MINERALOGY, GEOCHEMISTRY, AND PETROLOGY OF A NEW GREEN GROSSULAR GARNET (“TSAVORITE”) FOUND IN MADAGASCAR, AND ITS TECTONIC IMPLICATION

Tsavorite, a green grossular garnet (Al_1-x-yCr_xV_y)₂(SiO₄)₃, initially discovered in Tsavo (Kenya), was found in Southern Madagascar in 1997. The green color is attributed to the presence of V and Cr in isomorphic substitution for Al. In this study, we aim to (1) provide mineralogical descriptions of tsavorite along with its geochemistry and (2) understand the petrology and the tectonic implication of the deposit.

Tsavorites from Madagascar have no perfect habits, and are typically anhedral or subhedral. They are usually found either (i) as nodules, (ii) as disseminated grains, or (iii) associated with quartz; and their deposits are always contained within graphitic gneiss/leptynite enriched in V and Cr. These rocks are usually associated with marble and amphibolite. The content of V and Cr in tsavorite is significant (1915 ppm and 309 ppm), and the V/Cr ratio varies from 5.2 to 6.8 with an average of 6.3. Isotopic analysis of some tsavorite samples shows a d¹⁸O range between 12.5 to 13.9, suggesting either metamorphic rocks/waters or sedimentary sources.

The protolith is an organic-rich sedimentary rock, within which vanadium and chromium were originally concentrated. These elements were incorporated within the grossular lattice during metamorphism and fluid-rock interaction. Tsavorite-bearing rocks are characterized by the mineralogical assemblage “tsavorite + titanite + K-spar + diopside”. Index minerals (kyanite, garnet, muscovite ± K-spar) indicate crystallization of tsavorite between the upper amphibolite and granulite facies (i.e. T~500-650ºC and P~3-5Kbar). The rock assemblage described herein is not only characteristic in Madagascar but it is also very typical of the Mozambican Orogenic Belt (MOB), a belt that formed during the amalgamation of Gondwana between 450 to 900 Ma. In addition, the content in V from Madagascar's tsavorite is intermediate compared to those found in the belt. This might probably explain the role of Madagascar as located in the middle of the MOB. The MOB is oriented North-South and it extends along the East African coast, Madagascar, Sri Lanka down to Antarctica. These geologic and structural correlations explain that tsavorite deposits found in Madagascar and other locations in the MOB are similar and have experienced the same tectonic history.