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 http://www.multitran.ru/c/m.exe?t=3254584_2_1Breton 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 http://www.multitran.ru/c/m.exe?a=110&t=1635707_2_1&sc=2submeridional 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 http://www.multitran.ru/c/m.exe?a=110&t=2877024_2_1&sc=517avlakogene 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, http://www.multitran.ru/c/m.exe?a=110&t=139422_2_1&sc=27hornblende, 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).