Partial melting evolution of the Variscan High-Grade Metamorphic Complex recorded in amphibolites from Asinara Island (Italy)

Partial melting processes occurring in mafic lithologies at low pressures and medium-high temperatures are not well understood and therefore may conceal crucial aspects of crustal evolution. Here, we target a composite lens, 150 x 50 m in size, comprising massive amphibolites, banded amphibolites and levels of amphibole-bearing granulites, hosted in a highly deformed, migmatitic metasedimentary sequence belonging to the Variscan High-Grade Metamorphic Complex at Punta Scorno in the northern Asinara Island (Sardinia, Italy). These amphibolites and granulites record a complex history of prograde metamorphism, and possibly, multiple stages of partial melting.  

The massive amphibolites do not display any evidence of partial melting and consist mostly of magnesio-hornblende (X_Fe = 0.26-0.36 and 0.46-0.51) and Ca-rich plagioclase (An_41-96) with later-crystallised biotite (X_Fe= 0.36-0.51). Banded amphibolites show a similar assemblage, although hornblende and biotite are Fe-richer (X_Fe = 0.54-0.65 and X_Fe = 0.55-0.63, respectively) and plagioclase contains less Ca (An_29-79). In addition, banded amphibolites are rich in quartz, and contain minor amounts of other amphiboles, in particular grunerite (X_Fe = 0.55-0.59) overgrown by tschermakite (X_Fe = 0.59-0.68), and rare garnet (Alm_69-72Grs_11-14Prp_7-9Sps_8-10). Cuspate edges of quartz and melt pseudomorphs of plagioclase within the banded amphibolites provide evidence of partial melting. Also, the amphibole-bearing granulites are characterised by a significant amount (ca. 10%) of grunerite (X_Fe = 0.51-0.64) with tschermakitic rims (X_Fe = 0.62-0.76), and garnet (Alm_71-76Grs_11-14Prp_6-12Sps_4-7). These rocks are more massive with respect to the banded amphibolites, and are much richer in quartz and plagioclase of variable composition (An_49-86), suggesting that they might be the result of partial melting of the banded amphibolites. The former presence of melt is also supported by several crystallised melt inclusions found in the garnets.

We used a combination of single-element thermometry (Ti-in-Amp (Liao et al., 2021; Bartoli et al., 2024) and Ti-in-Qz (Osborne et al., 2022)) and phase equilibrium modelling (Connolly, 2005) to constrain the metamorphic evolution. Our results suggest prograde growth of hornblende in the massive and banded amphibolites, followed by crystallisation of grunerite in the partially molten rocks – possibly as a peritectic phase – subsequently replaced by garnet and overgrown by tschermakite. The early prograde conditions are recorded by small hornblende crystals yield ca. 650 °C, whereas the peak P-T conditions during which grunerite grew, are ca. 0.5 GPa/730 °C. Retrograde crystallisation of tschermakite occurred at ca. 600 °C. As the peak conditions occurred at much lower temperature than those required for dehydration melting of amphibole, it is likely that a fluid influx during the prograde part of the P-T path lowered the solidus temperature of the system (Weinberg and Hasalová, 2015).

This work was funded by Fondazione di Sardegna Progetto RAWEX  and Horizon Europe programme, grant 101131765 (EXCITE2).

 

Bartoli, O. et al. (2024). Contributions to Mineralogy and Petrology, 179, 65

Connolly, J. A. D. (2005). Earth and Planetary Science Letters, 236, 524-541

Liao, Y. et al. (2021). American Mineralogist, 106(2), 180-191

Osborne, Z. et al. (2022). Contributions to Mineralogy and Petrology, 177, 31

Weinberg, R. F. and Hasalová, P. (2015). Lithos, 212-215, 158-188