Exotic origin of the ruby deposits of the Mangari area in SE Kenya

Mercier, Alain; Debat, Pierre; Saul, John M.

doi:10.1016/s0169-1368(99)00002-5

Cited by 50 publications

(34 citation statements)

References 47 publications

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“…These rocks were metamorphosed at lower grade than the Kurase-HT gneisses, under amphibolite-facies conditions, and in a very static regime. Ages close to 615 Ma are only recorded in sample Voi35, which is strained, and at Rockland, where local metasomatism and higher metamorphism occurred (granulite-facies conditions, Mercier et al 1999). Our results show that these three units, Kasigau Group, Kurase-HT gneisses, and Kurase-TB metasediments, were metamorphosed at the same time (620 to 600 Ma), but under different P-T and strain conditions.…”

Section: Synchronous Metamorphism Of Kasigau and Kurase Groupsmentioning

confidence: 59%

“…This is in agreement with P-T conditions estimated for the Rockland mine. Thermobarometry applied to the garnet-biotite-sillimanite-graphite-bearing gneisses yields temperature and pressure in the amphibolite facies (650 8C, 7 kbar, Mercier et al 1999). Locally, parageneses of the ruby-bearing plagioclasite (e.g., sapphirine embedded in micas) and the associated metamorphosed ultrabasites are consistent with their formation under granulite-facies conditions (T ¼ 700-750 8C and P ¼ 8-10.5 kbar, Mercier et al 1999).…”

mentioning

confidence: 85%

“…2) dated at 580-550 Ma (Hauzenberger et al 2007, Bauernhofer et al 2008. Detailed geological descriptions of the Kasigau and Kurase Groups are available in several reports and associated maps (Sanders 1963, Pohl et al 1980, Saggerson 1962, Frisch & Pohl 1986, as well as in more recent works (Mercier et al 1999, Simonet 2000, Hauzenberger et al 2007, Bauernhofer et al 2008. We have summarized and added to them in the following sections.…”

Section: Geological Settingmentioning

confidence: 99%

“…Ages of the ruby-bearing plumasites (Rockland mine) are interesting in two ways: first, they are associated with a rare example of metasomatism associated with gem formation, and second, the desilication occurred under local granulite-facies conditions (T ¼ 700-750 8C and P ¼ 8-10.5 kbar, Mercier et al 1999). In this case, separated zircon and monazite crystals display similar results with ages of 612.4 6 0.5 and 609.0 6 3.3 Ma, respectively, attesting to their synchronous crystallization.…”

Section: Kurase Tsavorite-bearing Metasedimentsmentioning

confidence: 99%

“…Thermobarometry applied to the garnet-biotite-sillimanite-graphite-bearing gneisses yields temperature and pressure in the amphibolite facies (650 8C, 7 kbar, Mercier et al 1999). Locally, parageneses of the ruby-bearing plagioclasite (e.g., sapphirine embedded in micas) and the associated metamorphosed ultrabasites are consistent with their formation under granulite-facies conditions (T ¼ 700-750 8C and P ¼ 8-10.5 kbar, Mercier et al 1999). The ruby formation is associated with local metasomatism, with fluids that transformed the pegmatite in ruby 6 apatite 6 uvite 6 phlogopite-bearing plagioclasite, and the adjacent ultramafic rocks in phlogopitites.…”

mentioning

confidence: 99%

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Chronological Constraints On Tsavorite Mineralizations and Related Metamorphic Episodes In Southeast Kenya

Martelat¹,

Paquette²,

Bosse³

et al. 2017

Can Mineral

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Tsavorite is exclusively hosted in the Neoproterozoic Metamorphic Mozambique Belt (NMMB). The gemstone mines, widespread between Kalalani (Tanzania) and Mgama Ridge (Kenya), define a continuous corridor over a hundred kilometers in length. The tsavorite is hosted by a metasedimentary sequence defined as the Kurase tsavorite-bearing metasediments (Kurase-TB metasediments) that also hosts rubies. These metasediments underwent amphibolite-facies metamorphism and are surrounded by granulitic gneisses that are also of sedimentary origin (the Kurase high-temperature gneisses). All these rocks lie below the Kasigau Group, a unit dominated by granulite-facies metamagmatic rocks.To constrain the timing of events that led to this peculiar occurrence of tsavorite, we have performed geochronological analyses of thin sections and of separated grains of zircon, monazite, and rutile using LA-ICP-MS and ID-TIMS, as well as 40 Ar/ 39 Ar of muscovite and phlogopite from various lithologies. The results show that the different terranes were metamorphosed synchronously between 620-580 Ma but under different P-T strain conditions. The Kurase-HT gneisses and the rocks from the Kasigau Group are highly strained and underwent granulite-facies metamorphism with abundant partial melting and emplacement of felsic melts between 620 and 600 Ma. Textural observations also underlined a late regional water flux controlling the occurrence of V-free muscovite and monazite mineralizations at 585 Ma. The latter event can be related to the 1 activity of the Galana shear zone, in the east. The Kurase-TB metasediments escaped strain and partial melting. They record amphibolite-facies conditions with static heating, since initial sedimentary structures were locally preserved. The age of the tsavorite mineralization was inferred at 600 Ma from metamorphic zircon rims and monazite from the closest host-rocks, sampled in the mines. Hence, tsavorite crystallization occurred statically at the end of the metamorphic event, probably when the temperature and the amount of volatiles were at maximum levels.Conversely, the ruby formed by local metasomatism of felsic dikes and isolated ultramafic bodies. The rubies are older and zircons and monazites from a ruby-bearing felsic dike (plumasite) were dated at 615 Ma. Finally, data from rutile and micas indicate a global cooling below 430 8C of the whole region between 510 and 500 Ma.

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Section: Synchronous Metamorphism Of Kasigau and Kurase Groupsmentioning

confidence: 59%

mentioning

confidence: 85%