Some geochemical and mineralogical peculiarities of alkaline rocks of the Ukrainian Shield

Kryvdik S.G., Dubyna A.V.

N.P. Semenenko Institute of geochemistry, mineralogy and ore formation NAS of Ukraine, Kyiv, Ukraine

kryvdik@igmof.gov.ua

 

At present there are known above 40 massifs and small occurences alkaline and subalkaline rocks, carbonatites and kimberlites within the Ukrainian Shield (USh). Most of them are Proterozoic (1,7-2,1 Ga) and only  in Azov area they are as well as Proterozoic and Devonian. Alkaline rocks of USh belong to two main complexes: alkali-ultrabasic or carbonatitic and gabbro-syenitic. They are also different age: first – 1,9-2,1 Ga, and second – 1,7-1,8 Ga

Proterozoic alkaline granites (Perga complex in North-West of USh) and Devonian dykes of grorudite (Easr Azov) have a limited spread. On the basis of those data it is possible to distinguish alkali-granite complexes in USh too.

It is established or assumed that some alkaline and subalkaline syenites are spatial and genetically related to anorthosite  rapakinigranite plutons. Such syenites are significantly enriched in REE, Y and Zr. Two unusual rare metal deposits – Yasrebeckoe and Azovskoe (rich in Zr, TR, Y ores) are related to those syenites.          

Geochemical and mineralogical peculiarities of alkaline rocks of USh depend on two main factors: 1) geotectonic setting; 2) depth of erosion in their massifs.

In deep eroded massifs of alkali-ultrabasic complex, dark-colored minerals are more enriched in iron and less alkaline than same minerals from hypabyssal massifs. About considerable deep erosion cut (to 20 km) of Chernigovka (Novo-Poltavka) massif an assumption was expressed in 1989 yet. Following investigations of alkaline-ultrabasic massifs and their occurrences of USh showed the similarity to Chernigovka massif (deep eroded Proskurovka and Antonovka ones) hypabyssal Gorodnica, Glumcha and Bolarka small intrusions as well as dykes of Pokoshivka zone. The dark-colored minerals from alkaline rocks in Proskurovka and Antonovka massifs are similar by composition to ones in Chernigovka. Essentially enriched in iron clinopyroxene with negligible alkalinity (10-30% aegirine component) while content of hedenbergite component reach to 30-35% are typical for them. The higher content of  hedenbergite component (up to 63%) is fixed in aegirine-ferrosalite from fenites in Mala Tersa massif. Increased content of hedenbergite component is observed in pyroxenes from alkaline rocks in considerably eroded carbonatitic complexes of other regions.

In fact aegirine pyroxenes and alkaline (calciumless) amphiboles in alkaline rocks of Chernigovka, Proskurovka and Antonovka massifs are not revealed. Amphiboles in this rocks are represent by enriched in iron hastingsite, rarely cataphorite and edenite, often – by intermediate varieties of hastingsite-cataphorite, edenite-cataphorite series, occasionally by richterite.

Fluorite and primary rare-earth-fluorine carbonates in rocks of compared massifs are not revealed, although apatite, amphibole and mica from those rocks have increased or high fluorine content.

At the same time in hypabyssal occurrences of alkali-ultrabasic rocks (Gorodnica, Glumcha and other) the pyroxene in alkaline rocks belong to diopside, aegirine-diopside or aegirine. The diopside and aegirine-diopside are typical for jacupirangite-ijolite series of this rocks but aegirine is representative in fenites. It is observed alkaline amphibole in fenites of these occurences.

 

Table. Concentrations of indicated dash-elements (ppm, ТіО₂, %) from alkaline-ultrabasic complexes of Ukrainian Shield

 

 

The alkaline rocks mentioned of deep eroded complexes are also characterized by relatively low magnesity (Mg#) of biotite (50-60%). Perhaps, unusual very enriched in iron olivine (up to Fa₇₀) from carbonatites of Chernigovka massif can be explain by a deep erosion cut.

In fenites of such complexes sodium commonly prevails over potassium that is also typical for considerably eroded carbonatitic massifs.

