distinguished: (1) the early magmatic stage closely related to intrusion of agpaitic magma; (2) late-magmatic fenitization induced by pegmatoids and veins in the host gneisses. The metasomatic alteration which took place at the different stages of fenitization directly reflected the behavior of closing conditions of Sr and Nd isotope systems: whereas the former exhibit significant crustal contamination by radiogenic Sr, the latter show stability during metasomatic process (Fig. 1).

Fig.1. Sr and Nd isotopic compositions of the agpaitic syenites and the rocks forming contact zones of the Khibiny and Lovozero massifs. The Khibiny massif: 1 - endocontact nepheline syenites ("khibinites") and 2 - alkaline syenites ("umptekites"), 3 - nepheline syenites of the main phases of intrusion. Lovozero massif: 4 - endocontact nepheline syenites ("lujavrites"), 5 - albite-microcline veins in host gneisses, 6 - nepheline syenites of the main phases of intrusion. 7 - exoconatact Ar gneisses of the basement. All data recalculated to 365 Ma. Data for (3) and (6) from (Kramm and Kogarko, 1994).

Variations of trace elements along the profiles crossing the contact zones provide evidence of high mobility of Nb, Ta, Zr, Hf and REE during contact metasomatism. Experimental modelling of the agpaite syenite - gneiss interaction in the presence of fluid (Zaraisky et al., 2008) supports this result. In order to decipher characteristics of metasomatic alteration a numerical model using isocon mass balance method (Grant, 1986) was created for several pairs of alkaline rock samples and gneisses. According to the degree of gains and losses, three groups of trace elements can be distinguished: (1) the most mobile elements are Nb and Ta, which gains exceed 10; (2) Zr, Hf, and REE comprise the group of intermediate mobility (gains within 3-4); (3) Rb, Sr, Ba exhibit poor mobility. Experimental data and geological observations (Giere, 1990; Rubin et al., 1993; Van Baalen, 1993, Salvi et al., 2000; Zharikov, Gorbachev, 2004) show important role of F^-, Cl^-, SO₄^2-. Taking into account relatively high average content of F in agpaitic magmas (Kogarko, Krigman, 1981) the F^- ligand is suggested to be the dominant agent responsible for the HFSE and REE transfer. Penetration of F-rich fluid into the Ca-rich gneisses have led to joining F and Ca to form fluorite which resulted in precipitation of REE and HFSE on the chemical barrier. The SO₄^2- ligand may have played the similar role in REE and HFSE transfer, but in the case of the Khibina and Lovozero contacts sulphates (barite) were as a result of extraction Ba from the Ba-rich feldspar which is the main rock-forming phase of the Archean Gneisses.

This work was supported by the Russian Foundation for Basic Research (Grant no. 09-05-00224).

References

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