HIGH-Ba PHLOGOPITE FROM KIMBERLITE OF THE MALOKUONAPSKAYA PIPE (Yakutia): COMPOSITION AND 40Ar/39Ar DATING

Résumé fourni par la source

We present new data on high-Ba phlogopites first discovered in kimberlite of the Malokuonapskaya pipe. The studied phlogopites can be divided into two groups: (1) megacrysts up to 2 mm in size with a BaO content of up to 0.58 wt.%, characterized by an increase in the content of this component from core to edge (with direct zoning for BaO), and (2) zoned crystals up to 200 μm in size, containing inclusions of minerals of the kimberlite groundmass, with a BaO content of up to 14.6 wt.% in the cores, regularly decreasing to 1 wt.% toward the edges. The two generations of mica formed in different and unrelated processes. Crystallization of megacrysts took place at the earliest stages of the evolution of kimberlite melt. The slight increase in Ba content from the core to the edges of phenocrysts might be the result of the evolution of kimberlite melt leading to its enrichment in incompatible elements. Phlogopites of group 2 formed at a later stage, together with the groundmass minerals. High Ba contents in these phlogopites are possible in the case of the strong enrichment of the kimberlite melt with light elements. The refertilized lithosphere under the influence of the Siberian traps might have been the source of these elements. 40Ar/39Ar dating of the phlogopite yielded an age of 232.1 ± 2.6 Ma.

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