The geodynamic evolution of the Southern European Variscides: constraints from the U/Pb geochronology and geochemistry of the lower Palaeozoic magmatic-sedimentary sequences of Sardinia (Italy)

The high-grade metamorphic complex of northern Sardinia consists of a strongly deformed sequence of migmatitic ortho- and paragneisses interlayered with minor amphibolites preserving relic eclogite parageneses. The protolith ages and geochemical characteristics of selected gneiss samples were determined, providing new constraints for reconstructing the Palaeozoic geodynamic evolution of this sector of the Variscan chain. The orthogneisses are metaluminous to peraluminous calcalkaline granitoids with crustal Sr and Nd isotopic signatures. One orthogneiss from the high-grade zone and one metavolcanite from the volcanic belt in southern Sardinia were dated by LAM-ICPMS (and SHRIMP) zircon geochronology. The inferred emplacement ages of the two samples are 469 ± 3.7 and 464 ± 1 Ma, respectively. The analysed paragneisses are mainly metawackes with subordinate metapelites and rare metamarls. Three paragneiss samples were dated: zircon ages scatter between 3 Ga and about 320 Ma, with a first main cluster from 480 to 450 Ma, and a second one from about 650 to 550. Variscan zircon ages are rare and mostly limited to thin rims and overgrowths on older grains. These data indicate that the high-grade complex principally consists of middle Ordovician orthogneisses associated with a thick metasedimentary sequence characterised by a maximum age of deposition between 480 and 450 Ma. The association of nearly coeval felsic-mafic magmatic rocks with immature siliciclastic sedimentary sequences points to a back-arc setting in the north Gondwana margin during the Early Palaeozoic. The Variscan metamorphic evolution recorded by the high-grade gneisses (Ky-bearing felsic gneisses and mafic eclogites) testifies to the transformation of the Late Ordovician–Devonian passive continental margin into an active margin in the Devonian–Early Carboniferous.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

TiO2 II: The high-pressure Zr-free srilankite endmember in impact rocks

TiO₂II, a high-pressure polymorph of titanium dioxide, is a diagnostic indicator of shock metamorphism in impact rocks. Due to its typical micro-to-nanometer scale, there are no ab initio structure solutions of natural TiO₂II, thereby generating uncertainty about its crystal structure and its known similarity with srilankite (Ti_0.67,Zr_0.33)O₂. Nanoscale electron diffraction investigation of TiO₂II from the Australasian tektite strewn field provides the first ab initio structure solution revealing a primitive orthorhombic lattice with cell parameters a = 4.547 Å, b = 5.481 Å, c = 4.891 Å, and space group Pbcn, that is, the same as srilankite and scrutinyite α-PbO₂. The linear a and c decrease, and b increase with Ti content indicate TiO₂II as Zr-free srilankite endmember in the binary system ZrO₂-TiO₂. Thereby the name srilankite should be used referring to TiO₂II, according to the International Mineralogical Association recommendations. We provide the first evidence for a topotactic subsolidus rutile-to-TiO₂II transition, founding their finely intermixing nanocrystals in the same TiO₂ crystal, where TiO₂II is within the crystal and surrounded by rutile in direct contact. They also show recurrent iso-orientation, with TiO₂II [100] parallel to rutile [100], TiO₂II [010] parallel to rutile [011], and TiO₂II [001] parallel to rutile (0–11). The rutile-TiO₂II iso-orientation suggests the compression of rutile (0–11) planes as a possible transition mechanism from rutile to TiO₂II, with a consequent shortening of ~0.5 Å per cell. The presence of TiO₂II in the distal (~1200 km) impact ejecta from the Australasian tektite strewn field indicates shock pressures of ~12–15 GPa and post-shock temperatures below 500°C followed by rapid quenching.