Phase equilibria constraints on melting of stromatic migmatites from Ronda (S. Spain): insights on the formation of peritectic garnet

Abstract

Stromatic metatexites occurring structurally below the contact with the Ronda peridotite (Ojen nappe, Betic Cordillera, S Spain) are characterized by the mineral assemblage Qtz+Pl+Kfs+Bt+Sil+Grt+ Ap+Gr+Ilm. Garnet occurs in low modal amount (2–5 vol.%). Very rare muscovite is present as armoured inclusions, indicating prograde exhaustion. Microstructural evidence of melting in the migmatites includes pseudomorphs after melt films and nanogranite and glassy inclusions hosted in garnet cores. The latter microstructure demonstrates that garnet crystallized in the presence of melt. Re-melted nanogranites and preserved glassy inclusions show leucogranitic compositions. Phase equilibria modelling of the stromatic migmatite in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2– H2–O2–C (MnNCaKFMASHOC) system with graphite-saturated fluid shows P–T conditions of equilibration of 4.5–5 kbar, 660–700 °C. These results are consistent with the complete experimental re-melting of nanogranites at 700 °C and indicate that nanogranites represent the anatectic melt generated immediately after entering supersolidus conditions. The P–T estimate for garnet and melt development does not, however, overlap with the low-temperature tip of the pure melt field in the phase diagram calculated for the composition of preserved glassy inclusions in garnet in the Na2O– CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system. A comparison of measured melt compositions formed immediately beyond the solidus with results of phase equilibria modelling points to the systematic underestimation of FeO, MgO and CaO in the calculated melt. These discrepancies are present also when calculated melts are compared with low-T natural and experimental melts from the literature. Under such conditions, the available melt model does not perform well. Given the presence of melt inclusions in garnet cores and the P–T estimates for their formation, we argue that small amounts (<5 vol.%) of peritectic garnet may grow at low temperatures (≤700 °C), as a result of continuous melting reactions consuming biotite

Wealth of P–T–t information in medium-high grade metapelites: Example from the Jubrique Unit of the Betic Cordillera, S Spain

“…However, these authors locate this event at UHP conditions. Bartoli et al (2013) also suggested garnet formation by melting in their migmatites occurring, in fact, below the Ronda peridotite, but being very similar to the studied ones. The formation of peritectic garnet (see also Barich et al, 2013) is compatible with melt inclusions (see Figs.…”

“…3). Especially the inclusions consisting of quartz and feldspar resemble those reported by Bartoli et al (2013) as melt inclusions in garnet from a nearby occurrence below the Ronda peridotite. As the melt inclusions in garnet from the migmatites of the Jubrique Unit (see also Ruiz Cruz and Sanz de Galdeano, 2014) are currently under detailed study (Barich et al, 2013) the variability of these inclusions were not further investigated here.…”

“…According to Bartoli et al (2013) this process should have occurred at a pressure of 0.5 GPa, although higher pressures and temperatures of up to 800°C were previously estimated (Balanyá et al, 1997;Tubía et al, 1997), in Alpine times. Time constraints for the Alpine event come from radiometric dating of the rims of zircon in metasediments (e.g.…”

“…Among the new diamondiferous rocks from the Betic-Rif orogen are those from the Jubrique Unit (Ruiz Cruz, 2013;Ruiz Cruz and Sanz de Galdeano, 2014), which were studied here again considering the wealth of P-T information especially from zoned garnet and geochronological information from monazite. Bartoli et al (2013). The location of the studied samples is indicated by a star.…”

Age of anatexis in the crustal footwall of the Ronda peridotites, S Spain

“…Hence, melting could have occurred at the wet granite solidus in the presence of H 2 Orich fluids and/or during the progressive fluid-absent breakdown melting of Bt (see Bartoli et al, 2013). The low proportion of hydrous minerals in the orthogneiss and relatively low T of melting for the generation of diatexitic migmatites point to water-fluxed melting (e.g.…”