Extreme chromite alteration in Antanimbary chromitites from the Maevatanana belt, Tsaratanana sheet (Northern Madagascar)

The Maevatanana Belt, located in the north-central Madagascar, is the westernmost of three belts (Andriamena, Beforona and Maevatanana) belonging to the Tsaratanana Sheet. This tectonic unit, individuated by Collins (2006), is composed of mafic gneisses, tonalites, chromite-bearing ultramafic rocks and meta-pelites, some of which were metamorphosed to ultra-high temperatures at 2.5 Ga (Goncalves et al., 2004). Chromitites were found close to the village of Antanimbary in the northern part of Maevatanana Belt, but unlike nearby chromitites, hosted within ultramafic bodies of Andriamena belt, they have never been studied. Seven separate chromitite lenses, forming a 3 km long, NE to SW trending, alignment, outcrop close to the Ikopa river bank. They are included within a metamorphic unit, known in literature as orthoamphibolite, composed of sodic plagioclase, horneblenditic amphibole and minor biotite and quartz. All chromitites are massive with more than 70 and up to 90 modal % chromite and have a cumulus texture where the cumulus phase is always chromite with intercumulus silicates. Chromite grains, completely altered in ferritchromite, ranging from 0.5 to 0.1 mm in size, are euhedral, with fractured cores and porous rims, and enclosed in an anhedral silicate gangue. The most common silicate mineral is chlorite, whose composition falls in the fields of sheridanite and clinochlore and shows detectable Cr2O3 contents (1-2 wt%), which are anyway low if compared to kammererite usually associated to ferritchromite. Tremolitic to actinolitic amphibole and orthopyroxene also occur, often in intergrowth. Serpentine and titanite are present as accessory silicate phases. Ilmenite, as tiny inclusions in chromite grains, is the most common non silicate gangue mineral, followed by rutile, monazite, magnetite and rarely pyrrhotite. Chromite crystals (isolated or in aggregates) preserve the original shape even if they are completely altered in ferritchromite. A slight core to rim zonation occurs with broken and porous ferritchromite cores surrounded by a more porous corona showing a stronger alteration. Grains never preserve composition of primary chromite. Generally ferritchromites are very low in Cr2O3, never exceeding 43 Cr2O3 wt%. FeO is high, ranging between 29.23 and 32.81 wt%, calculated Fe2O3 is never below 6.60 wt% and reaches very high values, up to 28 wt%, in more altered ferritchromite grains. MgO is extremely low, systematically below 2.14 wt%. Al2O3 content is strongly variable, with the lowest and highest limits at 3.33 and 21.96 wt%. Ferritchromite composition, plotted in XFe vs XCr and XFe vs XFe3+ “fried egg diagrams”, shows a best fit with layered mafic-ultramafic intrusions (Barnes and Roeder, 2001), confirmed by composition of chromite alteration rims from Bird River Sill (Ohnenstetter et al., 1986). Quite constant high values of XFe, ranging between 0.89 and 0.94, together with variable XCr evidence that alteration of primary chromite is complete and occurred firstly at reducing conditions with a primary substitution of Fe2+ on Mg. Only after almost complete loss of Mg iron is oxidized to Fe3+ and substitutes for Al and Cr.