Petrochemical peculiarities of Devonian magmatic complex in Dnieper-Donetsk avlakogene

Donskoy N. A.

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


In the region studied the Devonian magmatic rocks are widespread. They are separated into three subcomplexes. The most old basic-ultrabasic is represented pyroxenites, dunites, gabbros and vein rocks. Their age is D2-D3. The basaltic subcomplex is most common. The recent subcomplex is alkaline nepheline-leicite one (upper D3).

Main magmatic stages are related with Hercynian orogenisis ones. In orogenesis stage the renewing of old faults occurred. The most intensive volcanic occurrences and mineral resources related have been discovered in zone of intersection of sublatitudinal and faults.

In junction zone of Dnieper-Donetsk depression (DDD) with Azov block of Ukrainian Shield (Ush) the all of three subcomplexes are represented to the best advantage. The Devonian magmatic occurrences have been discovered at South part of the Dnieper-Donetsk frame in Khomuto-Rostov fault zone, intrusive stocks next to Zirka township (city of Mariupol), Kichiksu, Mariupolskiy and Primorskiy massifs and other. There predominantly are 1st intrusive subcomplex rocks at that area. The 3d subcomplex effusive rocks are predominantly located in central part of DDD (Belaya Tserkov▓ bowing and other small occurrences) and Pripyat depression.

Appointed petro- and geochemical characteristics are specific for Devonian volcanic complex: the 1st subcomplex rocks are enriched in TiO2, less in Fe2O3 and FeO, depleted in Al2O3 and SiO2.

The 2nd subcomplex rocks correspond average basaltoid, but they have more SiO2, Al2O3, alkali, titanium and less SiO2 and Na2O. The natrium prevails the sodium.

The 3rd subcomplex rocks are enriched in K2O, TiO2, Al2O3 and depleted in SiO2 и Na2O. K2O+Na2O/Al2O3 coefficient range is 0,88-1,65. The effusive rock compositions are close to intrusive rock ones, but they have more CaO and alkali, depleted in MgO.

The main geochemical feature of Devonian magmatic complex is high titanium, copper (predominantly in pyroxenites), fluorine, rare-earth elements of cerium group, relatively low SiO2. According this data the 3d subcomplex rocks are attributed at ultrabasic and alkaline-basaltic magmatism.

Features of metasomatic processes. The Devonian magmatic occurrences are associated with metasomatic processes. Alkalization is prevailed. Chloritization, serpentinization and ore serpentinite formation associate the 1st subcomplex rocks. Scarnization and ore polymetallic sulfides formation processes associate the 2nd subcomplex rocks. Some alkaline metasomatic stages (fenitization) such as first, medium and final are distinguished. In the fist stage a recrystallyzation of primary rocks and sodium, potassium, iron, magnesium input have occurred. The new aggregates are represented biotite,, albitite, potassium feldspar, apatite and ore minerals. The medium stage is different for primary rocks. In syenite and granosyenite hornblende and biotite have disappeared. Aegerine, sphene and nepheline have appeared. The final stage is specified by uniform paragenesis: aegerine- augite, potassium feldspar, nepheline, sphene, ore mineral and apatite. The process of potassium metasomatism, which related with beryllium, niobium and rare-earth mineralization, is actively displayed. In comparison with average composition of rock of same type, increased amount of copper (0,3%), zinc (up to 1%), nickel and molybdenum have been established. In appointed locations the explosive rocks have been transformed into mica rocks.

Alongside with 1st magmatic subcomplex rocks prevalence, 2nd magmatic subcomplex rocks of latite- trachyte type have been noticed. And the 3rd subcomplex rocks are represented small vein, jacupirangite, syenite- porphyry, bostonite and nepheline syenite bodies. Study of Devonian magmatic rocks in Pripyat depression has shown that they and magmatic rocks in junction zone of DDD with Azov block of Ush are the same type. General feature is trachytization, silica depletion and high trace elements, such as zirconium, niobium and rare-earth elements. The volcanogenic rocks are represented tuffs, more rarely lava flood. Abyssal igneous rocks are absent. Simultaneous presence of nepheline and leicite is typical difference of nepheline rocks in Pripyat depression. They are strong altered and contain a lot of sedimentary material.

In central part of DDD has also discovered a number of Devonian magmatite occurrences. Ancient dome, next to city of Khorol, structural characteristics of tuffs and lava breccias indicate nearest crater presence. The volcanic rocks compose effusive beds and tuff fields. Plutonic rocks have occurred into debris only. Three volcanogenic layers are distinguished. They are probably to correspond volcanic stages. In the bottom layer, into debris the basic-ultrabasic subcomplex rocks prevail over. Alkaline rocks dominate into debris of second layer and into thin beds of third one. Nepheline rocks often occur than leicite ones. Meltegites, iolites and nepheline pyroxenites have been observed. In another Devonian magmatism locations of studied region they are not noticed.

The volcanogenic rocks in central part of DDD are enriched in rare-earth metals, predominantly in high potassium and sodium rocks. High titanium, vanadium, zirconium, copper and lead are noted. On petrochemical and geochemical criteria these rocks are similar to Devonian magmatites in Pripyat depression.

Devonian magmatic rocks specialization. The 1st subcomplex -titanium ores (ore pyroxenites, serpentinization and chloritization products), the 2nd one √ skarns with polymetalic sulfides, the 3rd one √ complex nepheline ores, carbonate metasomatites (enriched in copper, nickel and molybdenum, beryllium and niobium mineralization).

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