THE CHROMITITES OF TEHUITZINGO SERPENTINITES (ACATLÁN COMPLEX, SW MEXICO): AN EXAMPLE OF NO MOBILIZATION OF THE PGE DURING HIGH-PRESSURE METAMORPHISM

The Tehuitzingo serpentinites and their associated chromitites (Acatlán Complex, South of Puebla State, SW Mexico) are associated with eclogitic rocks, and embedded in a metasedimentary matrix, suggesting that the serpentinites, chromitites and associated eclogitic rocks underwent the same metamorphic history. The chemical composition of primary chromite of the Tehuitzingo chromitites is representative of ophiolitic chromitites and typical of refractory-grade chromitites (Al-rich). Our data suggest that the Tehuitzingo serpentinites and associated chromitites are a fragment of oceanic lithosphere formed in an arc/back-arc environment, and equivalent to a mantle sequence of ophiolites from a supra-subduction zone. The Tehutizingo chromitites have low PGE contents, ranging between 102 and 303 ppb. The chondrite-normalized PGE patterns are characterized by an enrichment in Ir-subgroup elements (IPGE=Os, Ir, Ru) relative to the Pd-subgroup elements (PPGE=Rh, Pt, Pd). In addition, all the chromitite samples display a negative slope from Ru to Pd [(Os+Ir+Ru)/(Pt+Pd)=4.78-14.13]. These patterns (plus the PGE abundances) are typical of ophiolitic chromitites elsewhere. All the analyzed samples roughly exhibit the same chondrite-normalized PGE patterns of non-metamorphosed ophiolitic chromitites. Thus, the PGE distribution of the Tehuitzingo chromitites has not been significantly affected by high-pressure metamorphism (eclogitic facies). The chondrite-normalized PGE patterns of the Tehuitzingo serpentinites also indicate that the PGE distribution in the residual mantle peridotite exposed in this area has not been affected by the different serpentinization events, as the serpentinites show a similar pattern to that of the peridotites that were only partially serpentinized, present in mantle sequences of non-metamorphosed ophiolites In conclusion, our data suggest that in the chromitites and serpentinites from Tehuitzingo, no significant redistribution (or concentration) of PGE occurred neither during metamorphism, nor during other alteration processes. If some PGE mobilization occurred, it was restricted within individual chromitite bodies without changing the bulk-rock PGE composition.