The analysis on ultrahigh temperature rocks from Lützow Holm Complex, in Antarctica, shed new light on the A-type granites, one of the most debated in terms of origin.

By adopting an in-source perspective, rather than focusing on the final product (granite), the occurrence of pristine melt inclusions (MIs) in garnet from residual metapelitic ultrahigh- temperature (UHT) granulite from East Antarctica was documented.

The research was conducted in the framework of the HOT ANTARCTICA project, funded by the Programma Nazionale di Ricerca in Antartide, whose goal is studying the hottest metamorphic rocks from Antarctica. The results were recently published in Geology.

“Through a collaboration with the University of Siena, Museo Nazionale dell’Antartide (Italy), the University of Kyoto, the University of Niigata and the National Institute of Polar Research (Japan) and the University of Edinburgh in Scotland, I got access to samples from several important ultrahigh temperature locations in Antarctica”, says Bruna Borges Carvalho, researcher of the Department of Geosciences of the University of Padova and principal investigator of the HOT ANTARCTICA project.

The new article deals with the extensively debated origin of A-type granites and sheds new light on a long-standing enigma in crustal petrology – whether or not anatexis at ultrahigh temperature conditions may impact crustal differentiation.

“We found tiny melt droplets in garnet, preserved as beautiful glassy and crystallized melt inclusions which coexist with sapphirine and quartz and with rutile. Sapphirine and quartz is a diagnostic assemblage for ultrahigh temperature metamorphism (i.e. T>900°C). Additionally, we applied the Zr-in rutile thermometer using the composition of rutile coexisting with the melt inclusions and coupled that with phase equilibria modelling allowed us to constrain the entrapment of the melt at temperatures around 1000°C and 8 kbar (that is roughly 27 km depth). Major and trace element composition of the melt inclusions were obtained and revealed that these tiny melt droplets share several geochemical features with A-type granites from landmark papers in the literature. This is an important finding because metasedimentary sources have only rarely been invoked in the petrogenesis of A-type granites. Furthermore, since UHT anatexis can be linked to granite magmatism, we can conclude that UHT anatexis plays a role on crustal differentiation”, Bruna Borges Carvalho explains.

In summary this study of the hottest rocks from Antarctica funded by PNRA opened new doors into the origin of these particular granites which occur in a variety of geodynamic contexts on Earth but also in extraterrestrial bodies.

Link to research: https://doi.org/10.1130/G51097.1

Title: “Revealing the link between A-type granites and hottest melts from residual metasedimentary crust” - Geology (2023)

Authors: B.B. Carvalho; O. Bartoli; B. Cesare; M. Satish-Kumar; M. Petrelli; T. Kawakami; T. Hokada; M. Gilio