High pressure metamorphic conditions in garnet amphibolite from a collisional shear zone related to the Tapo ultramafic body, Eastern Cordillera of Central Peru

Abstract

A discontinuos belt of elongated ultramafic rock bodies (mostly serpentinites) occurs in the Eastern Cordillera of the central Peruvian Andes. One of the main occurrences is the Tapo Massif, a lense-shaped serpentinite body, ~2 km x 5 km, comprising small podiform chromitite deposits (Castroviejo et al., 2009) and bands or lenses of garnet-amphibolite, both strongly sheared and thrust upon the upper Paleozoic sediments of the Ambo Group (Fig. 1). Metabasite geochemistry suggests a mid-ocean ridge or an ocean island protolith. The whole sequence can be interpreted as a disrupted ophiolitic complex (Castroviejo et al., 2010). The geological setting of the Tapo occurrence is described by J.F. Rodrigues et al. (2010). To get information about its geotectonic setting we applied new geothermobarometric techniques to the garnet amphibolite. Finding representative samples with an adequate mineralogy to apply these techniques is in this case a difficult task. A common problem is the almost ubiquitous overprinting by serpentinisation or retrograde metamorphism and, locally, by metasomatism or alteration enhanced by deformation, producing a variety of rock types, as rodingites, birbirites and listvaenites. Nevertheless, careful sampling followed by petrographic examination of the rocks allowed to identify some samples in which useful assemblages are present.