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Chromitite Formation in a Post-Collisional Setting: Insights from the Ranomena Chromite Deposit, Madagascar
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DOI：10.1093/petrology/egag040.png)
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Abstract
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The Precambrian basement of Madagascar hosts numerous yet poorly studied chromitite occurrences. The Ranomena deposit, a representative example in the North Toamasina chromite district, is hosted within lenticular mafic-ultramafic bodies of the Betsimisaraka/Anaboriana-Manampotsy domain. The chromitites exhibit disseminated to massive textures with cumulus chromite and orthopyroxene as the primary intercumulus silicate. Chromite compositions are high-Cr (Cr# mostly > 60) with elevated TiO2 (0.18–0.78 wt.%) and trace-element signatures (enrichments in Ti, Zn, Co, Mn, V, Sc; depletions in Ga, Ni) resembling those of stratiform chromitites. Whole-rock PGE patterns show relative enrichment in Os, Ir, Ru, and Rh over Pt and Pd, akin to the Lower Group chromitites of the Bushveld Complex. Zircons have been both identified in situ in thin sections and separated from mineral concentrates. Zircons from chromitite samples yield concordia U–Pb ages of 530.4 ± 2.2 Ma and 525.3 ± 2.3 Ma, respectively. Their δ18O values (4.7–6.1‰) and εHf(t) values (−5.4 to + 6.8) indicate variable degrees of crustal contamination. A binary mixing model suggests that the parental magmas formed from mantle-derived melts variably hybridized with crustal material (∼15–20% in the slightly earlier pulse). The Ranomena chromitites are coeval with post-collisional granitic magmatism in the central East African Orogen. We propose that they crystallized from mafic magmas generated during lithospheric delamination and asthenospheric upwelling in a post-collisional extensional setting, which may represent a previously unrecognized type of chromitite formation setting in orogenic belts.
Keywords:
chromitite
post-collisional setting
Madagascar
U–Pb zircon dating
mantle-derived magma
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