2011

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Metasomatism and melting in fenite-migmatite zones of the Vishnevye Gory miaskite massif

S.S. Abramov and I.T. Rass

Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences, Moscow, Russia; westabra@ya.ru

Chemistries of minerals in fenites and migmatites from the root part of the Vishnevye Gory miaskite intrusion and the systematic variations in the mineral assemblages of the rocks are consistent with a model according to which the miaskites were generated by the melting of fenites that developed in a gneiss protolith.

The genesis of the miaskite-carbonatite association in the Vishnevye Gory Range, Southern Urals, is explained by either (i) the in-situ derivation of alkaline magmas from a crustal source material under the effect of alkaline fluids coming from the mantle (Levin et al., 1997; Rass et al., 2004) or (ii) the emplacement of alkaline-carbonatite magma coming from melting zones of a mantle plume (Nedosekova et al., 2009).

Although the petrology and geochemistry of magmatic replacement processes (i.e., the origin of miaskite magma via the replacement of crustal rocks under the effect of alkaline fluid) was closely examined by many geologists (Ronenson, 1066; Levin et al., 1997; Rass et al., 2004), our recently obtained data on the chemistries of minerals in the fenite-migmatite portion of the complex (in the Potatiny Mountains, Central Alkaline Stripe) allowed us to specify certain aspects of magma-generating processes.

The fenite-magmatic root zone exhibits a zonal transition from plagiogneiss and plagioamphibolite through fenite to migmatite of miaskite composition. The fenitized gneisses consist of the Pl-Cpx-± Amph-Bt-Ilm±Qtz-Ap association, which gives way to the Pl-Kfs-Bt-Cpx-Mag-Ap-Tnt of fenite and is further replaced (with progress in the fenitization process) by the Kfs -Pl-Bt-Cal-Mag-Ap-Tnt association of biotite-feldspar rocks. Miaskite melts are generated in the migmatite zone, which includes lenses and segregations of miaskite composition (Ne+Bt+Kfs+Ab+Mag+ Tnt +Zrc) in feldspar-biotite rocks. The thickness of the migmatization zones increases, and miaskite develops first as swarms of dike-shaped bodies and then also as relatively small intrusions (autochthonous miaskite, according to L.Ya. Levin).

We examined systematic variations in the chemical composition of biotite, pyroxene, and accessory minerals in this zonal complex. From the outer (fenite) to rear (miaskite) zones, biotite becomes richer in Ti and Al, and its Fe mole fraction also increases (Fig. 1). Pyroxene in the frontal zones (fenitized gneiss) is aegirine-salite and becomes progressively more magnesian and aluminous with the transition to fenite. Al-rich varieties contain up to 15 mol % of the jadeite end member, whereas their aegirine concentration decreases (Fig. 2), i.e., the isomorphic substitution was isovalent Fe³⁺>Al ,and the concentrations of the tschermakite end member remained unchanged (3-6 mol %) in all of the zones.

An increase in the Al concentration in the minerals is strongly correlated with the increase in the Al content in the metasomatites from the gneiss to migmatite (Levin et al., 1997; Rass et al., 2006), with Ca and Si removed from the migmatization zones.

An increase in the Fe mole fraction of the biotite in association with magnetite suggests a decrease in the oxygen activity with increasing temperature. The aegirine concentrations in the clinopyroxene in the metasomatites should have thereby decreased according to the reactions

aegirine + potassic feldspar → jadeite + Fe-biotite + O₂,

aegirine + albite → jadeite + magnetite + O₂.

The jadeite stability field should have thereby also expanded because of an increase in the Al mole fraction of the rocks. Correspondingly, the transition from fenite (i.e., pyroxene-bearing metasomatites) to feldspar-biotite rocks can be explained in terms of the replacement of jadeite by aluminous biotite. Thermodynamic simulations in the Ca-Al-Fe H₂O-CO₂-O₂ system at 5 kbar indicate that this replacement could proceed in an H₂O-CO₂ fluid (0.7-0.6H2O + 0.3-0.4CO2) at a temperature of 630-660°С. The six-mineral association Pl (An-Ab)-Bt(Ann-Sd)-Cpx(Hed-Jd)-Kfs-Cal-Mag was proved to be highly susceptible to the concentration of the jadeite end member in the pyroxene, and a decrease in this concentrations expanded the stability field of nepheline-pyroxene associations at the sacrifice of biotite.

The F and Cl concentrations of biotite in the metasomatites varies within a few wt %, which testifies that the metasomatizing fluid did not contain any unusually high concentrations of these elements and, hence, should have an aluminous aqueous-carbon dioxide composition. Experimental data indicate that this fluid cannot serves as an efficient transporter of alumina and metals (because of the absence of their high-solubility complexes) but can readily transfer silica and calcium in alkaline and carbonate complexes. Because of this, metasomatic processes in any fenite-migmatite complex should be associated with the removal of silica and calcium and a complementary increase in the aluminum mole fraction of the rocks. Consequently, the biotite and clinopyroxene should become progressively more aluminous and the rocks progressively more leucocratic.

The transition from fenite to migmatite is pronounced in the replacement of pyroxene and amphibole by a biotite-feldspar association. This suppresses the temperature at which miaskite melt can be derived from these rocks, and the increase in the alumina content is favorable for the derivation of agpaitic miaskites.

It follows that the Vishnevye Gory miaskite complex resulted from the magmatic replacement of a source gneiss material with the derivation of miaskite and further magmatic evolution of the newly derived melt (Abramov and Rass, 2009). The root portions of the complexes have long been affected by the percolation of alkaline fluid and the ensued desilification of the rocks. The relatively low temperature of the fluid predetermined that the rocks could start melting only after significant removal of mafic components (easily solvable in H₂O-CO₂ fluid) from the metasomatized rocks. As a result, leucocratic alkaline miaskite melts were generated, and Ca was removed with the fluid to higher levels of the complex. Ca enrichment in the allochthonous miaskite intrusion did not induce liquid immiscibility with the generation of carbonatite melts (Rass et al., 2006), and carbonatite in the Vishnevye Gory Massif itself and calcite in the miaskite are of postmagmatic nature.

References

Levin, V.Ya., Ronenson, B.M., and others. Alkaline-Carbonatite Complexes in the Urals. Yekaterinburg, UralGeokom, 1997, 274 pp. (in Russian)

Nedosekova, I.L, Vladykin, N.V., and others The Il’mensky–Vishnevogorsky Miaskite–Carbonatite Complex, the Urals, Russia: Origin, Ore Resource Potential, and Sources, Geology of Ore Deposits, Vol. 51, No. 2, 2009, pp. 139-162.

Rass, I.T., Abramov, S.S., and others. Role of Fluid in the Genesis of Carbonatites and Alkaline Rocks: Geochemical Evidence, Geochemistry International, Vol. 44, No. 7, 2006, pp. 636-656.