Zoned chromite records multiple metamorphic episodes in the Calzadilla de los Barros ultramafic bodies (SW Iberian peninsula)

Abstract

Podiform chromitites occurring in the ultramafic bodies of Calzadilla de los Barros, in the Ossa-Morena zone of the Iberian Massif (SW Iberian Peninsula), were deformed and metamorphosed together with their host rocks, leading to the development of variably complex patterns of zoning in chromite grains. These patterns consist of cores with variable composition surrounded by thin rims of porous chromite. Two types of zoned chromite are observed in chromitites: (1) crystals with zoned cores characterised by progressive Mg# [=Mg/(Mg + Fe 2+ )] decrease from inner to outer core, surrounded by Fe 2 O 3 -poor, porous rims, and (2) crystals showing the opposite chemical trend in cores (progressive Mg# increase from inner to outer core) and Fe 2 O 3 -rich porous rims. Mg-rich chlorite is the only silicate mineral forming the matrix of all chromitites and filling most pores in chromite rims. Accessory chromites in dunites show complete transformation to ferrian chromite and Cr-rich magnetite. However, some of them exhibit complex chemical and textural zoning with three concentric zones (from inner to outer core and to inner rim) characterised by progressive Mg# increase and Cr# [=Cr/(Cr + Al)] decrease, surrounded by an outer rim with almost the same composition as the inner core. Some chromite grains from massive chromitites (defined here as higher than 85 vol.% chromite) still preserve inner core compositions unaffected by metamorphic transformation. These inner cores show high-Al composition (Cr# = 0.48–0.52) with Mg# ranging from 0.65 to 0.70. These compositions resemble those reported for Al-rich, podiform chromitites in ophiolites elsewhere and indicate that chromitites from Calzadilla de los Barros derived from a MORB melt type in a MORB or supra-subduction zone setting. The chemical and textural variations in zoned chromite from chromitites and dunites can be interpreted in terms of a multistage process characterised by two episodes of retrograde metamorphism separated by a high-temperature heating event. This interpretation suggests a correlation with the tectono-metamorphic evolution of the Neoproterozoic basement of the Ossa-Morena zone.