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Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

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



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 = 610) 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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