Diversity of potassium-bearing tourmalines in diamondiferous Kokchetav UHP metamorphic rocks: A geochemical recorder from peak to retrograde metamorphic stages

Chemical variations and inclusion mineralogy of tourmaline in diamond-bearing ultrahigh-pressure (UHP) gneisses and related siliceous rocks from the Kokchetav Massif, Kazakhstan are extensively investigated, in conjunction with a review of previously discovered K-bearing tourmaline with microdiamond inclusions. Tourmalines in these Kokchetav UHP rocks have a wide compositional range (dravite - " potassium-oxy-dravite" - schorl - uvite - feruvite - foitite, and some of their oxy-variants) with potassium content ranging from 0 to 0.576 atoms per formula unit (apfu, based on Y+Z+T cations=15) (or 0-2.76wt.% K₂O) and decreasing from core to rim. The variation in potassium is spatially related to the inclusion mineralogy and is attributed to changes in metamorphic grade. Four stages of tourmaline growth in the Kokchetav UHP rocks are recognized based on the potassium contents and occurrences; in order of decreasing P and T: (1) K-dominant tourmaline (K>Na, Ca, and X-vacancy) with microdiamond and high-SiTi phengite inclusions and without quartz inclusions; (2) K-rich tourmaline (K: 0.2-0.05apfu) with graphite, phengite, kyanite and quartz inclusions; (3) K-poor tourmaline (K: 0.05-0.01apfu) with matrix quartz and biotite; and (4) K-free anhedral or overgrown tourmaline associated with low-P-T minerals such as amphibole or chlorite. Several lines of evidence also suggest that K-dominant tourmaline crystallized in the diamond stability field. Tourmalines in the Kokchetav UHP rocks recorded multi-stage growth history from peak UHP conditions through a granulite facies overprint event including partial melting and ultimately to the latest retrograde metamorphism during exhumation.