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Boletín de la Sociedad Geológica Mexicana Volumen 72, núm. 3, A280719, 2020 |
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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
1 Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, 04510, CDMX, México.
2 Universidad de Zaragoza, Departamento de Ciencias de la Tierra, Pedro Cerbuna 12, 50009 Zaragoza, Spain.
† Rest in peace.
3 Departamento de Mineralogía y Petrología, Universidad de Granada, Facultad de Ciencias, Avenida. Fuentenueva s/n, 18002 Granada, Spain.
4 Departament de Mineralogia, Petrologia i Geologia Aplicada, Universitat de Barcelona. C/Martí i Franquès s/n, 08028 Barcelona, Spain.
5 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.
6 Instituto Andaluz de Ciencias de la Tierra (Universidad de Granada-CSIC), Avenida las Palmeras 4, Armilla, 18100 Granada, Spain.
7 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. http://dx.doi.org/10.18268/BSGM2020v72n3a080420
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 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.
