Grospydite level of the initial melts of the Ilmeny Mountains alkali-ultramafic association, South Urals

Rusin A.I.,* Krasnobaev A.A.,* Medvedeva E.V.,** Baneva N.N.,* Valizer P.M.**

* Institute of Geology and Geochemistry UB RAS, Yekaterinburg, Russia

** Ilmeny State Reserve UB RAS, Miass, Russia

rusin@igg.uran.ru

 

The fragments of the Ilmeny Mountains alkali-ultramafic central type intrusion, disintegrated in the postcollisional deep shear zone, attributed them to the genotype of the “fenite(miaskite)-carbonatite formations of the linear zones” (Rusin et al., 2006). An absence of relationship with mantle mafic-ultramafic magmatism and their location in the “deep faults” which control the alkali-carbonatite fluids are the principal features of this type. Authors showed that numerous mafic-ultramafic blocks in the central shear zone and quartzite-shale strata of the cover have a complex of geological and geochemical features of the primary substrate, multiphase differentiation of which could form miaskite-carbonatite association. All mafic-ultramafic blocks are not related to the country rocks, have tectonic boundaries and contain abnormal high trace and REE concentrations.

The total REE content in ultramafic rocks is up to 360 ppm. They have higher concentrations of Rb, Ba, K, Nb, La, and Ce and low contents of Ti, V, Ni, and Cr in comparison with primitive mantle. REE distribution along with indicator ratio (Nb/Ta = 16, Zr/Hf = 43, U/Th = 16, and Sm/Nd = 0.0997-0.2071) point to the probable affinity of the Ilmeny ultramafic rocks to the mantle enriched. Nd and Sm isotopy definitely indicates correlation of ultramafic rocks with EM1 and EM2 mantle reservoirs which formation is explained by the global alteration caused by the deep mantle plums (Kogarko, 2006). Plume processes in the Late Precambrian formed picrite, picrite-diabase, and dolerite dike swarms and sills could be the possible reason of an appearance of the enriched mantle reservoir being the deep matter source of the Ilmeny Mountain alkali-ultramafic association. The Sm-Nd age of phlogopite-richterite olivinite and dolomite carbonatite of the Buldym massif is about 600 Ma (unpublished data of I.L. Nedosekova) and it correlates with the Vendian activization of rifting (plume) processes.

The presence of metafoidolite is important to establish the deep sources of the Ilmeny Mountain alkali-ultramafic association. Firstly, it was described by K.I. Postoev as «gabbro-amphibolite» from the exocontact of serpentinite massifs (Nyashevo and others) and then A.G. Bazhenov distinguished the «anortite amphibolite». Both ultramafic rocks and metafoidolite form boudines and blocks grouped in the central shear zone along with tectonic flow and are chaotically spread in quartzite-shale strata because of their contrasting rheological properties. “Anortite amphibolite” has unusual high trace and REE contents (to 517 ppm) with invariable presence of nepheline, olivine, and larnite in normative composition.

High concentrations of Sr (up to 841 ppm) and Ba (up to 1379 ppm) and Ba/Sr ratio (1.5) comparable with that of the deep kimberlite magmas could indicate formation conditions (Lutz, 1975). Low SiO₂ content (30-43 wt %), high concentration of Al₂O₃ (up to 30 wt %) and СаО (up to 20 wt %), and moderate content of Na₂O+K₂O (1-2.5 wt %) are the important features of the rocks as well. «Anortite amphibolite» is regarded to be an alkaline rock similar to ultramafic and mafic foidolite on the basis of petro- and geochemical data and normative composition. The weakly amphibolized feldspar ijolite (malinyite) is an evidence of typical alkali-ultramafic platform intrusions of ijolite-jacupirangite series.

Research of metafoidolite («anortite amphibolite») revealed the diversity of their mineralogical variety and principal features of mineral compositions (Korinevsky, Korinevsky, 2006; Medvedeva, 2006), for example, the presence of the high-calcium garnets (Pyr_4-20Alm_30-62Sp_0,6-9Gros_32-61) in the association with diopside, zoisite, kyanite, and corundum. Firstly, O’Hara and Mercy (1966) suggested that garnets with grossular end member of 35-70 % are characterized by the absence of miscibility gap and they could rarely appear in unusual low silica rocks at corresponding physical and chemical conditions. This supposition was confirmed after detail study of grospydite and kyanite eclogite from kimberlite pipes Zagadochnaya, Yakutia (Sobolev, 1974) and Victoria Roberts, South Africa (Lappin, 1978; Dowson, 1983; Sharp at al., 1992). Experimental study of subsolidus associations which form after grospydite (Green, 1970) showed that association of high-calcium garnet with clinopyroxene and plagioclase is stable at pressure interval of 22.5 to 27 kbar. Plagioclase disappears, kyanite forms, garnet portion relatively to clinopyroxene and grossular content in garnet increase with increasing in pressure up to 36 kbar.

