Abstract
Abstract The present work reports the first anatectic melt inclusions found so far in the Mesoarchean basement in East Greenland. Using optical microscope observations and MicroRaman spectroscopy, we show that garnets in metasedimentary migmatite contain primary polycrystalline aggregates which can be confidently interpreted as former droplets of anatectic melt, i.e. nanogranitoids. In some cases, they coexist with coeval fluid inclusions under conditions of primary fluid-melt immiscibility. The re-evaluation of the metamorphic pressure and temperature conditions with up-to-date phase equilibria modelling, combined with the identification of nanogranitoids and fluid inclusions, suggests metamorphic peak equilibration and partial melting in presence of a COH-fluid at T ~1000°C and P > 7 kbar. To date, this is the oldest verified occurrence of nanogranitoids and fluid-melt immiscibility during garnet growth in a partially molten environment.
Highlights
Primary crystallised melt and fluid inclusions (MI and FI) in metamorphic minerals such as garnet or zircon constitute an invaluable archive to identify and investigate partial melting processes and crustal differentiation (e.g. Cesare et al 2015; Nicoli, Ferrero 2021). Nicoli and Ferrero (2021) suggested there might be a link between the pesence of MI and FI, volatile recycling/reworking in the crust, the evolution of plate tectonics and surface habitability
Using optical microscope observations and MicroRaman spectroscopy, we show that garnets in metasedimentary migmatite contain primary polycrystalline aggregates which can be confidently interpreted as former droplets of anatectic melt, i.e. nanogranitoids
They coexist with coeval fluid inclusions under conditions of primary fluid-melt immiscibility
Summary
Primary crystallised melt and fluid inclusions (MI and FI) in metamorphic minerals such as garnet or zircon constitute an invaluable archive to identify and investigate partial melting processes and crustal differentiation (e.g. Cesare et al 2015; Nicoli, Ferrero 2021). Nicoli and Ferrero (2021) suggested there might be a link between the pesence of MI and FI, volatile recycling/reworking in the crust, the evolution of plate tectonics and surface habitability. Nicoli and Ferrero (2021) suggested there might be a link between the pesence of MI and FI, volatile recycling/reworking in the crust, the evolution of plate tectonics and surface habitability. If this correlation works for the Phanerozoic eon, there is a lack of data for the Precambrian period. Because of its exceptional exposure, age and pristine aspect, the Mesoarchean lithologies exposed in the Uttental Plateau (Fig. 1a) represent an ideal natural laboratory to investigate crustal reworking processes and the remobilisation of volatile elements (e.g. H, C, N) in active continental margin during the onset of plate tectonics
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