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Elvira Ibragimova, A.P. Karpinsky Russian Geological Research Institute (VSEGEI) (Russian Federation)
Tatiana Epifanova, Saint-Petersburg State University (Russian Federation)
Oleg Kazanov, Kola Mining and Geological Company (Russian Federation)
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This paper deals with REE data on the platinum-bearing intervals of the East Pana layered massif. As REE are known to be one of the most efficient indicators of the petrologic processes, they were used to reveal possible sources of the PGE-bearing rocks as well as rocks underlying and overlying them.
East Pana layered massif is the part of the Fedorovo-Pansky Igneous Complex (Fennoscandian Shield, NW Russia), aged 2491±1.5 Ma. It is hosted by Archean granitoids and Paleoproterozoic volcanic rocks of the Imandra-Varzuga riftogenic structure. Based on cumulus stratigraphy the massif has been divided into 4 cyclic zones (from bottom to top): Lower Marginal Zone (LMZ), composed of noncumulus fine-grained mafic rocks; Gabbronorite Zone 1 (GNZ1), represented by various gabbronorites; Gabbronorite Zone 2 (GNZ2), mainly composed of trachytic gabbronorites and Gabbro Zone (GZ) which consists of coarse-grained gabbro with olivine gabbronorites in its footwall part.
As significant low-sulfide PGE mineralization is strictly confined to the boundary of GNZ1 and GNZ2, rocks of these cyclic units were tested for REE distribution. Also rocks of the chilled margin of the massif (LMZ) were considered as they are believed to represent an initial melt of the intrusion.
The distribution of REE in rocks of LMZ corresponds to moderately fractionated magma melt, the rocks are enriched in light REE and have high REE concentrations compared to the upper units.
There is a distinct difference in distribution of REE in the lower and upper parts of GNZ1. Rocks of the lower part of GNZ1 correspond to less fractionated melt than LMZ rocks. There is a distinct negative Eu anomaly which could be explained by the selective removal of Eu from magma which happened between the formation of LMZ and GNZ1. The only process to explain this could be primary crystallization of cumulus plagioclase and its removal to the upper levels of the magma chamber because of gravitation or convection processes. This resulted in a complementary positive Eu anomaly in the upper part of GNZ1. Thus, we observed 2 subzones in GNZ1 which are complementary to each other with respect to REE distribution. The crystal differentiation seems to be the main process of the formation of this unit. This supposition is consistent with Sm-Nd isotopic data; according to them the rocks of GNZ1 have similar and values, varying from -1.1 to -1.5.
Rocks of GNZ2 are very different from the rocks of GNZ1 with respect to REE distribution, though they are close to the rocks of LMZ. It is evident that between GNZ1 and GNZ2 formation the composition of the intrusive melt had significantly changed. The reason for this could be a replenishment of the intrusive chamber by a large portion of magma which was close in composition to the initial melt of the intrusion. In this case both these portions of magma could have had a common source as the rocks of GNZ2 have uniform εNd values (-1.2) very similar to those of GNZ1.
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