A MESOZOIC BACK-ARC IN THE GUERRERO COMPOSITE TERRANE (MEXICO): EVIDENCE FROM TRACE ELEMENTS AND RE-OS ISOTOPES IN PODIFORM CHROMITITES FROM LOMA BAYA

Podiform chromite deposits occur in ultramafic-mafic bodies scattered along the Pacific coast of Mexico. However, some important features of these chromite deposits, such as the nature of their parental melts, their tectonic setting and timing of formation are completely unknown. In this communication we will start to fill this gap in our knowledge, reporting the first comprehensive study of the Loma Baya chromite deposit, one of the chromite deposits found along the boundary of the Guerrero Composite Terrane (GCT).

Chromite from the Loma Baya deposit is characterized by low Cr# [atomic Cr/(Cr+Al)] =0.56-0.49 and high Mg# [atomic Mg/Mg+Fe²⁺]=0.73-0.68 with very low Fe₂O₃ [Fe³⁺# = atomic Fe³⁺/(Fe³⁺+Al+Cr)] < 0.06]. Minor and trace elements, obtained using in situ LA-ICP-MS, occur within the following ranges: Mn = 2528 to 2544 ppm, Ni = 1115 to 1837, Zn = 773 to 3936 ppm, Ti = 628 to 1013 ppm, V = 735to 971 ppm, Co = 201 to 306 ppm, Ga = 41 to 65 ppm, Sn = 8 to 17 ppm, Sc = 1 to 13 ppm. In situ Re-Os isotopes on 12 grains of laurite [Ru,Os)S₂], obtained using LA-MC-ICP-MS, reveal ¹⁸⁷Os/¹⁸⁸Os ratios ranging 0.1250 and 0.1271 (average 0.1260 ± 0.0004, 2SE) corresponding to T_Ma ≈ T_RD model ages in the range between 0.14 and 0.45 Ga, and clustering around two peaks at ca 0.3 Ga (predominant) and at ca 0.13 Ga (minor).

The compositional range shown by chromite of the Loma Baya deposit is typical for high-Al podiform chromitites like those hosted in the mantle sequence of ophiolite complexes worldwide. The melts that produced these chromite deposit had 15.3-15.8 wt% Al₂O₃ and relatively high TiO₂ (0.3-0.5) with FeO/MgO ratios varying between 0.7 and 0.8, broadly similar to those of the basalts that precipitate chromite in back-arc settings (González-Jiménez et al., 2011, Lithos 125). The clustering of Os model ages around two peaks ~0.3 Ga and ~ 0.13Ga is interpreted as the result of the emplacement of the PGE-bearing podiform chromitite bodies during Mesozoic in a back-arc setting, thus reworking a piece of lithospheric mantle that had already experienced melting during the Carboniferous. Identical ages obtained by independent geochronological methods in correlative ophiolitic rocks of the area support our interpretation. Our results therefore provide evidence of an active back-arc basin in the CGT during Mesozoic.