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Petrology of the mafic magmatism of the Korosten anorthosite- rapakivi
granite plutonic complex
Leonid Shumlyanskyy
M.P. Semenenko Institute of geochemistry, mineralogy and ore formation,
Kyiv, Ukraine
lshumlyanskyy@yahoo.com
Korosten anorthosite – rapakivi granite plutonic complex is one of the
most “classical” complexes that belong to the
anorthosite-mangerite-charnockite granite (AMCG) association. It occurs
in the North-Western region of the Ukrainian shield and has intruded
Paleoproterozoic (c. 2100-1970 Ma) metamorphic and igneous rocks.
Korosten AMCG pluton occupies an area in c. 12 000 km2 and is
composed by wide variety of rocks: peridotite, gabbro, anorthosite and
granite with subordinate amounts of syenitic and alkaline rocks. About
20 % of the area is occupied by mafic rocks and anorthosites while the
rest belong to various granites. However, geophysical data indicate
prevalence of mafic rocks at depth (Bogdanova et al, 2004).
Mitrokhin (2001) has subdivided mafic rocks of the Korosten plutonic
complex (KPC) into five rock series: (1) early anorthosite A1;
(2) main anorthosite A2; (3) early gabbroic G3;
(4) late gabbroic G4; and (5) dyke series D5.
Geochronological data (Amelin et al., 1994; Verkhogliad, 1995; Scherbak
et al., 2008) indicate that igneous rocks of this complex were formed
during a prolonged time interval – 1800-1740 Ma. The earliest rocks in
the pluton are represented by the early anorthosites that were formed at
mid-crustal level between 1800 ± 1.3 and 1778.1 ± 8.7 Ma. During that
period dolerites of the Bilokorovychi dyke swarm – 1799 ± 10 Ma were
intruded.
In spite of the widely held opinion that most of the rapakivi and
rapakivi-like granites of the KPC were formed after emplacement of the
huge A2 anorthosite bodies, available U-Pb data indicate that
these have crystallized between 1780 ± 6 and 1764.6 ± 5.1 Ma, or even
earlier.
The main stage of the magmatic activity within the KPC occurred between
1765 and 1755 Ma. Up to now, the only dated Volodarsk-Volynskyy
anorthosite massif crystallized between 1761 ± 0.6 and 1757.1 ± 5.7 Ma.
Simultaneously were emplaced some of the plagiophiric dolerite dykes and
layered titaniferous gabbroic intrusions. Rhyolites at the bottom part
of the Ovruch depression outpoured at 1761 ± 13 Ma (Shumlyanskyy and
Bogdanova, 2009). Small bodies of the late granite-porphyre crystallized
at 1758.2 ± 2.3 Ma.
The latest stages of the magmatic activity within the KPC are
represented by the Lizniky granite massif (1752 ± 8 Ma) and veins of
rare-metal granites (1752.4 ± 8.6 Ma). The youngest known mafic rocks
are represented by sheet-like dolerite bodies revealed in the Bondary
quarry (c. 1750 Ma).
Composition and origin of the initial melts for the anorthosites and
related rocks of AMCG complexes is one of the most intriguing question
in the modern petrology. We consider that such initial melts were of
jotunitic composition. This idea is supported by wide occurrence of
jotunite dykes and chilled margins in many AMCG complexes in Norway (Duchesne
et al., 1985; Robins et al., 1997; Vander Auwera et al., 1998), Canada
(Owens et al., 1993), Poland (Wiszniewska et
al., 2002), and Ukraine. In the KPC jotunites (or jotunite-like rocks)
occur as numerous dykes within and outside the Korosten pluton and as
chilled margins of the layered gabbroic intrusions.
There are two main opinions about origin of jotunites (Owens et al.,
1993, and references therein): (1) as a result of fractional
crystallization of more primitive melt of basaltic or oxidised
“dioritic” composition with complementary crystallization of anorthosite;
(2) as a result of melting low-crustal mafic rocks. As was noted by
Duchesne et al. (1985), the geochemistry of jotunitic rocks in the
Rogaland anorthosite province (Norway) contradicts the hypothesis about
their residual origin since fractionation of huge volume of plagioclase
from the tholeiitic melt must inevitably lead to depletion of residual
melt with respect to Sr and to large negative Eu anomaly. Both features
are absent in Rogaland jotunites. In the KPC most primitive jotunites do
not display any Eu anomaly, and this feature is quite common in
fractionated jotunites enriched with REE.
