Early stage evolution of the mafic-ultramafic belt at La Melada, Sierra de San Luis, Argentina: P–T constraints from metapyroxenite pseudosection modelling

At La Melada, Sierra de San Luis, Argentina, two lenses of mafic-ultramafic rocks, which underwent Famatinian metamorphism followed by shear deformation, form part of a NNE–SSW-trending belt along the eastern flank of the Sierra. Metapyroxenites from the lenses are granoblastic rocks made up of igneous and metamorphic minerals, with varying degrees of deformation and mylonitization. These rocks preserve evidence of post-magmatic re-equilibration that followed the emplacement of the mafic-ultramafic rocks. Microstructural investigation and P–T pseudosection modelling suggest that La Melada metapyroxenites experienced a temperature and pressure decrease from the M0 (800 °C < T < 920 °C; 0.7 GPa < P < 0.9 GPa) to the M1 stage (720 °C < T < 780 °C; 0.6 GPa < P < 0.7 GPa). Further re-equilibration led to the formation of talc + cummingtonite during the M2 stage and to the development of metamorphic plagioclase and dolomite in the post-M2 stage. The P–T segment reconstructed for La Melada metapyroxenites is consistent with a back-arc setting and with P–T paths previously reported in the literature.

► Ultramafic rocks from La Melada, Sierra de San Luis, preserve M1, M2, and post-M2 stages of metamorphic re-equilibration.
► M1 stage is defined by the formation of amphibole + spinel on igneous minerals.
► M2 is defined by talc and cummingtonite and post-M2 stage by formation of plagioclase and dolomite.
► La Melada ultramafic rocks experienced a P–T decrease from igneous to post-M2 stage.