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The Petrologic Nature of the "Medellín Dunite" Revisited: An Algebraic Approach and Proposal of a New Definition of the Geological Body
Antonio GARCIA–CASCO, Jorge Julián RESTREPO, Ana María CORREA–MARTÍNEZ, Idael Francisco BLANCO–QUINTERO, Joaquín Antonio PROENZA, Marion WEBER, and Lidia BUTJOSA
https://doi.org/10.32685/pub.esp.36.2019.02
ISBN impreso obra completa: 978-958-52959-1-9
ISBN digital obra completa: 978-958-52959-6-4
ISBN impreso Vol. 2: 978-958-52959-3-3
ISBN digital Vol. 2: 978-958-52959-8-8
Citation is suggested as:
Garcia–Casco, A., Restrepo, J.J., Correa–Martínez, A.M., Blanco–Quintero, I.F., Proenza, J.A., Weber, M. & Butjosa, L. 2020. The petrologic nature of the “Medellín Dunite” revisited: An algebraic approach and proposal of a new definition of the geological body. In: Gómez, J. & Pinilla–Pachon, A.O. (editors), The Geology of Colombia, Volume 2 Mesozoic. Servicio Geológico Colombiano, Publicaciones Geológicas Especiales 36, p. 45–75. Bogotá. https://doi.org/10.32685/pub.esp.36.2019.02
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Abstract
The “Medellín Dunite", the main ultramafic body of the Central Cordillera of Colombia, constitutes a fragment of oceanic lithospheric mantle formed at a back–arc basin/incipient arc scenario emplaced onto the western continental margin of Pangaea during Triassic time. This body has been classically, and is still considered, mainly of dunite composition. However, in spite of two subsequent metamorphic imprints that obscure the primary mantle mineralogical composition, there is petrographic and geochemical evidence that points to a harzburgitic nature of the unit. In order to overcome the petrographic effects of medium–T metamorphism, metasomatism, and serpentinization, we analyzed published and new major–element geochemical data by means of algebraic methods to approximate the mantle mineralogical composition of ultramafic rocks. The restored mantle mineralogy clearly indicates that the body is mainly of harzburgitic composition, and therefore we propose that the term “Medellín Dunite" should no longer be applied to avoid terminological confusion. Furthermore, a phase–relation approach in simple systems for the metamorphic evolution allows identifying the main reason for the contradictory terminology used so far: olivine is paragenetic (stable) with tremolite and talc during medium–T (ca. 600 °C) metamorphic imprint undergone by the body. During this initial metamorphic event, characterized by full hydration (as opposed to the late–stage serpentinization), mantle pyroxenes reacted out and medium–T olivine formed while high–T olivine persisted metastably as a likely consequence of moderate temperature and sluggish diffusion kinetics. On the other hand, we analyze two likely geodynamic scenarios to provide a common context of metamorphism for the ultramafic body and associated metabasites (Aburrá Ophiolite): (i) ocean–floor metamorphism and (ii) intra–backarc subduction–initiation metamorphism. The latter allows a new tectonic view of the Aburrá Ophiolite, formed by tectonic units from the upper and downgoing plates on a nascent active plate margin. For all these reasons, we propose the new term “Medellín Metaharzburgitic Unit" in order to combine in a single term the original high–T mantle composition, its subsequent metamorphic transformation, and the independent tectonic character of the ultramafic body.
Keywords: Medellín Dunite, Medellín Metaharzburgitic Unit, metaharzburgite, phase relations, ophiolite.
La “Dunita de Medellín", principal cuerpo ultramáfico de la cordillera Central de Colombia, constituye un fragmento de manto litosférico oceánico formado en un ambiente de cuenca de retroarco/arco incipiente emplazado sobre el margen continental occidental de Pangea durante el periodo Triásico. Este cuerpo es y ha sido clásicamente considerado de composición esencialmente dunítica. Sin embargo, y a pesar de dos eventos metamórficos subsecuentes sufridos que han enmascarado su composición mineralógica mantélica primaria, existe evidencia petrográfica y geoquímica que indica una naturaleza harzburgítica generalizada de la unidad. Para evitar los efectos petrográficos del metamorfismo de T–media, metasomatismo y serpentinización, analizamos datos geoquímicos publicados y nuevos de rocas ultramáficas mediante métodos algebraicos para deducir su composición mineralógica mantélica primaria. La mineralogía mantélica reconstruida claramente indica que el cuerpo es principalmente de composición harzburgítica y, por tanto, proponemos que el término “Dunita de Medellín" no debe aplicarse en el futuro para evitar confusiones terminológicas. Aún más, un análisis de las relaciones de fases en sistemas simples durante la evolución metamórfica sufrida permite identificar la razón principal por la cual se ha llegado a esta terminología contradictoria: el olivino es paragenético (estable) con tremolita y talco durante el evento metamórfico generalizado de T–media (ca. 600 °C) sufrido por el cuerpo. Durante este evento metamórfico inicial, caracterizado por hidratación completa (a diferencia de la serpentinización tardía), se consumieron los piroxenos primarios mantélicos y se formó olivino, en tanto que el olivino de T–alta persistió de forma metaestable probablemente como consecuencia de una cinética de difusión lenta a temperatura moderada. Por otro lado, analizamos dos posibles ambientes geodinámicos para ofrecer un contexto común para el metamorfismo del cuerpo ultramáfico y las rocas básicas asociadas (Ofiolita de Aburrá): (i) metamorfismo de fondo oceánico y (ii) metamorfismo de inicio de subducción intra cuenca de retroarco. Este último modelo permite una nueva conceptualización tectónica de la Ofiolita de Aburrá, conformada por unidades tectónicas pertenecientes al techo y muro del incipiente margen de placa activo. Por todo ello, proponemos el nuevo término “Unidad Metaharzburgítica de Medellín" para unir en una sola expresión la composición mantélica original de T–alta, la subsiguiente transformación metamórfica y la naturaleza tectónica independiente del cuerpo ultramáfico.
Palabras clave: Dunita de Medellín, Unidad Metaharzburgítica de Medellín, metaharzburgita, relaciones de fases, ofiolita.
An Anorthite
Atg Antigorite
Ath Anthophyllite
Brc Brucite
Cal Calcite
Chl Chlorite
Chu Clinohumite
CIPW Cross, Iddings, Pierson, and Washington
CMAS CaO–MgO–Al2O3–SiO2
CMS CaO–MgO–SiO2
CNPq Conselho Nacional de Desenvolvimento Científico e Tecnológico
Cpx Clinopyroxene
Di Diopside
Dol Dolomite
En Enstatite
Fl H2O–fluid
Fo Forsterite
Grt Garnet
IAT Island–arc tholeiite
ICP–OES Inductively coupled plasma optical emission spectrometry
LOI Loss on ignition
Lz Lizardite
Mgs Magnesite
MORB Mid–ocean ridge basalt
Ol Olivine
Opx Orthopyroxene
P Pressure
Per Periclase
Qz Quartz
Spl Spinel
T Temperature
Tlc Talc
Tr Tremolite
* Total Fe expressed as FeO
** LOI Loss on ignition
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