A reappraisal of the metamorphic history of the Tehuitzingo chromitite, Puebla state, Mexico

The Tehuitzingo ultramafic body (Acatlán Complex, southern Mexico) is a strongly serpentinized harburgitic body that encloses chromitite bodies and blocks of eclogitic rocks. Hydrous retrograde metamorphism on chromitite bodies resulted in the formation of: i) partly altered chromite characterized by unaltered cores surrounded by Fe²⁺-rich and Al-depleted porous chromite containing chlorite; ii) porous chromite corresponding to a chromite that was entirely transformed to Fe²⁺-rich and Al-depleted porous chromite; and iii) zoned chromite formed by modified cores surrounded by Fe³⁺-rich non-porous chromite and magnetite rims. The content of minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in the cores of partly altered chromite preserves the magmatic fingerprint of back-arc-bearing chromitites, while the cores of zoned chromite are enriched in Zn, Co and Mn but depleted in Ga, Ti, Ni and Sc, and display a metamorphic signature. P-T pseudosections performed in the system CrMFASH allow us to constrain the temperature of formation of Fe²⁺-rich chromite by reaction of magmatic chromite with olivine between 584 and 449 °C; while the new thermodynamic calculations performed in the FMASH system shows that the Fe³⁺-rich non-porous chromite and magnetite rims in zoned chromites were formed at lower temperatures (i.e. 270–340 ºC and < 20 kbar). Temperatures of alteration obtained using our new thermodynamic model are in agreement with the qualitative estimation that use minor and trace elements in Fe²⁺-rich chromite (i.e. 560–700 ºC). These temperatures are more restricted than those estimated for chlorite hosted in partly altered (193–481 ºC) and porous chromite (158–255 ºC), and those reported in host serpentinite and associated eclogitic rocks (210–399 ºC). A comparison of P-T conditions estimated by our new thermodynamic models and the distribution of minor and trace elements in the different zones of chromites allow us to infer that the alteration took place during the hydrous retrograde metamorphism, and that the high-pressure metamorphism did not modify the composition of chromites. Thus, the microstructural zoning displayed by chromite grains was formed during the exhumation of a segment of a back-arc-related oceanic lithosphere, at similar temperature conditions than the Tehuitzingo serpentinite.