STATIC UHT METAMORPHISM AND PARTIAL MELTING IN THE MID-CRUST: A SEM AND TEM STUDY OF MELT-RELATED METAMORPHIC REACTION TEXTURES AT THE MICRO- TO NANO-SCALE

The Tasiuyak gneiss of northern Labrador is a belt of migmatitic granulite-facies paragneiss formed during the late stages of the Paleoproterozoic Torngat orogeny. During the Mesoproterozoic, it was host to the c. 1.35-1.29 Ga Nain Plutonic Suite, a mid- to upper-crustal anorogenic granite, anorthosite, diorite to troctolite intrusive complex. Intrusions in the mid-crust produced aureole temperatures of up to 900°C. In the vicinity of Anaktalik Brook, wedges of this paragneiss lie within multiple contact aureoles.

The paragneiss in these aureoles are unique in having reached granulite-facies conditions and undergone partial melting twice. Due to deformation during the Torngat orogeny, there was significant melt connectivity and melt loss during regional metamorphism, leaving behind a relatively dry bulk composition. Later, contact metamorphism locally reached UHT conditions (≥900°C) based on thermodynamic modeling of the PT conditions for melting, combined with the observation of melt-related microstructures, and the evidence for the presence of the UHT mineral osumilite.

Detailed SEM mineral maps of thin sections document metamorphic reaction textures, including evidence for melt producing reactions related to the breakdown of course-grained regional metamorphic porphyroblasts. The combination of SEM and TEM imaging provides a detailed look at the grain boundaries of static melt-related microstructures. This is the first study to image evidence for partial melting in granulite-facies rocks at the nano-scale. At this scale, thin films and 'bubbles' of feldspathic minerals provide evidence for melt along grain boundaries of melt-related microstructures, enhancing our understanding of the possible extent of melt connectivity, and the nature of melt crystallization. It is likely that melt partially coated the majority of grain boundaries associated with melt-producing reactions, allowing a relatively continuous network for the exchange of elements required during many of the observed metamorphic reactions. Given the impact of partial melting on the rheological properties of rocks, the ability to detect nano-scale evidence for melt coating grain boundaries could be extremely important in the understanding of the physical processes associated with granulite-facies metamorphism.