Natural zircon crystals often show complex secondary textures that cut across primary growth zones. In zircon showing structural damage caused by self-irradiation, such textures are the result of a diffusion- reaction process in which a hydrous species diffuses inwards and “catalyzes” structural recovery. Nanoscale pores develop, solvent elements such as Ca, Al and Fe are gained, and radiogenic Pb is lost. In both aqueous fluids and melts, replacement of zircon with undamaged structure by a coupled dissolution- reprecipitation process can produce similar textures. The reacted domains usually have lower trace element contents and may contain micrometer-sized pores and inclusions of uranium, thorium and/or yttrium phases, originally in solid solution. Both processes have considerable implications for zircon geochronology

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The chemical and structural alteration of metamict zircon crystals from a 619 ±17 (2σ) Ma old, posttectonic granite in the southern part of the Eastern Desert, Egypt was studied. The crystals show simple oscillatory growth zones with metamictization–induced fractures, which provided pathways for fluid infiltration. Electron and ion microprobe analyses reveal that metamict, i.e. U and Th–rich, areas are heavily enriched in Ca, Al, Fe, Mn, LREE, and a water species, and have lost Zr and Si as well as radiogenic Pb. These chemical changes are the result of an intensive reaction with a low–temperature (120—200°C) aqueous solution. The chemical reactions probably occurred within the amorphous regions of the metamict network. During the zircon–fluid interactions the metamict structure was partially recovered, as demonstrated by micro-Raman and -infrared measurements. A threshold degree of metamictization, as defined empirically by an α–decay dose, Dc, was necessary for zircons to undergo hydrothermal alteration. It is proposed that Dc marks the first percolation point, where the amorphous domains start to form percolating clusters in the metamict network and where bulk chemical diffusion is believed to increase dramatically. The time of the hydrothermal alteration is determined by a lower intercept age of a U-Pb SHRIMP discordia of 17.9 (2σ) Ma, which is in good agreement with an apatite fission track age of 22.2 (2σ) Ma. The hydrothermal alteration event occurred contemporaneously with the main rifting phase of the Red Sea and widespread low- temperature mineralizations along the Red Sea coast.

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Permian basin formation and magmatism in the Southern Alps of Italy have been interpreted as expressions of a WSW-ENE-trending, dextral megashear zone transforming Early Permian Pangea B into Late Permian Pangea A between~285 and 265 Ma. In an alternative model, basin formation and magmatism resulted from N-S crustal extension. To characterize Permian tectonics, we studied the Grassi Detachment Fault, a low-angle extensional fault in the central Southern Alps. The footwall forms a metamorphic core complex affected by upward-increasing, top-to-the-southeast mylonitization. Two granitoid intrusions occur in the core complex, the synmylonitic Val Biandino Quartz Diorite and the postmylonitic Valle San Biagio Granite. U-Pb zircon dating yielded crystallization ages of 289.1 ± 4.5 Ma for the former and 286.8 ± 4.9 Ma for the latter. Consequently, detachment-related mylonitic shearing took place during the Early Permian and ended at~288 Ma, but kinematically coherent brittle faulting continued. Considering 30°anticlockwise rotation of the Southern Alps since Early Permian, the extension direction of the Grassi Detachment Fault was originally~N-S. Even though a dextral continental wrench system has long been regarded as a viable model at regional scale, the local kinematic evidence is inconsistent with this and, rather, supports N-S extensional tectonics. Based on a compilation of >200 U-Pb zircon ages, we discuss the evolution and tectonic framework of Late Carboniferous to Permian magmatism in the Alps.