There are some another mineralogical and geochemical peculiarities that corroborated the expressed opinion about considerable erosion cut of these massifs. Although the exact estimation of erosion cut of Dnestrov-Bug’s alkaline-ultrabasic massifs (Proskurovka and Antonovka) seems impossible at this time. However it could be assume this value was no less then 10 km (possibly up to 20 km for Chernigovka massif). It should be noted that these massifs located within granulite’s rocks and level of erosion cut of separated blocks in this area achieved from 18-26 up to 34-37 km (dates of mineralogical geothermobarometry). 

Consequently there are indirect and direct data about considerable erosion cut of Chernigovka, Proskurovka and Antonovka alkaline-ultrabasic massifs.

As preliminary conclusion can be  surmise about essential level of erosion cut is reflecting  of geochemical peculiarities of alkaline rocks. At any case, the hypabissal intrusions (Gorodnica, Glumcha, Pokoshevka's dykes) are significant poorer in Zr, Nb and REE than deep-eroded Proskurovka and Antonovka massifs (Tabl.). The similar pattern is exhibited in the line-elongated Chernigovka carbonatite complex. The results of the last and previous analytical investigations and  papers allow to reveal a considerable distinctions in the north, central and south parts of these line-elongated massifs. Even greater distinctions are in Begim-Chokrak occurrence which is additionally studying now in respect of mineralogy and geochemistry.

The most full collection of rocks - alkaline pyroxenites, nepheline and alkaline syenites with various carbonatites are represent in central and northern parts of Chernigovka massif. Only carbonatites and fenite-syenite rocks occur in south part of this massif.

The dark-colored silicate minerals from all types of carbonatites at south part are more magnesian than ones from same type of rocks at northern and southern parts of massif. Sometimes in carbonatites from southern part of these massif tetrafferiphlogopite occurs, enough rare mineral for carbonatites in Azov area. There is relatively magnesian olivine Mg# 78-85%, whereas in carbonatites from northern part (including their dolomitic varieties) the magneseti achieves of this minerals only 30%. The clinopyroxenes from carbonatites and xenolith of melteigite in southern part belong to aegirine-diopside while in central part – aegirine-salite and aegirine-fferosalite. Moreover, carbonates (calcite and dolomite) of carbonatites from southern part of the massif contain noticeably less (up to total disappearing) iron than from ones central and northern parts where content FeO in calcite from sövites achieve 2,3% and in dolomite of beforsites – 4,4-5,8%. Still lower FeO content is in calcite of carbonatites from  Begim-Chokrak occurence.

By iron distribution in coexist carbonates the pressure (up to 6 kBar) and correspondingly erosion cut (up to 20 km) as well as temperature by A.S. Talantsev, J.R. Goldsmith geobarometers were calculate.

The investigation of C and O isotope composition in carbonates from carbonatites also confirms the available regularity: in the southward occurring oxygen isotopes become more heavy  (in northern and central part of massif δ¹⁸О is 5-10‰, in  southern – 10-13‰, and in Begim-Chokrak – achieve to 15-17‰ i.e. overstep the limits of typical carbonatite data).

Similar directional dependence is found by distribution rare metals. All type of carbonatites in southern part of massif contain much less Nb (10-100, occasional 200 ppm) in comparision with northern and central parts (200-3000 ppm). The monazite is an usual accessory mineral (occasional significant content) in carbonatites of northern part, whereas in southern part it isn’t found or occurs rarely, i.e. REE completely are incorporated in carbonates and apatites. As a whole Chernigovka carbonatite massif essentially differ from overwhelming majority of carbonatite complexes in other regions.

However determining of optimal deep of the highest possible rare metal concentration (or some of them)  for carbonatitic complexes isn't possible now. It is possible to predict that the richest ores can be massifs with medium (5-15 km?) erosion cut. Anyway, the hypabissal carbonatitic complexes are rarely ore-bearing in comparison with eroded ones although deep erosion cut can destroy thickness carbonatitic bodies (stocks). However, the geochemical specialization of alkaline complexes is caused by geodynamic conditions too. We think the alkaline-ultrabasic massifs of western part of the USh had been formed by compression conditions of earth crust (in the blocks between sutural zones), while in eastern part of these region (Azov area) the massifs of alkaline rocks had been confined to rift zones.