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Martelat J. E., Paquette J. L., Bosse V., Giuliani Gaston, Monie P., Omito E., Simonet C., Ohnenstetter D., IchanG'I D., Nyamai C., Wamunyu A. (2017). Chronological constraints on tsavorite mineralizations and related metamorphic episodes in southeast Kenya. Canadian Mineralogist, 55 (5), 845-865. ISSN 0008-4476

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Lien direct chez l'éditeur doi:10.3749/canmin.1700019

Titre
Chronological constraints on tsavorite mineralizations and related metamorphic episodes in southeast Kenya
Année de publication2017
Type de documentArticle référencé dans le Web of Science WOS:000412799400002
AuteursMartelat J. E., Paquette J. L., Bosse V., Giuliani Gaston, Monie P., Omito E., Simonet C., Ohnenstetter D., IchanG'I D., Nyamai C., Wamunyu A.
SourceCanadian Mineralogist, 2017, 55 (5), p. 845-865. ISSN 0008-4476
RésuméTsavorite 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 (KuraseTB 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 theactivity 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 degrees C of the whole region between 510 and 500 Ma.
Plan de classementGéologie et formations superficielles [064] ; Géophysique interne [066]
Descr. géo.KENYA
LocalisationFonds IRD [F B010071370]
Identifiant IRDfdi:010071370
Lien permanenthttp://www.documentation.ird.fr/hor/fdi:010071370

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