Metamorphic fingerprints of Fe-rich chromitites from the Eastern Pampean Ranges, Argentina

Abstract

Chromitites hosted in the serpentinized harzburgite bodies from Los Congos and Los Guanacos (Eastern Pampean Ranges, north Argentina) record a complex metamorphic evolution. The hydration of chromitites during the retrograde metamorphism, their subsequent dehydration during the prograde metamorphism and the later-stage cooling, have resulted in a threefold alteration of chromite: i) Type I is characterized by homogeneous Fe3+- and Cr-rich chromite; ii) Type II chromite contains exsolved textures that consist in blebs and fine lamellae of a magnetite-rich phase hosted in a spinel-rich phase; iii) Type III chromite is formed by variable proportions of magnetite-rich and spinel-rich phases with symplectitic texture. Type I chromite shows lower Ga and higher Co, Zn and Mn than magmatic chromites from chromitites in suprasubduction zone ophiolites as a consequence of the redistribution of these elements between Fe3+-rich non-porous chromite and silicates during the prograde metamorphism. Whereas, the spinel-rich phase in Type III chromite is enriched in Co, Zn, Sc, and Ga, but depleted in Mn, Ni, V and Ti with respect to the magnetite-rich phase, due to the metamorphic cooling from high-temperature conditions. The pseudosection calculated in the fluid-saturated FCrMACaSH system, and contoured for Cr# and Mg#, allows us to constrain the temperature of formation of Fe3+-rich non-porous chromite by the diffusion of magnetite in Fe2+-rich porous chromite at <500 °C and 20 kbar. The subsequent dehydration of Fe3+-rich non-porous chromite by reaction with antigorite and chlorite formed Type I chromite and Mg-rich olivine and pyroxene at =800 °C and 10 kbar. The ultimate hydration of silicates in Type I chromite and the exsolution of Type II and Type III chromites would have started at ~600 °C. These temperatures are in the range of those estimated for ocean floor serpentinization (<300 °C and <4 kbar), the regional prograde metamorphism in the granulite facies (800 °C and <10 kbar), and subsequent retrogression to the amphibolite facies (600 °C and 4-6.2 kbar) in the host ultramafic rocks at Los Congos and Los Guanacos. A continuous and slow cooling from granulite to amphibolite facies produced the exsolution of spinel-rich and magnetite-rich phases, developing symplectitic textures in Type III chromite. However, the discontinuous and relatively fast cooling produced the exsolution of magnetite-rich phase blebs and lamellae within Type II chromite. The P-T conditions calculated in FCrMACaSH system and the complex textural and geochemical fingerprints showed by Type I, Type II and Type III chromites leads us to suggest that continent-continent collisional orogeny better records the fingerprints of prograde metamorphism in ophiolitic chromitites.