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Servicio Geológico Colombiano

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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

By Antonio GARCIA–CASCO, Jorge Julián RESTREPO, Ana María CORREA–MARTÍNEZ, Idael Francisco BLANCO–QUINTERO, Joaquín Antonio PROENZA, Marion WEBER and Lidia BUTJOSA

Pages 33–51

Manuscript accepted October 29, 2018 


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 Pangea 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 us to identify 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.