When the continental crust melts: a combined study of melt inclusions and classical petrology on the Ronda migmatites

Melt inclusions hosted in peritectic minerals of migmatites represent a novel and powerful small-scale tool to investigate the anatexis of the continental crust. In this thesis, taking advantage of a new experimental approach developed during this work, a combined study of classical petrology and melt inclusions in migmatites was performed to characterize in detail the composition and the physical properties of anatectic melts, the fluid regimes, and the melting mechanisms and conditions during the anatexis of the metasedimentary crust located below the Ronda peridotite (Betic Cordillera, S Spain). Here, the tectonic emplacement of mantle rocks within the continental crust produced high-temperature metamorphism and partial melting in the underlying metasedimentary rocks. The investigated migmatites are quartzo-feldspathic metatexites located towards the base of the crustal sequence and quartzo-feldspathic mylonitic migmatites found close to the contact with the peridotite. The petrologic study was made by petrographic observations, compositional characterization of minerals and bulk rocks, conventional thermobarometry and pseudosection calculations. The quartzo-feldspathic migmatites are mainly composed of Qtz + Pl + Kfs + Bt + Sil + Grt and probably derived from a greywacke protolith. Muscovite, very rare in the metatexites, is absent in the mylonites. Graphite is present in all migmatites. The former presence of melt is recorded by melt inclusions and melt pseudomorphs at the microscale, and by peraluminous leucogranitic leucosomes at the mesoscale. The changes in phase compositions and modal contents from the metatexites to the mylonites, along with thermobarometric calculations and pseudosection modelling, are consistent with an increase of melting conditions and degree towards the contact with the peridotite. Melt inclusions have been found in peritectic garnets of the investigated quartzo-feldspathic migmatites. A detailed study was carried out to characterize the microstructural features of melt inclusions and their chemical composition by microscope observation, FESEM imaging, piston cylinder experimental remelting, Raman and EMP analyses. The inclusions are primary in origin and were trapped within peritectic garnet during crystal growth. They are very small in size, mostly ≤ 10 µm, and typically show a well-developed negative crystal shape. Three types of inclusions were identified: totally crystallized (nanogranites), partially crystallized and preserved glassy inclusions. Crystallized melt inclusions contain a granitic phase assemblage with quartz, feldspars and micas. In this study, a new approach for the experimental re-homogenization of melt inclusions was developed, performing remelting experiments at 5 kbar pressure using a piston cylinder apparatus. At the minimum re-homogenization temperature of 700 °C, most melt inclusions are completely re-homogenized. Conversely, melt inclusions display decrepitation cracks and CO2 bubbles when remelted at higher experimental temperatures. For the first time, a pseudosection was constructed using the bulk composition of the fully re-homogenized melt inclusions from the metatexites. This approach constrained the melt entrapment at temperature close to the minimum re-homogenization temperature (T ~700 °C). The composition of those melt inclusions re-homogenized at 700 °C is comparable to that of preserved glassy inclusions in the same rock. In general all the studied melt inclusions in the migmatites and mylonites have peraluminous leucogranitic compositions. However, inclusions in the mylonites (mostly glassy) display quite variable Na2O/K2O and have lower primary H2O contents (1.0-2.6 wt%) than melt inclusions in the metatexites (3.1-7.6 wt%). Combining information from the melt inclusions and from the classical petrology allowed a better understanding of the melting processes occurred in the crustal sequence below the Ronda peridotite. The data collected in this study suggest that the crustal melting at Ronda mainly occurred under H2O-undersaturated conditions by the continuous Bt dehydration melting reaction and at temperatures that did not exceed ~800 °C. In the metatexites, towards the base of the crustal sequence, crustal anatexis likely started at the solidus (T ~700 °C) in presence of H2O-rich intergranular fluids. The growing peritectic garnets entrapped melt droplets produced at slightly different temperatures, whose composition was mainly diffusion-controlled. Melt inclusions seem to record the evolution of melt composition during prograde anatexis. Conversely, most of the peraluminous leucogranitic leucosomes are primary melts produced at, or close to, the metamorphic peak. The anatectic melts at Ronda have viscosity values greater than those commonly considered for granitic melts formed at the same P-T conditions, implying much longer timescales for melt extraction and ascent through the metasedimentary crust at Ronda, as well as much greater strength of the migmatites. The consistency of the collected compositional data along with the careful multidisciplinary approach adopted in this research, indicate that melt inclusions in peritectic minerals from migmatites represent a unique microstructure where anatexis is recorded and can be characterized in situ, in its earliest stage.