Fluid-melt immiscibility during lower crustal melting recorded in the orthogneiss of Punta de li Tulchi, Sardinia (Italy).

Garnet in high grade rocks often contains primary melt (MI) and fluid inclusions (FI), fundamental to reconstruct the suprasolidus history of the rocks in which it occurs. MI and FI-bearing garnets have been recently found at Punta de li Tulchi, on the NE coast of Sardinia, Italy. They were identified in a single, decimetre-thick leucocratic body of overall granitic composition, hosted in migmatitic orthogneiss. Whereas in the host rock garnet is generally rare or absent, in the leucocratic body it reaches up to 5 vol% and occurs as subhedral grains up to several mm in size. Garnet composition is mainly almandine-spessartine (Alm_60-70, Sps_20-30 Pyr_~7 Grs_~3).

The inclusions occur in clusters in the inner part of the garnet, i.e., they are primary in nature and therefore trapped during garnet growth. A preliminary investigation via optical microscopy and MicroRaman Spectroscopy (MRS) shows the presence of (at least) two types of inclusions. Type 1 inclusions are polycrystalline, ≤30 microns in size, and contain cristobalite +white mica ±carbonate, an assemblage overall consistent with their interpretation as nanogranitoids, or crystallized melt inclusions of anatectic origin. Type 2 inclusions contain two phases, i.e., liquid and vapour (70-30 vol% approximately). MRS display the presence of liquid H₂O and a CH₄-rich bubble, with minor amounts of CO₂ and N₂ detected in some cases. Often type 2 inclusions are characterized by the presence of a carbonate – either ankerite (CaFe carbonate) or rhodochrosite (Mn carbonate), based on the features of its Raman spectrum.

Previous studies in the area suggest that these crustal rocks underwent high T metamorphism and re-melting in presence of fluid at 1.0 -1.1 GPa (Fancello et al., 2018) during the Variscan orogeny. The finding of melt-bearing garnets suggests that anatexis here also involved dehydration melting, at least locally, with formation of peritectic garnet. Moreover, this took place under conditions of primary fluid-melt immiscibility, as testified by the coexistence of MI and FI in the same cluster, a feature already found in many other nanogranitoid studies (see Ferrero et al., 2023). Finally, this is the first case study where nanogranitoids are found in almandine-spessartine garnet rather than in almandine-pyrope.

References

Fancello, D., et al. Trondhjemitic leucosomes in paragneisses from NE Sardinia: Geochemistry and P-T conditions of melting and crystallization. Lithos, https://doi.org/10.1016/j.lithos.2018.02.023

Ferrero, S., et al. H₂O and Cl in deep crustal melts: the message of melt inclusions in metamorphic rocks. EJM, https://doi.org/10.5194/ejm-35-1031-2023