Melt segregation in late syn-tectonic anatectic migmatites: an example from the Onawa contact aureole, Maine, USA

We present field and petrological observations on the nature of layer-parallel leucosomes in late syntectonic migmatites in the contact aureole of the Onawa pluton, central Maine, USA, and use the observations to constrain a model for the formation of these leucosomes by deformation-controlled melt segregation during anatexis. Observations include: 1) the preferred position of lit-par-lit leucosomes near or at contacts between graded sedimentary beds; 2) the presence of symmetric melanosomes adjacent to leucosome layers; 3) evidence for asymmetric inflation of leucosomes by melt; 4) wide variability in leucosome modal mineralogy; 5) the similarity of plagioclase compositions and zoning trends in leucosomes and adjacent mesosomes; 6) quartz and plagioclase clusters in mesosome aligned parallel to the axial surface of regular folds of the layers; 7) symmetric alkali element depletion trends in mesosomes adjacent to lit-par-lit leucosomes; 8) textural evidence indicating that melt was involved in leucosome formation. Based on these observations, we propose a model for lit-par-lit leucosome formation by differential stress-driven melt segregation from less competent, micarich layers, or portions of graded layers, to more competent quartzo-felspathic layers, or portions of layers, leading to layer inflation by melt inflow, rather than segregation of melt in shear or tensile structures. Syn-anatectic contractional folds in part controlled the patterns of melt migration to the leucosomes. Variable proportions of unmelted material initially present in the leucosome layers, of melt added to these layers and minerals crystallized from this melt, and of melt lost from the layers to external sinks explain the variability in lit-par-lit leucosome compositions in these rocks.