Phlogopite in mantle xenoliths and kimberlite from the Grib pipe, Arkhangelsk province, Russia: Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

• Published in: Geoscience Frontiers
• Main Authors: A.V. Kargin, L.V. Sazonova, A.A. Nosova, N.M. Lebedeva, Yu.A. Kostitsyn, E.V. Kovalchuk, V.V. Tretyachenko, Ya.S. Tikhomirova
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2019
• Subjects: Geology; QE1-996.5; Arkhangelsk
• Online Access: https://doi.org/10.1016/j.gsf.2018.12.006; https://doaj.org/article/bb571ee6ec624d859dfd8f727e394dbd

We present petrography and mineral chemistry for both phlogopite, from mantle-derived xenoliths (garnet peridotite, eclogite and clinopyroxene–phlogopite rocks) and for megacryst, macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle (SCLM) and the origin of phlogopite in kimberlite. Based on the analysed xenoliths, phlogopite is characterized by several generations. The first generation (Phl1) occurs as coarse, discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene–phlogopite xenoliths. The second phlogopite generation (Phl2) occurs as rims and outer zones that surround the Phl1 grains and as fine flakes within kimberlite-related veinlets filled with carbonate, serpentine, chlorite and spinel. In garnet peridotite xenoliths, phlogopite occurs as overgrowths surrounding garnet porphyroblasts, within which phlogopite is associated with Cr-spinel and minor carbonate. In eclogite xenoliths, phlogopite occasionally associates with carbonate bearing veinlet networks. Phlogopite, from the kimberlite, occurs as megacrysts, macrocrysts, microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts. Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains, which indicates that they are the disintegrated fragments of previously larger grains.Phl1, within the garnet peridotite and clinopyroxene–phlogopite xenoliths, is characterised by low Ti and Cr contents (TiO2 < 1 wt.%, Cr2O3 < 1 wt.% and Mg# = 100 × Mg/(Mg + Fe) > 92) typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences. They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene–phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before ...