Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Temperature evolution of early Proterozoic ultrabasite intrusion of the Belomorsky region (Karelia)

Akimenko M.I., Asavin A.M.

GEOHI RAS

Development of large Cu-Ni deposits of the Kola peninsula (Monchetundra) causes significant interest in the Early Proterozoic intrusions of basic and ultrabasic composition, which are known as “Belomorsky druzite complex”, for the first time described more than of a century ago. We have investigated one of the manifestations of ultrabasite discovered by the geophysic and confirmed before field geological studies.

As a result a petrographical study it is established that ultrabasite are orthopyroxene -clinopyroxene of species. Chemical analysis and conversion on CIPW(Table 1). Additional interest is dispersed sulfide impregnations in the sample (Fig. 1). Perhaps further work will clarify the composition and the paragenesis of these sulfides.

Table 1. The chemical composition of the ultrabasic sample and recalculate on the CIPW norm

Primary large (modify) orthopyroxene crystals occur in the rock. It forms large grains or porphyroblasts with the decay structures, often with ingrowths of other minerals – olivine, clinopyroxene (Fig. 2). The composition of orthopyroxene is quite ferruginous enstatite up to 75% and 24% ferrosilite. Apparently, these inclusions are the products of the reaction changes the primary intrusive harzburgite and conversion them into orthopyroxene lherzolites. Olivine in thin section is represented by two generations of large porphyritic crystals and fine aggregate intergrowths with clinopyroxene (Fig 3-4).

For a given species rock is characterized by the absence of serpentinization of olivine, which indicates high-T sub-magmatic processes after conversion. Such processes could be associated with a common regional stage of high-temperature metamorphism (Early Proterozoic ?), or with intra-chamber postmagmatic processes – pyroxenezatiton, amphibolization.

The low degree of weathering of rock raises the problem of estimating the age of these manifestations. If we consider them as part of the Proterozoic igneous complexes, then they should have by now be strongly serpentinized. Absence of serpentinization can be attributed to either the younger age of the intrusions, or younger age post-magmatic processes and metamorphism. In any case, the development of small area of that type of rocks this in the evidence of either their affinity to the apical or the root of the plutonic bodies.

Clinopyroxene in the rock in the main forms euhedral small crystals, in contrast to orthopyroxene in it is not observed decay structures (Fig. 3) Individual clinopyroxene crystals are larger and with more rough borders, probably formed at the magmatic stage. Sulphides form rims around orthopyroxene crystals and single grain (Fig. 1). The chemical composition of minerals is presented in Table 2.

Table 2. The chemical composition of minerals.

Spinel group minerals contain 50% magnetite component and about 20% of chromite component, you can also note the high titanium content (up to 12.9% ulvoshpinel component). The calculation of spinel-olivine equilibrium shows [Fabri 1975] a temperature of about 300C. Calculation of bipyroxene thermometer [Perchuk, 1976], the formation temperature of clinopyroxene was 800.

The history of the rock, so apparently included several stages. The primary magma - olivine-orthopyroxene (formed from the large rock structure panidiomorf). Secondary magmatic (? Metasomatic) phase of 800-900 ° C - pyroxenezatiton: Clinopyroxene -olivine-orthopyroxene equilibrium. Metasomatic transformation - metamorphism of 300-400 ° C - clinopyroxene-olivine-spinel.

Reference

1.        Perchuk LL, Ryabchikov ID (1976) Phase correspondence in mineral systems // Cyrillic. M. "Nedra", 1976, 287 pp.

2.        Fabri Cs J (1979) Spinel-olivine geothermometry in peridotites from ultramafic complexes // Contrib Mineral Petrol 69:329-336