Serpentinites from Central Cuba: petrology and HRTEM study

Abstract

We have sampled serpentinites and the closely associated metabasites in the paleosubduction zone context of Central Cuba in order to characterise the microstructures of serpentine minerals as a function of metamorphic grade. The samples were collected in the eclogitic unit of the Escambray Massif and in the Zaza Zone, where eclogitic conditions were locally attained. Serpentinites are associated to the metabasites in lenses embedded in a metasediment matrix (Escambray) or form the matrix that embeds the metabasites (Zaza Zone). Field and petrological evidence suggests that serpentinites and associated metabasites underwent the same metamorphic history. Serpentinites from the Escambray Massif have preserved high-grade structures. In the Zaza Zone, most of the sampled serpentinites only underwent low-grade metamorphism and the sample that underwent eclogitic conditions is strongly retrogressed. Low-grade serpentinites show pseudomorphic textures in thin section, a mixture of chrysotile, poorly crystallised serpentine and minor lizardite. High-grade samples, characterised by non-pseudomorphic textures, mainly consist of antigorite, associated to minor chrysotile. Transmission electron microscopy shows that antigorite from preserved samples (Escambray Massif) displays few microstructural defects such as stacking faults or modulation dislocations, in contrast to the higher density of defects encountered in highly retrogressed serpentinites (Zaza Zone). We thus propose that the record of high metamorphic grade in matrix antigorite is best characterised by the elimination of structural defects.