Vander Auwera et al. (1998) have experimentally shown that jotunite
melts can not be obtained by fractionation (+ crustal contamination)
from more primitive mafic melts. According to these authors, jotunite
melts can be considered as initial for the Rogaland andesine anorthosite.
Massive crystallization of plagioclase (first solidus phase) leads to
formation of cumulative anorthosite; further development of this system
leads to formation of melanogabbro, often enriched with Fe-Ti oxides and
apatite.
The present author has analyzed Sr and Nd isotope composition in 12
samples of jotunite dykes and chilled margins of gabbroic massif of the
KPC. In general, their isotope composition closely corresponds to the
isotope composition of gabbroic rocks. Two dykes revealed rather
“primitive” Nd isotope composition (εNd1760 = 0.5 and 1.6),
i.e. they are close to the “early anorthosite” of the KPC.
The composition of the main rock forming minerals of anorthosites of the
main phase of emplacement is very close to the composition of minerals
that compose jotunite dykes. This may indicate that composition of the
melts from which anorthosites have crystallized was close to the
composition of jotunitic dykes. However, composition of rock-forming
minerals of gabbroids of the ilmenite-bearing layered intrusions is
slightly different – plagioclases are more sodic that may indicate more
evolved composition of the parental melt (Duchesne et al., 2006). At the
same time mafic minerals are more magnesian that may be explained by
“concurrency” with Fe-Ti oxides that effectively extracted iron from the
melt.
Both chilled margins and dykes form a single field on the variation
diagrams of chemical composition which indicates their close
relationships. However, these do not form a single trend with gabbroids
of the layered series as may be expected if these are regarded as
cumulates (gabbro of the layered series) and residual melts after their
crystallization (jotunite). Instead, jotunitic, gabbroic and
anorthositic trends converge to a single point that probably corresponds
to the composition of the initial melt. This point closely resembles
composition of the Zvizdal-Zalissya dyke. Evolution of the initial
“primitive” jotunitic melt was caused by fractional crystallization of a
“regular” gabbroic assemblage that includes plagioclase, mafic (pyroxene
and olivine), opaque (ilmenite and Ti-magnetite) minerals and apatite.
However, these minerals were effectively separated in the dense
jotunitic melt due to difference in density – plagioclase was removed
due to flotation while mafic and opaque minerals sedimented on the floor
of magmatic chamber.
Geochemistry and isotope composition of the KPC dykes and chilled
margins may shed some light of the origin of initial melts. Level of REE
fractionation ((La/Yb)N = 6…10) probably indicates presence
of garnet in the source of the melts, so the source might be represented
by garnet-bearing mafic lower crust. Chondrite-normalized pattern of the
trace elements indicate low-crustal origin of the initial melts. In
particular, these are characterized by obvious Nb-Ta anomaly that was
probably inherited from the source rock. The same origin attributed to
the negative Th anomaly. All but one studied rocks reveal deep Sr
negative anomaly that can not be explained solely by plagioclase
fractionation since this anomaly does not depend on Eu anomaly.
Probably, this feature was also inherited from the source rock.
On the ground of Sr and Nd isotope compositions, Shumlyanskyy et al.
(2006) suggested that the initial melts of the KPC might have inherited
their isotope composition from the low crust formed during the Osnitsk
orogenic event at 2000-1970 Ma. At 1800-1760 Ma these low-crustal rocks
had ɛNd = 0.0… -1.9. So, to explain positive εNd values found in some
samples one has to invoke more depleted material in the source. Such
material may be represented by the depleted mantle melts that intruded
north-western region of the Ukrainian shield at virtually the same time
(1790-1780 Ma, Shumlyanskyy et al., 2008). These melts were
characterized by positive εNd values and their input into the low crust
at c. 1800 Ma may have caused abundant melting; some mixing between
intruded melts and old low crustal material might resulted in the
slightly positive εNd values in the initial for the KPC melts.
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