Dunite tectonites of Medellín, department of Antioquia, Colombia
DOI:
https://doi.org/10.32685/0120-1425/bolgeol28.3.1987.305Keywords:
Serpentinization, deformational flow, subsolidus, thrust contact
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Copyright (c) 1987 Servicio Geológico Colombiano
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Abstract
To the east and north of Medellín, there is an elongated body of metamorphic dunite with a northwest direction and an approximate area of 60 km2, associated with orthoamphibolites of the late Precambrian (?) early Paleozoic.
The dunite tectonite is uniform in composition, highly magnesian, and associated with podiform chromite. It shows variable degrees of serpentinization, but this is more intense towards the tectonized marginal zones. Primary mineral foliation, lineation, and microfolding indicate that the tectonite was subject to deformational flow under high-pressure, high-temperature conditions and the metamorphic textures suggest recrystallization under subsolidus conditions.
The dunitic body has partly subhorizontal tectonic contacts (thrust contact) with the orthoamphibolites, or Quaternary cover masks them. No evidence of igneous contacts is observed. However, locally metasomatic rocks are present.
The petrological and structural characteristics of the dunite body suggest that it is the basal part of a dismembered ophiolitic sequence, which possibly formed as a refractory residue during the partial melting event in the mantle. Tentatively, a tectonic emplacement is postulated, apparently by abduction onto the orthoamphibolites in the Late Jurassic-Late Cretaceous.
During or after emplacement, the dunite tectonite was locally subjected to low-grade metamorphism, with serpentinization and formation of mineral associations of the green shale facies.
The metabasites (amphibolites) belong to a regionally metamorphosed sequence in amphibolite facies. Their chemical characteristics are like those of the toleiitic basalts, and they are in the field delimited for unaltered gabbros of the Atlantic ridge. The rocks follow the differentiation trend of the toleiitic magmas using an AFM diagram.
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