Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Minettes of Porja Guba, White Sea: new mineralogical, isotopic and geochemical data

Koreshkova M.Yu.*, Lokhov K.I.*^,**, Kornakov A.S.*, Presnyakov S.L.**, Kapitonov I.N.**, Bogomolov E.S.**

*Saint-Petersburg state university, Saint-Petersburg, Russia

**A.P. Karpinsky Russian Geological Research institute, Saint-Petersburg, Russia

mk6456@mail.ru

In the area of Porya Guba in Kandalaksha bay of the White Sea several dikes of micaceous lamprophyres occur. Their similarity to lamproites in mineralogical and bulk rock composition was shown by (Proskuriakov, Uvadiev, 1992; Nikitina et al., 1999). L.P. Nikitina and coworkers (1999) obtained Rb-Sr isochron age of 1719±8 Ma for these rocks that was not supported by Sm-Nd data. In the same area, there are a few dikes of nepheline picrites presumably Devonian in age. The intersections of dikes of these 2 kinds are absent. The lamprophyre dikes are cut by sulfide-bearing carbonate veins, well-known for their silver mineralization, with the age of 802 Ma (Lokhov et al., 2010). We have carried out a study of mineralogical and bulk rock compositions of the lamprophyres, U-Pb dating of zircons separated from these rocks using SIMS SHRIMP II and a study of Lu-Hf isotope system using LA-MCICPMS Finnigan Neptune/DUV-193 following the method of (Lokhov et al., 2009).

In addition to data presented by (Proskuriakov, Uvadiev, 1992; Nikitina et al., 1999; and others), it is necessary to note that the lamprophyres range from rocks with phenocrysts of diopside (Di) and phlogopite (Phl) and calcite-Di-Phl ground-mass to those having Phl phenocrysts and K-feldspar, K-richterite, Sr-apatite and Phl in the ground-mass. Olivine and leucite are absent. Among 22 dikes we have studied, the rocks with abundant calcite in the ground-mass predominate. One of the bodies is a carbonatite dyke that intruded a previously existing lamprophyre dike. Phlogopite both in phenocrysts and in the ground-mass is zoned. Although the compositional variations are not wide (10-12 wt.% Al₂O₃, 2-4 wt.% TiO₂), general tendency is towards tetraferriphlogopite but an increase in Al₂O₃ and BaO contents is observed in some cases. Rhythmic zoning and xenogenic cores are frequently present. Diopside is poor in Ti, Na and Al (up to 2 wt.% Al₂O₃); it has aegirine-augite overgrowths. Potassic richterite is poor in Ti (0.3-1.3 wt.% TiO₂). Potassic feldspar (Kfs) is present as orthoclase and microcline; it contains up to 3 wt.% FeO and 13 wt.% BaO. Several dikes contain albite along with Kfs. Its appearance is probably due to secondary alteration as well as hematite and Ba-rich feldspar exsolution and the transition of orthoclase into microcline. Accessory minerals are rutile, monazite and zircon. The latter is the mineral of ground-mass that crystallized before calcite and Kfs but later than apatite and Phl. This explains irregular shape of zircon crystals and appearance of edges and faces when zircon contacts with calcite and Kfs. The oscillatory zoning is seen in transmitted light and in BSE but not seen in CL because of low luminescence which is due to high Th (440-3880 ppm) and U (330-1380 ppm) contents and high concentration of related lattice defects. Zircon is characterized by high Th/U ratio (0.7-3.8). Most grains are fractured; thin alteration zones occur along cracks.

The study of isotopic composition of U and Pb in 10 zircon grains has shown that 8 of them are reversely discordant. This is probably due to zircon alteration and enhanced ion yields from a radiogenic labile Pb and/or the big difference in composition between the analyzed zircon and the standard zircon. Most grains have ²⁰⁷Pb/²⁰⁶Pb age in the range of 1760-1830 Ma. A normally-discordant grain is much younger. Intercepts of discordia and concordia give age values of 1738±22 and 827±150 Ma that correspond to intrusion of the dikes and to the superimposed alteration respectively. The last value is close to the time of silver-bearing veins formation. Intrusion of alkaline picrites and/or calcite veins could produce the observed alteration of minerals in lamprophyres and the partial loss of radiogenic Pb in zircon. The Lu-Hf isotopic system has shown that zircons have low ¹⁷⁷Lu/¹⁷⁶Hf <0.001 and contain excess radiogenic Hf. The initial isotopic composition varies broadly: eHf(Т) from -7 to +42 that is not possible in case of single magmatic zircon generation. No correlation between ¹⁷⁷Hf/¹⁷⁶Hf and ¹⁷⁷Lu/¹⁷⁶Hf is observed. The radiogenic Hf hence is not related to inclusions of minerals having high ¹⁷⁷Lu/¹⁷⁶Hf ratios, for example, apatite, garnet and carbonates. This suggests that a component containing radiogenic Hf was captured by zircon when it crystallized or reacted with a fluid during late-magmatic stage. Hf-Nd correlation diagram (Fig. 1) demonstrates heterogeneous distribution of excess radiogenic Hf in zircon. Same situation was described in a carbonate-silicate metamorphic rock (Lokhov et al., 2009). Partial recrystallization or replacement of minerals with high Lu/Hf in a deep-seated source is necessary to produce melts or fluids having an excess of radiogenic Hf. Garnet from Phl-eclogites that are present as xenoliths in these dikes is a probable source of radiogenic Hf. Phlogopite in eclogites replaces garnet giving rise to a fluid with radiogenic Hf. Isotopic data for lamprophyres: εNd(T) = -8 and (⁸⁷Sr/⁸⁶Sr)_I = 0.7027 support our interpretation.

Fig. 1. Hf-Nd correlation diagram. Dotted grey lines demonstrate a terrestrial array – a correlation band for magmatic rocks, ellipses present fields of kimberlites of first (1), second (2) and transitional (3) type and lamproites and minettes (4) (Davies et al., 2006; Nowell et al., 2004).

Despite the obvious similarity to lamproites, the presence of calcite and Na-rich feldspar excludes the possibility to use the term “lamproite” for these rocks. The term “minette” is probably best. Minettes from Churchill province, Canada (Peterson et al., 2002), including diamond-bearing bodies, are compositional analogues of Porja Guba lamprophyres. These authors as well as L.P. Nikitina and coworkers (1999) relate the origin of the minettes to postcollisional melting of a mixed crustal-mantle source. A further study of magmatic evolution of the minettes is necessary to understand their origin. The complex zoning of minerals and the presence of a carbonatite in this suite point to possible interaction of melts of different origin.

This study was performed within the scope of Scientific Research Work funded by federal budget № 3.37.81.2011 and № 3.37.86.2011.

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