2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Late-Proterozoic boninite-like dikes of dolerite, as a new type of intraplate magmatism in the Kola province

Petrovsky M.N.*, Petrovskaya L.S.*, Bazay A.V.*, Travin A.V**,

Elizarova I.R.***

*Geological institute KSC RAS, Apatity, Russia

**United Institute of Geology, Geophysics and Mineralogy SB RAS, Novosibirsk, Russia

***I.V.Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials KSC RAS, Apatity, Russia

petrovskiy@geoksc.apatity.ru

According to the established view in the geological literature, boninites series rocks includes the boninites and marianites and are the products of magmatism in the initial stages of ensimatic island arcs formation (Velinsky, Pavlov, 2002; Dobretsov et al., 1986; Simonov et al, 1988; Crawford et al., 1989, etc.). Recently boninites were also determined in the Phanerozoic ophiolitic complexes and Precambrian greenstone belts (Gelman et al., 1988; Popov, 1988, etc.).

Heaving studied doleritic dikes forming a small swarm in area rivers Lylyok and Iokanga – Lake Iokangskoe We established that they conform the typical boninites by petrochemical, geochemical and mineralogical characteristics. The isotope-geochemical studies shown that this doleritic dikes are Late-Proterozoic age (Fig. 1).

Fig. 1. Results of 40Ar/39Ar – research boninite-like doleritic dikes by step heating mono-fraction plagioclase.

Boninites and marianites are effusive and dike rocks with phenocrysts of high-magnesium olivine, clinoenstatite (it occures in modern boninite), orthopyroxene (enstatite, bronzite), clinopyroxene (augite, pigeonite) which immerse in a hyalopilitic or glassy quartz-normative or quartz-modal matrix separately and less together. Dyke rocks is characterized by full, less partial crystallization of the glassy matrix with formation of a very thin plagioclase and clinopyroxene "felt" with the quartz presence (Gelman et al., 1988; Dobretsov et al., 1988; Meijer, 1980). The studied dykes are composed of greenish-gray or dark gray to black porphyritic rocks with aphanitic bulk. The phenocrysts are presented by olivine (Fo75-94Fa6-25), bronzite less enstatite, magnesian augite and sometimes there are phenocrysts of plagioclase (bytownite). The bulk rocks is formed by micrograined fibrous aggregate of augite and plagioclase (labradorite less andesine-labradorite) with an admixture of magnetite, chromite, and quartz. The hydroxyl-bearing minerals are absent. The teardrop-shaped glass separation phenocrysts are rarely observed in the dark-colored minerals.

Petrochemical classic boninites and marianites are characterized by a high content of SiO2 equal to 50-60 wt.% with synchronous MgO high content: a boninites 7-15 wt. %, and in marianites 15-25 wt. % (Popov, 1988; Crawford et al., 1989). The studied dyke rocks are characterized by high concentrations of SiO2 53-56 wt. % and MgO 13-18 wt. % as well as low concentrations of TiO2 0.16-0.45 wt. % (Table 1). The quartz-normative composition (q = 2.41-9.09 wt. %.) is petrochemical characteristics of the studied rocks.

With studied doleritic rocks characterized by SiO2 high content, Mg high content (Mg / (Mg + Fe) equal to 0.73-0.84), as well as the ratio CaO/Al2O3 equal to 0.60-0.73 and 0.80-1.07, these dolerites conform petrochemicaly to High-calcium boninites and Low-calcium boninite III-type, according to the Crawford classification (Crawford et al., 1989).

Graphs of the distribution of rare earth elements (REE) in Lylyok-Iokangskih dolerites have typical boninites characteristic and its came close most boninites from New Caledonia (Fig. 2).

Table 1. The chemical composition of rocks, wt. %.

Note. 1-8 – dikes area rivers Lylyok and Iokanga – Lake Iokangskoe, 9 – boninite Low-Ca III-type (Crawford et al., 1989), 10 – boninite High-Ca (Crawford et al., 1989), 11 – melt inclusions in augite of dikes (analysis 1), 12 – the glass in the boninites (Popov, 1989).

Fig. 2. The distribution REE normalized to chondrite in the studied boninites-like dikes in comparison with the distribution of boninites and marianites of the classic areas of their distribution. Normalizing factors by Taylor and McLennan, 1985. Legend: 1 – Iok-545/03dike; 2 – Iok-545/03 dike; 3 – Field boninites from the Bonin islands 4 – field boninites from the Mariana Trench, 5 – field boninites formation Des Bazaltes from New Caledonia, field 3 , 4 and 5 are based on data (Hickey and Frey, 1982; Boninites, 1989).

Magmatic formations Late-Proterozoic time within the Fennoscandian Shield predominantly localized in the south-western part and the rest of the manifest is negligible. They are connected with Sveconorwegian (Grenville) stage of orogeny, which lasted in the range of 1200-930 million years ago. These manifestations are presented both typical mantle tholeiitic magmatism and typical crustal granitoidic magmatism (Arzamastsev et al., 2006; Brewer et al., 2002; Daly et al., 1983; Gower, 1985; Knudsen and Andersen, 1999). The finding of the doleritic dikes with the age of 1334 ± 11.4 million years on the Kola Peninsula boninite-like may indicate that the specific conditions of tension arose in north-eastern Fennoscandian Shield during the previous top Sveconorwegian orogeny. This conditions led to the emergence of boninite-like melts similar to the oceanic boninitic melts, but fundamentally different geodynamic conditions of generation. The identification of the boninites dikes formed in continental conditions is an important point to address the genesis boninitic melts.

This study was supported by RFBR, grant 09-05-00224.