Boletín de la Sociedad Geológica Mexicana

Volumen 72, núm. 3, A280719, 2020


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


Evidencias del metamorfismo en cromititas ricas en Fe del oriente de las Sierras Pampeanas, Argentina


Vanessa Colás1,*, Ignacio Subías2 †,, José María González-Jiménez3, Joaquín A. Proenza4, Isabel Fanlo2, Antoni Camprubí1, William L. Griffin5, Fernando Gervilla3,6, Suzanne Y. O’Reilly5, Monica F. Escayola7


Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, 04510, CDMX, México.

Universidad de Zaragoza, Departamento de Ciencias de la Tierra, Pedro Cerbuna 12, 50009 Zaragoza, Spain.

† Rest in peace.

Departamento de Mineralogía y Petrología, Universidad de Granada, Facultad de Ciencias, Avenida. Fuentenueva s/n, 18002 Granada, Spain.

Departament de Mineralogia, Petrologia i Geologia Aplicada, Universitat de Barcelona. C/Martí i Franquès s/n, 08028 Barcelona, Spain.

ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia.

Instituto Andaluz de Ciencias de la Tierra (Universidad de Granada-CSIC), Avenida las Palmeras 4, Armilla, 18100 Granada, Spain.

Instituto de Ciencias Polares y Ambientales ICPA, Universidad de Tierra del Fuego-CONICET, Fueguia Basket 251, 9410-Ushuaia, Tierra del Fuego, Argentina.

* Corresponding author: (V. Colás) This email address is being protected from spambots. You need JavaScript enabled to view it.



How to cite this article:

Colás, V., Subías, I., González-Jiménez, J. M., Proenza, J. A., Fanlo, I., Camprubí, A., Griffin, W. L., Gervilla, F., O’Reilly, S. Y., Escayola, M., 2020, Metamorphic fingerprints of Fe-rich chromitites from the Eastern Pampean Ranges, Argentina: Boletín de la Sociedad Geológica Mexicana, 72 (3), A080420.



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 ºC 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 ºC 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 ºC. These temperatures are in the range of those estimated for ocean floor serpentinization (<300 ºC and <4 kbar), the regional prograde metamorphism in the granulite facies (800 ºC and <10 kbar), and subsequent retrogression to the amphibolite facies (600 ºC 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.

Keywords: altered chromite, minor and trace elements, thermodynamic modelling, prograde metamorphism, Eastern Pampean Ranges, Argentina.