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Petrochemical features
of magmatites in various zones of Proterozoic subduction of Ukrainian
Shield
Agarkova
N.G., Setaya L.D., Sheremet Ye.M.
Ukrainian
State Research and Design Institute of Mining Geology, Rock Mechanics
and Mine Surveying (UkrNIMI) of the National Academy of Sciences of
Ukraine (NANU), Donetsk, Ukraine
EvgSheremet@yandex.ru
Petrochemical features of
Proterozoic magmatites from initial to final stages in different
geoblocks of Ukrainian Shield are considered from the view point of
plate tectonics.
The whole Ukrainian Shield,
as regards features of its geodynamic settings in Proterozoic, is
divided into east part (Priazovsky megablock), central part (West
Inguletsko-Krivorozhsko-Kremenchugskaya suture zone – WIKKSZ),
Inguletsky megablock and Golovanevskaya suture zone – GSZ), west part (Belotserkovsko-Bugsky
megablock, Nemirovsko-Kocherovskaya suture zone (NKSZ), and
Volyno-Podolsky megablock) (Sheremet
et al., 2011).
East part of Ukrainian
Shield (Priazovsky megablock)
Based on the diagram of
petrochemical compositions of magmatic rocks (Classification…,
1981) that shows values of average composition of intrusive complexes of
the Priazovie, the west part of subduction zone (West Priazovie) is
characterized by calc-alkali composition of magmatites and the east part
(Central and East Priazovie) ‑ by sub-alkali and alkali. Existence in
West Priazovie of magmatites of sub-alkali composition (Saltychansk and
Chernigov complexes) speaks, probably, that in the process of initial
movement of the plate under the Priazovsky megablock, together with
triggering of palingenic magmatism, there were deep cracks down to the
mantle along which carbonatite magma intruded. They were also channels
for conducting heat and fluids carried chemical (more alkali) moving
components and REE into upper layers of the earth’s crust, in
palingenesis of which Saltychansk orthite granites have formed.
Central part of
Ukrainian Shield
WIKKSZ.
Gabbro-monzonite
association is represented by gabbro-norites, enderbites, monzodiorites,
sub-alkali quartz monzodiorites, quartz monzonites and syenites.
Generally, based on their chemical composition, for these rocks typical
are large variations in SiO2 composition, calc-alkali and
sub-alkali content of alkali amount, low content of periclase with large
content of total iron and thus large iron content in rocks. Even if
judge only by chemical composition, we may say that these are not
typical magmatic but hybrid rocks formed by means of incomplete
palingenesis of substratum.
Ingulsky block (Kirovogradsky
orogen).
Therein developed are Kirovogradsky complex of quartz diorites and
tonalites, ultramafite-mafite and monzonite rocks of Korsun-Novomirgorod
complex, dyke complex of picrites and camptonites. As concerns
association reviewed in direction from ultrabasic rocks to monzonites,
we recorded increase in SiO2 in the range of 35‑55 %,
increase in the amount of alkalis from 1.2 to 6.6 % with constant
preponderance of Na2O compared with K2O and with
practically constant total iron content (70‑88 %). In this row the
content of TiO2 decreases from 5‑6 % in harzburgites to
0.8‑1.7 % in anorthosites and monzonites. Base rocks are characterized
by the highest total iron content in comparison with similar rocks of
Korosten Pluton, large content of TiO2 and increased for this
type of rocks content of K2O.
Comparison of dyke rocks
with ultrabasic rocks of Korsun-Novomirgorod massif shows that with
almost one and the same content of SiO2 ultrabasic rocks
contain more total iron content than picrites and camptonites. MgO
content in picrites is higher with almost one and the same content in
camptonites. By content of Na2O and K2O picrites
differ from ultrabasic rocks by lower values, while camptonites have
approximately comparable content of alkalis. Both picrites and
camptonites have lower total iron content in comparison with
harzburgites and troctolites of Korsun-Novomirgorod complex.
GSZ.
Ultramafite-mafites of
kapitansko-derenyukhinsky complex are represented by peridotites,
harzburgites, lherzolites, pyroxenites and gabbro. These rocks differ
from similar ultramafites of Korsun-Novomirgorod massif by almost twice
as large content of MgO and by lower content of alkalis and TiO2.
In the diagram SiO2
– (K2O + Na2O) all magmatic formations of the
central part of Ukrainian Shield by their petrochemical composition are
within the arrays of sub-alkali and calc-alkali rock
series, the latter are typically near sub-alkali series‑calc-alkali
series demarcation line. The exception is ultrabasic rocks of
kapitansko-derenyukhinsky complex of GSZ which belong to calc-alkali
series and represent derivatives of tholeitic magma.
West part of Ukrainian
Shield
NKSZ.
Therein represented are associations of
mafite-ultramafite rocks of
Bukinsky complex with crust rocks of middle and acid composition;
anorthosites, gabbronorites and leucocratic varieties of Korosten Pluton
granites; granites of Uman and Zhitomir complexes and also
gabbro-dolerites of Zvizdal-Zalesskaya dyke.
