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DYNA
Print version ISSN 0012-7353On-line version ISSN 2346-2183
Abstract
PEREIRA, EURICO; ORTIZ, FRANKLIN and PRICHARD, HAZEL. CONTRIBUTION TO THE KNOWLEDGE OF AMPHIBOLITES AND DUNITAS OF MEDELLÍN (COMPLEJO OFIOLÍTICO OF ABURRÁ). Dyna rev.fac.nac.minas [online]. 2006, vol.73, n.149, pp.17-30. ISSN 0012-7353.
At east and north of Medellín, a metamorphic dunite body of lensoid shape outcrops with northwest strike and an approximate area of 60 square kilometers, in tectonic contact with orthoamphibolites. These two bodies make part of a fragment of dismembered oceanic crust, being integral part of the Aburra Ofiolitic Complex (Correa and Martens, 2000), also fact admitted previously by Toussaint (1996) when considering to the amphibolites of Medellín like ophiolites and included them inside the Arquía Complex. Although up to now intermediate sectors of the such oceanic crust have not been identified yet, as the "flasser"-gabbros and the dikes complexes, the amphibolites shows a chemical print of a MORB-E enriched in the large ions lithofiles (LIL) and, in sporadic cases, of a MORB T of oceanic transitional basalts. The tectonic contact between the dunites and the amphibolites exhibits a variety of retromorphism situations and of hidrothermal alteration. The dunite is uniform in composition, highly magnesian and it contains several bodies of podiform chromite. From the metallogenic point of view it is important also to mention, for the first time, the determination of anomalous contents, overcoming in their sum 1.0 ppm, of the elements of the group of the platinum in this dunite.The most common PGM are antimonides, arsenides and alloys. Specifically they include most commonly Pd antimonides Hg-Cu-Au-bearing, sperrylite (PtAs2) and PtCu. Other PGM include one PtCuS, one Pt-Ni-Fe alloy, one Pd-Pt-Hg arsenide, one PdCu6SbAs, and one Os-Ir-Ni-sulf-arsenide. It is notorious on it the serpentinization that becomes more intense toward the areas where it is concentrated the tectonic movement. It is common to find structures of primary stratification, marked by the alignment of minerals, along the stretching lineation and microfolds that it has been interpreted as indicative that this unit it was subjected to flow deformational under low conditions of metamorphism of middle registered grade in the dunites and in the amphibolites. The intensity of the deformation and metamorphism is very difficult of explaining for the accretion of the Andean primitive arch, contemporary of the opening of the Atlantic, not Jurassic, like some authors recognize. The metabasites belongs regionally to a metamorphised sequence in amphibolite facies with garnet and they exhibit bent metamorphic foliation and shearing. Stretching lineatios and cinematic approaches of deformation indicate consistently that the address of the transport of the ophiolite would follow the azimuth N35° - 60°E and the sense of transport of the roof toward the NE, this is, on a continental border of Gondwanic affinity, hit on the edge of the Amazon craton. Being the interface oceanic crust - mantel is postulated that the dunites had a tectonic emplacement for obduction on the orthoamphibolites during a tecton-metamorphic episode, probably in the upper Paleozoic (Apalachian or Variscan orogenic cycle).
Keywords : Amphibolite; Ophiolite; Ophiolitic Complex; Platinum Group Elements (PGE); Dunite.