Evaluating the controls on the origin of tourmaline-bearing rocks in peraluminous and metaluminous systems: examples from Late-Variscan magmatism of Sardinia (Italy)

In magmatic systems, intensive and compositional parameters are commonly invoked to account for tourmaline saturation in the melt. To model the frequent occurrence of tourmaline-bearing rocks associated with peraluminous intrusions, a large emphasis is typically given to Al2O3 saturation index of magmas (ASI> 1.2). However, a major role played by partial melting processes of B-bearing protoliths has been recently proposed to control the B saturation of anatectic melts. In Sardinia, a B-bearing rock series is documented by tourmaline occurrence in the late-crystallization stages of Late-Variscan intrusives related to the older magmatic peak (305-300 Ma). Conversely, intrusives related to the younger magmatic peak (290-280 Ma) are almost made up of F-bearing granites and lack in tourmaline. The controls on the origin of tourmaline were investigated by comparing mineral assemblages from Mandrolisai (central Sardinia) and Arbus (SW Sardinia) igneous massifs, both emplaced at shallow depths in the frontal zone of a nappe edifice. Mandrolisai igneous massif results as a single pulse of a granodioritic metaluminous magma, while Arbus is a composite pluton made of several pulses reaching a peraluminous character in leucogranitic rock-units (ASI < 1.16). Arbus magmas belong to a true ilmenite series while the Mandrolisai straddles the ilmenite/magnetite field series. In Mandrolisai, tourmaline occurs in layered aplite/pegmatite dikes and in thin tourmalinite veins in roof pendants of metamorphic rocks, close to the western contacts of the intrusion. In Arbus, tourmaline occurs in pegmatitic layers, in granophyric dykes and within metamorphic country rock. The inclusive crystal-chemical study of tourmaline samples from pegmatites of both plutons highlights a comparable evolution from shorlitic to foititic composition; in addition, crystals from tourmalinite veins of Mandrolisai and dispersed within the country rock of Arbus show a dravitic composition, mostly related to magma-country rock interaction. The main differences regard the Mg-richest and Al-poorest composition of Mandrolisai tourmaline, which reflect a different composition of granodioritic magma, more magnesian and less aluminous (ASI = 0.93-0.95) The whole data sets of the two plutons allow the reconstruction of the same crystallization path: T = 650-400 °C, P ≤ 2.2 kbar and redox conditions reaching NNO buffer in the Mandrolisai massif. The uncommon occurrence of tourmaline-bearing rocks in a metaluminous intrusion such as Mandrolisai may be an evidence of the limited control exerted by Al2O3 saturation on the origin of tourmaline. Conversely, a more important role of B contents, likely coming from crustal sources, may be invoked for tourmaline saturation in the magma.