THE THERMAL EVOLUTION OF METAPELITIC GRANULITES FROM JUBRIQUE SEQUENCE (BETIC CORDILLERA, S SPAIN)

The trace-element thermometers Ti-in-quartz, Zr-in-rutile and Ti-in-zircon are successfully applied to polymetamorphic rocks to estimate their peak and post-peak temperatures. In this study we apply these thermometers to the metapelitic granulites from the Jubrique sequence (Betic Cordillera, S Spain), which contain quartz, zircon and rutile in equilibrium during most of the metamorphic evolution. The rocks contain Grt, Qtz, Kfs, Pl, Ky, Sil, Crd, Bt and accessory Rt, Ilm, Zrn, Mnz, Spl, Gr and Ap. These samples offer the opportunity to compare the results with those obtained in previous studies, through phase equilibria modeling and conventional thermobarometry. Most zircons imaged by cathodoluminescence show a complex core and an irregular, homogeneous or weakly zoned, rim. Rutile and zircon were analyzed by LA-ICP-MS, giving ranges of 1000-2800 ppm Zr and 7-25 ppm Ti, respectively. Ti in quartz was measured by EMP, and its content is 100 to 260 ppm. Pressure values used for calculations are 13 kbar for minerals included in garnet, 5 kbar for matrix quartz and zircon, 9 kbar for matrix rutile.

We recognized two types of quartz included in garnet: 1) large (≈800 μm) irregularly shaped inclusions, yielding an average Ti-in-quartz temperature of 878 ± 65 °C; 2) small (< 50 μm) single-crystal inclusions, at the garnet core, 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 (in garnet) and 810 ± 25 °C (in matrix). Average Ti-in-zircon 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).

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 dynamic recrystallization during deformation. These temperatures are much lower than those previously obtained.