Composition
arrays for basite-monzonitoid rocks of Bukinsky massif,
gabbro-anortosite-rappakivi rocks of Korosten and Korsun-Novomirgorod
Plutons in the diagram A-F-M differ from the arrays that characterize
composition of rocks of
Skaergaard Intrusion and trapps of Siberian platform. Here rocks have
higher sub-alkali composition and higher iron content (in the diagram
arrays are shifted to the left from angle M
(МgO)
and from the line of differentiation of compared rocks).
Gabbro-dolerites of
Zvizdal-Zalesskaya dyke have increased content of titanium (ТiO2
up to 3 %),
phosphorus (Р2О5
= 0.4-0.6 %) and alkalis (Na2О
= 3.1-3.8 %;
К2О
=
1.1-1.9 %).
In the diagram SiO2-(Na2O+K2O)
they are in the array of alkali basalts. Rocks have higher iron content
(80-90 %)
in comparison with basites of Korosten Pluton.
Uman complex.
By proportion of SiO2-(Na2O+K2O)
granitoids of this complex are distributed into three arrays. The first
array is formed by normal equigranular and porphyraceous granites with
67.24‑73.48 % of SiO2 and total content of Na2O+K2O
(6.76‑9.60 %); porphyraceous granites are the lowest in this array with
somewhat increased content of alkalis. The second array is formed mainly
by aplite-pegmatoid granites and also by varieties of equigranular and
porphyraceous granites which have the highest content of SiO2
(75.50‑77.09 %). Sub-alkali granites containing up to 11.87 % of Na2O+K2O,
where K content is three and more times as large as Na content, form the
third array.
Zhitomir complex.
According to the results of chemical analysis of granites, groups of
normal and leucocratic varieties are identified. SiO2 content
in normal varieties is 71.32‑73.0 % and 74.51‑75.02 % in leucocratic
varieties. These are sub-alkali rocks containing 7.5‑8.7 % of Na2O+K2O
with preponderance of К2O
(4.4‑5.4 %) compared with Na2O (3-4 %). Total iron content is
60-80 % and low
content (0.1-0.7 %)
of MgO.
Granites of Korosten
Pluton are
basically represented by
leucocratic varieties with
73‑75 % of
SiO2. These are
sub-alkali rocks which contain 8.5‑10 % of Na2O+K2O
with preponderance of К2O
(5‑7 %) compared with Na2O
(3-4 %). Total iron content is 60-80 % and
low content (0.1-0.4 %)
of MgO.
In the diagram SiO2
– (Na2O+ K2O) granitoids rocks are in the
array of sub-alkali leukogranites. Amid
monzonitoid associations we
can identify both sub-alkali and calc-alkali associations that may speak
for their hybrid origin.
According to Sheremet et
al.( 2011)
detection of suture zones
in Precambrian of Ukrainian Shield assumes existence in this region of
magmatic rocks which would conform to those or would be close to those
typical for the Phanerozoic island arc system (Zavarnitsky-Benioff
zones). Connected with such geodynamic settings in
Phanerozoic are
andesites, low-titanium
basalts of island arcs, absorakites, shoshonites, latites, trachytes and
some leucitic (lamproites, leucites) or leucite-containing rocks.
We think that similar
differences for magmatic rocks had to be also in Precambrian, although,
perfect analogy of geodynamic settings in
Phanerozoic
and Precambrian can not be expected. However, analysis of petrochemical
and geochemical features of single-type Precambrian magmatic rocks of
Ukrainian Shield enables us to identify their essential differences,
which can be explained by different geodynamic settings of their
formation in conditions of rifting crustal shortening.
In our case the most
interesting is the type of rocks formed in
conditions of crustal
shortening, which presumably are connected with formation of suture
(subduction) zones of Ukrainian Shield.
Outstanding feature of
gabbro of these zones is their low titanium content peculiar to basites
(volcanites) of shortening zones. More over, they have rather low
content of ore minerals (magnetite and ilmenite). The gabbros are highly
amphibolized, and their amphibolization can be explained by watering of
basalt melts in subduction zones. Low titanium content of gabbro as well
as their high amphibolization differ them from basites and mafites of
Korosten and Korsun-Novomirgorod Plutons had been forming in conditions
of crustal shortening.
References:
Classification
and Nomenclature of
Magmatic Rocks. (Handbook). / O. A. Bogatikov, V. I. Gonyshakova,
S. V. Efremov et al. – Moscow: Nedra, 1981. 159 p.
Sheremet E. M.,
Kulik S. N.,
Krivdik S. G. et al.;
Geologic-Geophysical Criteria of Ore Content and Metallogeny of
Subduction Zones of Ukrainian Shield / under the editorship of A. V.
Antsiferov. UkrNIMI. Donetsk: Knowledge (Donetsk Branch), 2011.
285 p. |