Geología de las ultramafitas pre-andinas de Tapo y Acobamba, Tarma, Cordillera Oriental del Perú

• Autores: Ricardo Castroviejo Bolívar, José Feliciano Rodrigues, J. Acosta, Eurico Pereira, Darwin Romero, Jorge Quispe, José Antonio Espí Rodríguez
• Localización: Geogaceta, ISSN 0213-683X, Nº. 46, 2009, págs. 7-10
• Idioma: español
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• Resumen

  • Ultramafic rocks occur scattered along a 300 km long NNW-SSE trading belt, parallel to the central Peruvian Andes in the Cordillera Oriental, from Tarma (Junín Dept.) to Huancapallac and Tingo María (Huánuco Dept.). The Tarma occurrences (Tapo and Acobamba) are dealt with here, as the first step of a broader research. The Tapo massif comprises strongly tectonised serpentinites with scarce peridotitic relics, amphibolites and podiform chromitites. It was overthrust on early Carboniferous metasedimentary rocks of the Andean basement (Ambo Group), and it shows evidences of a pre-Andean deformational history, not observed in the Ambo Group; the basal trust plane is folded by the Andean tectonics. The two smaller Acobamba occurrences are also allochtonous and show similar tectonic features. Major and trace element composition of amphibolites point to a tholeiitic basalt (to picrobasalt) protolith, compatible with an ocean-ridge or ocean-island environment. Small podiform chromitite lenses and chromite disseminations also occur; they are strongly deformed, metamorphosed and overprinted by hydrothermal alteration related to deformation, and were the subject of small scale mining. The ores comprise mainly chromite, ferritchromite, spinel, magnetite, ilmenite and scarce sulphides, as well as the secondary minerals stichtite and nimite. Results of this work exclude current interpretations of the Tarma ultramafites as autochtonus igneous intrusives, and point to a new interpreation for their emplacement.