Garnet zoning from the Devonian Littleton Formation, north-central Massachusetts, USA

Abstract

Compositional zoning patterns of garnet porphyroblast in kyanite-staurolite-garnet bearing sample from the Devonian Littleton Formation. north-central Massachusetts, reveal complex growth patterns_ Garnet porphyroblast exhibits asymmetric and irregular zoning patterns in Mn, Mg and Ca. The composition of the garnet at the textural boundaries is different for different traverses. and zoning reversals in XMn and Fe/(Fe+Mg) are coincident with development of inclusion trails within the porphyroblast. These observations suggest that the garnet zoning was not only modiljed from garnet-consuming reactions, but also influenced by pre-existing microfabrics through the effects of dissolution and precipitation. Relationships between chemical and textural truncations and garnet growth model proposed in this paper indicate that the zoning patterns of garnet were controlled by preferential dissolution and precipitation during the development of successive foliations that occurred in zones of shearing (high strain/stress) and zones of shortening (low strain/stress).