Serpentinites and epidote amphibolites from Ituango, department of Antioquia, Colombia
DOI:
https://doi.org/10.32685/0120-1425/bolgeol28.3.1987.329Keywords:
Ultramafite, Valdivia group, Paleozoic, harzburgite, ophiolitic, toleitic composition
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Copyright (c) 1987 Servicio Geológico Colombiano
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Abstract
The serpentinites and epidote-amphibolites of the Ituango area form a discontinuous body, very long and thin, north-south trending, and 35 km long, which is part of the mafic-ultramafic belt, alpine type, related to the Romeral tectonic zone. This body is embedded within a folded eugeosynclinal sequence (Valdivia Group), with low-grade e metamorphism and possibly Paleozoic age. The lithotypes that form it have tectonic contact with each other and with the encasing metamorphic rocks.
The sepentinite, sheared and internally fractured, consists of antigorite and/or lizardite and chrysotile. It has a MgO/MgO + FeO* ratio between 0.82 and 0.85 and MgO/SiO2 between 0.92 and 1.07, suggesting that serpentinization was an isochemical process, accompanied only by the addition of water and that the protolith was mainly a harzburgite. The relict chemical and mineralogical characteristics hint that this rock was a residue of partial melting in the mantle.
Gneissic and schistose epidote-amphibolites differing in mineralogy, degree of recrystallization, and shearing were distinguished; they are gradational from each other, the second type predominating towards the base of the ophiolitic thrust. The chemical composition of these rocks follows the toleiitic trend, is like that of the dredged intermediate gabbros of the Atlantic ridge and is in the field of high-Ti ophiolites. The protolith was, perhaps, a slightly magnesium-rich homogeneous gabbro generated in a marginal Basin ridge.
In the Late Cretaceous-Early Tertiary, the Ituango body was emplaced solidly, like a dismembered ophiolitic fragment along deep east-dipping reverse faults. During emplacement, the rocks were recrystallized and penetratively deformed. The emplacement at the present level is post-Early Miocene and continues today because of the active movement of the faults bounding these rocks.
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