Abstract Greywacke is a rock type pervasively present in orogens, and anatexis of greywackes is a pivotal process for crustal differentiation and granitoid formation. However, it remains challenging to characterize pristine compositions of anatectic melts derived from diverse lithologies. This study investigates nanogranitoids (i.e. crystallized melt inclusions) in metagreywackes from the Rauer Islands, eastern Prydz Bay, East Antarctica. This is the first time to recognize nanogranitoids in metagreywackes dominated by Bt-dehydration melting. Results of phase equilibrium modeling and Zr-in-rutile thermometry indicate that the peak metamorphic P–T conditions were ~ 0.95 GPa and ~ 850°C, and that the melts were trapped under near peak conditions. Zircon and monazite dating suggests that the granulite facies metamorphism and anatexis occurred during the Pan-African orogeny. Nanogranitoids (2 to 40 μm in size) occur in garnet, and are composed of cryptocrystalline daughter phases including phlogopite, muscovite, cristobalite, quartz, plagioclase, kokchetavite, and kumdykolite. They were experimentally re-homogenized under 850°C, with the glass showing silicic (SiO₂ = 74.2 ± 3.1 wt%), peraluminous (ASI = 1.1–1.6), K-enriched (K/Na = 1.1–2.6), and low-maficity (FeO + MgO = 1.7–4.1 wt%) compositions. Micro-Raman spectroscopy suggests that H2O contents in the melts vary from 0.44 to 1.08 wt%, with an average value of 0.75 ± 0.2 wt%. Compared with bulk rock compositions, the investigated melts show enrichment in Rb, Cs, and U, depletion in Ce and Sr, and elevated Rb/Sr ratios (3.8–13.7). From these geochemical characteristics of nanogranitoids, we conclude that: (1) K2O and Rb enrichment in melts points to incongruent melting involving dehydration of biotite; (2) Depletion of Sr reflects moderate melting degrees (i.e. plagioclase is still abundant at peak stage); and (3) Zircon saturation temperatures (775–837°C at 1.0 GPa) imply disequilibrium melting of accessory minerals during high temperature anatexis. Physical properties of melt (2.43 ± 0.06 g/cm3 density, and ~ 3.5 μW/m3 heat production) suggest that anatexis of metagreywackes would facilitate efficient melt extraction and redistribution of some large ion lithophile elements (Rb and Cs) and heat-producing elements (K and U). A detailed comparison between published data suggests that biotite-dehydration melting usually generates granitic melts with high heat production values. Considering the wide distribution and efficient melt productions, anatexis of metagreywackes may have a remarkable contribution to chemical differentiation across crustal sections, prompting stabilization of the crust.
Liu et al. (Tue,) studied this question.