U/Pb Ages of zircon and monazite from the tsavorite-bearing Neoproterozic rocks of southeastern Kenya, and the significance of static crystallisation of the tsavorite

Citation:
Martelat JE, G G, D O, Bosse V, Omito E, D I’i, Nyamai CM, Wamunyu A, Monie P. "U/Pb Ages of zircon and monazite from the tsavorite-bearing Neoproterozic rocks of southeastern Kenya, and the significance of static crystallisation of the tsavorite.". In: 13th SGA Biennial Meeting 2015. Nancy, France; 2015.

Date Presented:

August 2015

Abstract:

Tsavorite, the vanadian variety of green grossular, is hosted exclusively in metasedimentary formations in the Neoproterozoic Metamorphic Mozambique Belt. In order to understand the origin of tsavorite in southeastern Kenya we integrated field investigations, geochemical studies, and dating. Two units are seen in outcrop: 1) a metasedimentary sequence, and 2) a quartz-feldspar migmatitic group. This second group of rocks shows classic "Pan-african geometry" with vertical and horizontal foliation planes. The metamorphism attains granulite facies. The metasedimentary sequence is different and recorded lower metamorphism (high-amphibolite facies, 680 °C), and if static recrystallization occurs, few traces of melting was observed. Landslide with sediment deformation structures of the sedimentary deposition are preserved. Systematic LA-ICPMS U/Pb dating was done on monazite and zircon. The migmatitic rocks recorded ages from 615 Ma to 585 Ma. The metasedimentary rocks recorded ages from 600 Ma to 595 Ma. Older ages of 850-720 Ma seems to be inherited ages from magmatic minerals. The formation of tsavorite is the result of a sedimentary sequence preserved from strain but heated by surrounding granulitic rocks between 600 to 595 Ma. The regional associated tectonic process is unclear but efficient for vertical displacement. We suggest that it is a peculiar event and process, and was unable to completely recrystallize the sedimentary rocks. These last are exceptionally preserved from strain and melt, a case that may be unique in the world.

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