Applicazione dei termometri Ti-in-quarzo, Ti-in-zircone e Zr-in-rutilo alle granuliti metapelitiche della sequenza di Jubrique (Cordigliera Betica, Spagna Meridionale)

In this thesis, the new trace-element thermometers Ti-in-quartz, Zr-in-rutile and Ti-in-zircon are applied to the mylonitic gneisses from the anatectic sequence of Jubrique, which is located on the top of the Ronda peridotite slab (Betic Cordillera, Southern Spain). This sequence constitutes a strongly thinned crustal section. The rocks studied are granulite-facies migmatitic gneisses made up of Grt, Qz, Kfs, Pl, Ky, Sil, Crd, Bt and accessory Rt, Ilm, Zrn, Mnz, Spl, Gr, Ap and Py. The studied samples show the simultaneous presence of all three minerals required for the application of these thermometers (quartz, rutile, zircon) and offer the opportunity to compare the results with those previously obtained through phase equilibria modeling and conventional thermobarometry. Several crystals of quartz, zircon and rutile, both included in garnet porphyroclasts and present in the rock matrix, were investigated. Zircon was imaged by cathodoluminescence in order to reveal its internal structure: most zircons show a complex core and an irregular, homogeneous or weakly zoned, rim. Rutile and zircon were analyzed by LA-ICP-MS, giving contents of 1000- 2800 ppm Zr and 7-25 ppm Ti, respectively. Ti in quartz was measured by EMP, resulting in 100-260 ppm. Pressure values used for calculations are 13 kbar for minerals included in garnet, 5 kbar for matrix quartz, 9 kbar for matrix rutile. Two types of quartz included in garnet were recognized: 1) large (≈800 μm) irregularly shaped inclusions, both at the rim and at the core of garnet, yielding an average Ti-in-quartz temperature of 878 ± 65 °C; 2) small (< 50 μm) singlecrystal inclusions, at the garnet core only, yielding an average temperature of 906 ± 36 °C. Matrix quartz gave temperatures of 692 ± 44 °C (in melanosome) and 665 ± 24 °C (in leucosome). Crystallization temperatures calculated for rutile are 847 ± 50 °C (rutile in garnet) and 810 ± 25 °C (matrix rutile). Average Ti-inzircon temperatures for zircon included in garnet are 747 ± 34 °C (rim) and 750 ± 55 °C (core), whereas for matrix zircon the average temperatures are 766 ± 38 °C (rim) and 759 ± 46 °C (core). Quartz and rutile included in garnet seem to record peak metamorphic temperatures: Ti-in-quartz and Zr-in-rutile isopleths intersect at ≈850°C and 12 kbar, in agreement with peak conditions published in the literature. However, these rocks likely reached temperatures approaching UHT metamorphism conditions, as suggested by quartz in garnet (≈900 °C). Lower Ti-in-zircon temperatures probably record growth of zircon during cooling from peak temperatures. Lower temperatures calculated for matrix quartz (<700 °C) probably record crystallization from an anatectic melt and/or recrystallization under retrograde conditions and deformation. These temperatures are much lower than those obtained through phase equilibria modeling and conventional thermometry