Composition of trace elements in zircon of different genetic types from the Sakharjok syenite massif. Kola Peninsula
Vetrin V. R., Skublov S. G.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2015. V. 144. N 3. P. 1-13
Zircon is an accessory mineral in alkaline and nepheline syenites of the Neoarchean Sakharjok intrusion and, together with britolite and pyrochlore, forms ore-bodies in nepheline syenites of this massif. Zircon crystals have non-homogeneous zonal, someplace mosaic structure due to fragments and zones related to magmatic, hydrothermal and metamorphic stages of their formation. Magmatic zircon, relatively to other its genetic types, has the higher REE concentration (1769 ppm in average) and distinct Ce-maximum (Ce/Ce* = 105 in average) and Eu-minimum (Eu/Eu* = 0.19), without correlation between them. Composition of hydrothermal zircon is characterized by the low Ce/Ce* ratio (0.7—3.9, average 2.0), elevated LREE contents, and lowered ratios of MREE and HREE to La. Metamorphogenic zircon differs from magmatic one by the sharply lower REE concentration (385 ppm in average), lowered Th/U (0.32) and Ce/Ce* (31.9 in average) ratios. On diagrams in coordinates Ce/Ce* — MREE/La minimum values of these ratios are typical for hydrothermal zircon, medium and maximum — for metamorphic and magmatic zircons, respectively; these differences allow to delineate fields of their compositions credibly. Distribution of points of the zircon composition on these diagrams reflects the dependence of lanthanoid elements chemical activity on oxidation conditions of the zircon crystallization.
Key words: zircon; magmatic, hydrothermal, metamorphic genesis; trace elements, rare earth elements, basicity of lanthanides, redox conditions of crystallization.