Abstract

Pseudotachylytes, coseismic frictional melts, have formed in metagabbro and within a kinzigitic shear zone of the Ivrea Zone (Italy). In both rock types, amoeboid garnet has crystallized in the pseudotachylyte matrix, locally as a rim overgrown on pre-existing garnet clasts. This new garnet has been previously used to constrain the frictional melt formation in such rocks by applying geothermo-barometric calculations. Here we present a complete microstructural, textural and compositional characterization, by high-resolution scanning electron microscopy, focused ion beam tomography, electron backscatter diffraction and electron microprobe analysis. The garnet in pseudotachylyte exhibits subtle major element compositional and microstructural differences between the core and the overgrown rim. This suggests that the new garnet formed by rapid epitactic overgrowth on clasts of relic host rock garnet, during frictional melt cooling and solidification.

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