Chemical and normative composition of synthetic grospydite (Green, 1970) is similar to composition of the Ilmeny Mountains «anortite amphibolite» and composition (Pyr_9-11Alm_34-39Gros_50-55) of garnet, crystallized at pressure of 27-36 kbar, corresponds to almandine-grossular (Korinevsky, Korinevsky, 2006; Medvedeva, 2006). Ratio of Na₂O contents in clinopyroxene and grossular is a principal feature of clinopyroxene-garnet xenoliths in kimberlites.

«Sobolev’s empirical rule» (Sobolev, 1974) about (i) synchronous increase in grossular content in garnet and Na in pyroxene from grospydite and (ii) a decrease of grossular content in garnet and Na increase in pyroxene from eclogite, was confirmed by natural observations and thermodynamic calculations and experiments (Green, 1970; Dawson, 1983; Sharp et al., 1992). End member garnet composition from grospydite varies within a single sample and, at the same time, garnet composition from eclogite xenoliths do not change. Clinopyroxene, coexistent with garnet from grospydite and eclogite, is distinct by jadeite end member content. The same regularity could be found in amphibolite after grospydite in the Ilmeny Mountains. Clinopyroxene from these rocks contains insufficient Na content, Chermak’s calcium and Eskola’s pseudojadeite molecules. Paragenetic analysis showed that variety and features of mineral composition of amphibolite after grospydite of the Ilmeny Mountains is caused both by thermodynamic formation conditions and features of bulk rock composition. Both an absence of garnets in some amphibolite blocks and increase of pyrope end member content up to 15-20 % in almandine-grossular garnets from the Urazbaevo area are related to the higher Mg content of these rocks (X_F < 36 at %). Clinopyroxene compositions reflect the changing of the total Fe and Al mole fractions and appearance of high aluminum minerals (corundum, kyanite, and Al-spinel) is related to the low Cal/Al ratio in the rocks.

Formation of the Ilmeny Mountains «anortite amphibolite» after grospydite estimates a minimal depth of generation of initial alkali-ultramafic melts and Sharp’s with coauthors data (1992) point both to grospydite and coesite levels.

Nepheline in the melted grospydite xenolith (Lappin, 1978) confirms our normative calculations indicating that amphibolite after grospydite is an alkaline rock. Secondary minerals in grospydite xenoliths (plagioclase, zoisite and other), formed after their inclusion into kimberlite magma (Sobolev, 1974; Sharp et al., 1992), could be expanded by hydration reactions, related to the movement of the deep rocks to the low earth’s crust, for the «anortite amphibolite» association of the Ilmeny Mountains.

Zircon age (662±14 and 543±7.1 Ma) and first data on the REE distribution in garnet from «anortite amphibolite» will be discussed in the oral report.

This study was financially supported by the Program of BGS RAS No 4 and Integrational project of UB - SB - FEB RAS.

References

Dawson J.B. Kimberlites and their xenoliths. Springer-Verlag Berlin-Heidelberg-New York. 1980. 252 p.

Green Tr.H. An experimental investigation of subsolidus assemblages formed at high pressure in high-alumina basalts, kyanite eclogite and grospydite compositions //The origin of the principal series of the igneous rocks by the experimental date, 1970. P.21 - 52.

Kogarko L.N. Alkaline Magmatism and Enriched Mantle Reservoirs: Mechanisms, Time, and Depth of Formation // Geochemistry International, 2006. № 1. P. 3-11

Korinevskiy V.G., Korinevskiy E.V. New data in geology, petrography, and mineralogy of the Ilmeny Mountains // Miass, Institute of Mineralogy Urals Branch RAS, 2006, p. 102

Medvedeva E.V. Garnets from methamorphic rocks of the Ilmeny Mountains // Geology and mineralogy of Ilmeny complex. Miass, 2006. P.80-130

Rusin A.I., Krasnobaev A.A., Rusin I.A. et al. Alkali-ultramafic association of the Ilmeny-Vishnevy Mountains // Geochemistry, petrology, mineralogy, and genesis of alkaline rocks. Miass: UB RAS, 2006. P. 222-227.

Sharp Z.D., Essene E.J., Smyth J.R. Ultra-high temperatures from oxygen isotope thermometry of a coesite-sanidine grospydite // Contrib. Mineral. Petrol.,1992. V. 112. P. 358-370.

Sobolev N.V. The deep seated inclusions in kimberlites and the problem of the upper mantle composition. Novosibirsk: "Nauka" Siberian Branch, 1974. 264 p.