Evolution of Protherozoic and Paleozoic alkaline nepheline series in South-Western part of the Russian Platform

Donskoy A.N., Donskoy N.A., Legkaya L.I.

M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation, Kiev, Ukraine

donick_gg@mail.ru

 

There are two series of alkaline rocks which are wide spread on territory covered: Protherozoic of alkaline nepheline syenites; Paleozoic (Devonian) of trachyte basitic-ultrabasitic, leicite-nepheline series.

In recent time on the Ukrainian Shield (USh) and adjacent territories more than ten massifs of Protherozoic series containing nepheline rocks, ores have been established. These series' massifs, complex in construction and composition, were formed after Archaean and Early Protherozoic folding system had been finished. Their generations initiated by Early Protherozoic (PR₁³) tectonic-petrogenic energization of consolidated structures in South-Western part of the Russian Platform (mainly the USh). This period has been established like subplatform or late orogenic by different researchers. The alkaline massifs, which are complex in construction and have variety of rock association, confined to lifted consolidated blocks of subplatform stage of the USh. On border areas of Precambrian shields the relation of alkaline massifs to deep fault zones (usually their crossing zones) existing for a long time is a characteristic feature.

During this period in this geodynamic situation the complex Korosten and Korsun-Novomirgorod massifs, containing gabbro-anorthosite, rapakivi-granite, gabbro-wehrilete, subalkalic granitoid, lamprophyre, dyke, different metasomatic and other rock associations, were forming.

On the whole, every massif has particular geological and geochemical peculiarities:

1.      They have clear zonal construction and are confined to fault crossing zones of sublatitude and submeridional directions.

2.      There is no clear difference between magmatic and metasomatic formations on compositional and structure-tectonic criteria.

3.      They are generated in the form of layered inclined vein (bodies).

4.      Effusive and extrusive rocks are absent.

5.      From old rocks to new ones the temporal consecution is usually alkaline altered granite - granosyenite - quartz syenite - alkaline syenite - foyaite - mariupolite - alkaline pegmatite - albitite - carbonatite - lamprophyre.

6.      Regularity of formation of alkaline rocks depends on sodium and potassium behaviors. At the same time the distinct relation of increased contents of zirconium, tantalum, niobium, rare earth elements and alkali has been observed.

7.      Sodium has special role during mariupolitization and albitization processes. Main sodium concentrator minerals are nepheline, albite and aegerine. In the process of intensive metasomatism the sodium minerals as sodalite, cancrinite have been evolved.

8.      Potassium in comparison with sodium is dependent one. At the time of foyaite and alkaline pegmatite formation process the contents of potassium and sodium are approximately equivalent. Biotite substitutes aegerine as potassium metasomatism in mica rock.

9.      Isovalent isomorphism of potassium and sodium in feldspars is clear displayed.

10.   Metasomatic rocks belong mainly to sodium and sodium-potassium rock association.

11.   Aluminum to alkali sum ratio is prevailed (miaskite type). During alkaline rock formation the aluminum content has been increasing then decreases slightly.

12.   Increased aluminum content is related to alkali sum. Rare and rare-earth elements concentration is related to sodium metasomatism (Donskoy, 1982, 1985, 1988).

Devonian intrusive complex, including occurrences of alkaline magmatism, frames the Ukrainian Shield in north-east and north. They are widespread in Dnieper-Donetsk depression. But most large massifs are located in zone of junction between Donbass and Azov domain of the USh (Pokrovo-Kireevo), in the central part of Dnieper-Donetsk depression (Belotserkovka knoll), south-eastern slope of the USh (Khomutovka massif) and in Azov domain of the USh (Zirka massif) and others.

The rocks of basic-ultrabasic series are widespread in structures and stratified beds of the Dnieper-Donetsk depression. They are localized in particular structure zones occurred in the margins in the Dnieper-Donetsk depression.

On list of some features it may suppose to find new occurrences of these series’ rocks.

The rocks of upper Devonian basic-ultrabasic leucit-nepheline series belong to 3^rd final subcomplex of Devonian intrusive-volcanogenic association. Shapes of the rock bodies are different: circle-like (Pokrovo-Kireevo), stock (Khomtovka), flows (Pripyat Depression).

Data of isotopic age vary 250-383 Ma. Data based on paleontologic data are upper Devonian. Therefore the formation of alkaline rock association belongs to upper Devonian age.

These series rocks are aluminum, alkali liquor and may be diamonds ore bearing. They (principally Pokrovo Kireevo) have already huge resources of nepheline ores which may be used for nepheline-feldspar and other concentrate producing.

In the central part of Pokrovo-Kireevo structure the thick zones of orthoclasites with beryllium, niobium, rare-earth mineralization have been found. Beryllium contents vary from 0.0025 to 0.97%. Thickness of the zones is from 1 to 35 m. Distribution of beryllium is random. Beryllium-bearing orthoclisites are usually intensively altered. Beryllium minerals as gadolinite, bavenite, bertrandite, beryllite, barilite have been found. In beryllium orthoclasites have increased Nb contents from 0.1 to 1%. Concentrator minerals of niobium as columbite and ilmenoruthile have been determined. In beryllium metasomatic rocks increased contents of rare-earth elements, titanium and zinc (seldom) have been indicated. Concentrator minerals of rare-earth elements as rinkite and monazite have been found. In orthoclasites increased Cu (up to 0.3%) and Zn (up to 1%) contents have been indicated. Monazite is general concentrator mineral (Donskoy, 1993, 1992).

Comparison of alkaline rock associations to other regions shows that the massifs of nepheline rock association of syenite series of ancient platforms as the Ukrainian Shield and the Baltic Shield, occurrences of Southern Africa and Canadian platform are most ancient formations of the world. The middle Paleozoic alkaline occurrences are characterized by increasing of alkaline series areas and their variety. Among the Ural Hercynides the miaskite massifs have been formed. On the Baltic Shield Lovozero and Khibiny plutons are limited to paleorift zone. In Kazakhstan, center part of North Mongolia, Transbaikalia these rock series are related to middle Paleozoic rift formation.

 

References:

Donskoy A.N. Geochemical peculiarities of sodium and potassium behavior of alkaline rock association in Priazovie // Geochemistry and Ore Formation. 1982. No 10. P. 92-96 (in Russian).

Donskoy A.N. Geochemistry of metasomatic processes of alkaline rocks in the Ukrainian Shield // Geochemistry and Ore Formation. 1985. No 13. P. 63-69 (in Russian).

Donskoy A.N., Kulish E.A., Donskoy N.A. Nepheline rocks of Ukraine: complex aluminium-alumina and rare-metallic ores. Kiev: Logos. 1988. 222 p (in Russian).

Donskoy A.N., Sharkin O.P. Geochemistry and mineralogy of niobium and tantalum of alkaline rocks in the Ukrainian Shield // Geochemistry and Ore Formation. 1992. No 19. P. 118-136 (in Russian).

Donskoy A.N. Evolution of Devonian magmatism in Dnieper-Donetsk Depression // Geochemistry and Ore Formation. 1993. No 20. P. 72-90 (in Russian).