Unravelling the anatectic history of the lower conteninental crust through the petrology of melt inclusions and lu-hf garnet geochronology: A case study from the western Alpujárrides (Betic Cordillera, S. Spain)

Abstract

Partial melting (anatexis) plays a fundamental role in the generation, differentiation and the rheology of Earth’s continental crust. “Migmatitic” terranes constitute the main geological record of crustal anatexis throughout Earth’s history. Unravelling the mechanisms of crustal anatexis from these terranes has proven to be challenging particularly when it comes to unveiling the primary chemical composition of anatectic melts. The main aim of this thesis is to better understand lower crustal anatexis through the study of “nanogranite inclusions” —microscopic droplets of melt that formed via incongruent melting reactions— in metamorphic minerals, and its relationship with lithospheric scale tectonomagmatic processes. The originality of my Thesis resides in the combination of a petrological, thermodynamical and experimental study of anatexis on the basis of the study of nanogranite inclusions in garnets from high-pressure granulitic migmatites. The case study is migmatitic gneisses from the Jubrique unit, a complete —though strongly thinned— crustal section in the westernmost Alpujárrides (Betic Cordillera, S. Spain). These gneisses overlie the Ronda peridotites —the largest exposure of subcontinental lithospheric mantle on Earth— and provide a unique opportunity to investigate the nature and age of crustal melting events and their timing with mantle processes in the westernmost Mediterranean. Melt inclusions (≈ 30-40 μm) —now recrystallized to nanogranites— in Jubrique gneisses are present in garnet cores and rims throughout the entire sequence. Thermodynamic modeling and conventional thermobarometry provide peak conditions of ≈ 850 ºC and 1.2-1.4 GPa, corresponding to garnet cores with kyanite and rutile inclusions. Post-peak conditions of ≈ 800-850 ºC and c. 0.5 GPa are recorded in rims of garnet porphyroblast/clasts. The study of nanogranite inclusions shows that most garnet grew in the presence of melt. To constrain the primary composition and the P-T conditions of formation of nanogranitoids, we have carried out an experimental study of nanogranitoids in garnets, which were melted at 1.5 GPa and 850, 825 and 800 ºC in a piston cylinder apparatus. Experiments show that anatexis and entrapment of nanogranites occurred at c. 800 ºC. Electron microprobe and NanoSIMS analyses indicate that experimental glasses are leucogranitic and peraluminous and define two compositional groups: Type I corresponds to K-rich, Ca- and H2O-poor leucogranitic melts, whereas type II represents K-poor, Ca- and H2O-rich granodioritic to tonalitic melts. They are found, respectively, at cores and rims of garnet porphyroblasts/clasts, and show that Jubrique migmatites underwent two anatectic events under contrasting fluid regimes. To determine the age of crustal melting events and their timing with lithospheric mantle processes, we have analyzed Lu-Hf in whole rocks and garnets of Jubrique gneisses and garnet pyroxenites from the Ronda peridotite. The Lu-Hf isochrons confirm that the growth of garnet in Jubrique gneisses occurred in the Early Permian (c. 289 Ma) during the latest stages of the Variscan orogeny, most likely in a context of continental collision and overthickened continental crust. We found no Alpine Lu-Hf ages, indicating either that this event is not resolvable with our sampling and dating techniques, or that the Lu-Hf of garnet was not equilibrated in the Alpine orogeny. The Lu-Hf whole rock-garnet isochrons of mantle garnet pyroxenites provide Jurassic- Cretaceous (144 Ma), Paleogene (53 Ma) and Miocene (21 Ma) ages. We interpret early Miocene ages as recording the waning stages of an Alpine extensional-related thermal event before emplacement of peridotites. Mantle garnet pyroxenites do not record Lu-Hf Variscan ages that may suggest that this system was reset by later mantle events or that garnet in mantle rocks grew in geodynamic events later than the Variscan orogeny.