On the nature and composition of crustal sources for late Variscan magmatism – insights from southern Sardinia (Italy)

The external zone of the Variscan chain in southern Sardinia hosts several short-lived, both composite, and homogeneous plutons emplaced at shallow crustal levels during post-collisional extension. Except for Arbus composite pluton, dated 304 ± 3 Ma, the emplacement ages cluster within a narrow time interval between about 289 ± 1 and 285 ± 1 Ma. Granites show commonly a ferroan and F-bearing character (F in the range of 0.09 to > 0.01 wt.%) and are either sub-aluminous to peraluminous ilmenite (GS1 and MM), or metaluminous magnetite rock-series (GS2 and GS3). GS1 rocks contain dark siderophyllite as the only mafic phase, and ilmenite + xenotime (Y) as common accessory phases. GS2 granites contain biotite + allanite + magnetite. GS3 shows hastingsite + allanite + magnetite, with final crystallization of biotite + fluorite and rare fayalite . Finally, MM rocks are garnet and muscovite-bearing granites. The bulk-rock compositions of the granites are similar to experimental melts obtained at P < 8 kbar from crustal sources ranging from metagreywackes to meta-tonalites (Patiño Douce, 1997). Different proportions of metasediments / metaigneous crustal sources are also supported by Sr-Nd isotopic data (εNdt in the range of -5.4 to -7.5). The occurrence of a pre-Cambrian lower crust could fit the Variscan architecture of the external Nappe zone of Sardinia, as lower Cambrian metasandstones contain abundant clasts of amphibolite-facies gneiss likely derived from an unexposed crystalline basement possibly composed of TTG as commonly observed in Proterozoic basements (Shang et al., 2007; Nehring et al., 2009). Temperatures obtained from plagioclase/whole rock and apatite saturation thermometry are about 860- 870oC for both the ilmenite and magnetite-bearing granites and may be interpreted as near liquidus temperatures. Slight lower values between 730°C-785°C obtained with Crisp and Birks’s (2022) zircon saturation calibration may reflect the depleted character of meta-igneous sources. Overall, the whole data set suggests that anatexis involved the dehydration melting of biotite at temperature exceeding 850oC, as commonly argued for ferroan granite magmas (Scaillet et al., 1995; Frost and Frost, 2011). Finally, the chemical and isotopic signatures of the granites are likely inherited from an heterogeneous deep crust resulting from the juxtaposition of different crustal blocks during the late Carboniferous-Early Permian shearing and rotation of the Corsica-Sardinia massif. Late Variscan shearing, likely associated to delamination and lithospheric necking, appears as the most reliable mechanism to account for mantle upwelling, triggering biotite-dehydration melting of the lower crust.