Geochemical features of mafites from the mid-Paleoproterozoic Elet’ozersky layered complex of alkaline-mafic-ultramfic rocks in Northern Karelia (Russia)

 E.V. Sharkov*, V.V. Shchiptsov**, A.V. Chistyakov*, V.B. Belyatsky***

*Institute Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, Moscow’

** Institute of Geology, Karelian Scientific Center RAS, Petrozavodsk

***Institute of Precambrian Geology and Geochronology RAS, St-Petersburg

 

Elet’ozero massif in Northern Karelia is one of the largest (about 100 km²) layered mafic-ultramafic intrusions with alkaline core. Its peripheral  part composes mainly fine-grained marginal gabbros, and dominated inner portion – by Layered unit, represented alternation of peridotite, clinopyroxenite, gabbros (olivine gabbro, gabbro and gabbro-anorthosite) and their ore-bearing varieties. All rocks of this series in a variable degree enriched in Fe-Ti-oxides (titanomagnetite and ilmenite) of 10 vol.% on average till 30-40 vol.% and more in ore varieties. Central part of the massif (about 10% of the area) composed by alkaline rocks: nepheline syenites, Ne-bearing syenites and carbonatites. According to (Kogarko et al, 1995), age of the mafic rocks is about 2080 Ma (Sm-Nd isochrone); eNd = +3. Age of syenite body is 2071±40 Ma (zircon U-Pb method, SHRIMP-II data), i.e. rather close to mafic rocks.

          Specific feature of the mineral composition of gabbros, predominated in the Layered unit is presence of unusually high-calcic plagioclase (up to An₈₄) with high contents of  Ba and Sr, high-Ba biotite, Ti-augite and relatively ferruginous olivine (Fo_30-35); apatite represented by F-apatite.

According to our geochemical data, REE contents in the studied rocks are close to OIB which, probably, were parental magma. However, Elet’ozero cumulates on multicomponent diagrams in compare to lavas in coeval plateaus showed that concentration of many components often are not agree and sometimes opposite each other. For example, high contents of Ti, Eu, and Ba in cumulates are at variance with their contents in lavas, and U, Th, Nb, Eu and, essential, Sr, are opposite. We suggest that all these differences can be a result of composition of major cumulus minerals which remain in the crust when the melt was departed from the chamber and began to own live.