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

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doi: 10.18268/BSGM2020v72n3a080420
Authors:Colás, Vanessa; Subías, Ignacio; González-Jiménez, José María; Proenza, Joaquín A.; Fanlo, Isabel; Camprubí, Antoni; Griffin, William L.; Gervilla, Fernando; O'Reilly, Suzanne Y.; Escayola, Mónica F.
Author Affiliations:Primary:
Universidad Nacional Autónoma de México, Instituto de Geología, Coyoacan, Mexico
Pontifical Catholic University of Peru, Peru
Recursos del Caribe, United States
Universidad de Zaragoza, Spain
Universidad de Granada, Spain
Universitat de Barcelona, Spain
Macquarie University, Australia
National Scientific and Technical Research Council, Universidad de Tierra del Fuego, Argentina
Volume Title:Mineral deposits of Latin America and the Caribbean
Volume Authors:Proenza, Joaquín Antonio, editor; Torró, Lisard; Nelson, Carl E.
Source:Mineral deposits of Latin America and the Caribbean, edited by Joaquín Antonio Proenza, Lisard Torró and Carl E. Nelson. Boletín de la Sociedad Geológica Mexicana, 72(3). Publisher: Sociedad Geológica Mexicana, Mexico, D.F., Mexico. ISSN: 0366-1784
Publication Date:2020
Note:In English with English and Spanish summaries. 85 refs.; illus., incl. sects., 1 table, geol. sketch maps
Summary: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+1-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.
Subjects:Alkaline earth metals; Alteration; Amphibolites; Antigorite; Cambrian; Chain silicates; Chromite; Chromitite; Continental lithosphere; Cooling; Dehydration; Electron microscopy data; Exsolution; Genesis; High temperature; Hydration; Igneous rocks; Lamellae; Lithosphere; Lower Cambrian; Magnesium; Magnetite; Metals; Metamorphic rocks; Metamorphism; Metasomatism; Mineral composition; Models; Neoproterozoic; Nesosilicates; Olivine; Olivine group; Ophiolite; Orthosilicates; Oxides; P-T conditions; Paleozoic; Plate collision; Plate tectonics; Plutonic rocks; Precambrian; Prograde metamorphism; Proterozoic; Pyroxene group; Reconstruction; Retrograde metamorphism; SEM data; Serpentine group; Serpentinization; Sheet silicates; Silicates; Spinel; Subduction; Temperature; Textures; Thermodynamic properties; Trace elements; Ultramafics; Upper Precambrian; Argentina; Pampean Mountains; South America; Los Congos Argentina; Los Guanacos Argentina
Coordinates:S330000 S310000 W0640000 W0660000
Record ID:897043-4
Copyright Information:GeoRef, Copyright 2022 American Geosciences Institute.
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