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 Volume 2 Chapter 2

Chapter 2

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


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, 31 p. 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 down going 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.


Resumen 

L​a “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.



Abbreviations ​ 

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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