CRUSTAL MELTING FROM AMPHIBOLITE- TO GRANULITE-FACIES CONDITIONS: A MELT INCLUSIONS STUDY OF MIGMATITES, S SPAIN

Being trapped by growing peritectic phases at suprasolidus conditions, melt inclusions (MI) in migmatites represent a window into the pre-peak anatectic history of partially melted terranes. The crustal footwall of the Ronda peridotites consists of an inverted metamorphic sequence with migmatites and mylonitic diatexites at the top. A detailed microstructural and geochemical study has been conducted on primary MI hosted in peritectic garnet of i) metatexites at the bottom of the migmatitic sequence and ii) mylonitic diatexites close to the contact with the mantle rocks. Phase equilibria modeling shows that metatexites and diatexites underwent anatexis under amphibolite- and granulite facies conditions, respectively (660–700 °C, 4.5–5 kbar and 820-830 °C, 5.5-6.0 kbar).

Clusters of MI in the metatexites are rounded and preferentially located at the core of small garnet crystals, whereas these clusters may have a sigmoidal to spiral-like shape in garnets of diatexites. MI are small (2-10 µm) and show a variable degree of crystallization ranging from totally glassy to nanogranites. The latter consist of Qtz+Pl+Kfs+Bt+Ms aggregates. Piston cylinder remelting experiments led to the complete rehomogenization of nanogranites in metatexites at the conditions inferred for anatexis. Rehomogenized nanogranites in metatexites and glassy MI in diatexites are all leucogranitic, but systematic compositional variations have been observed. MI produced under amphibolite-facies conditions show higher H₂O and Na₂O/K₂O, lower FeO and higher concentrations of the trace elements controlled by feldspars: Sr and Ba. MI formed under granulite-facies conditions have higher concentrations of trace elements controlled by biotite (Cs, Rb and FRTE e.g. Zn, Sc) and accessory minerals (HFSE e.g. Zr, U, Th). The calculated Zrn saturation temperatures for the MI in the diatexite are 50-100 °C lower than temperatures obtained from phase equilibria modelling, suggesting that MI are likely undersaturated in the accessory phases. The MI in metatexites and diatexites are interpreted to record the composition of the anatectic melts produced 1) on, and immediately after crossing, the fluid-saturated solidus, and 2) during syn-kinematic anatexis via biotite dehydration melting at increasing temperature, respectively.