Formation of spinel-cordierite-feldspar-glass coronas after garnet in metapelitic xenoliths: Reaction modelling and geodynamic implications

Spinel + cordierite + K-feldspar + plagioclase + glass form coronas around garnet in metapelitic xenoliths at El Hoyazo and Mazarron, two localities of the Neogene Volcanic Province (NVP) of SE Spain. The presence of fresh glass (quenched melt) in all phases shows that corona development occurred under partial melting conditions. Algebraic analysis of mass balance in the NCKFMASH system suggests the reaction Grt + Sil + Bt + Pl = Spl + Crd + Kfs + melt as the most plausible model for the development of coronas in the El Hoyazo sample, and indicates that biotite was required as reactant for the formation of cordierite. The P-T conditions for the formation of coronas are estimated at similar to 820 +/- 50 degrees C, 4.5 +/- 0.6 kbar at El Hoyazo, and similar to 820 +/- 50 degrees C, 4.0 +/- 0.4 kbar at Mazarron. The El Hoyazo xenoliths record a complex P-T history, characterized by early melt production during heating and additional melting during decompression. A local cooling event characterized by minor retrograde reaction and melt crystallization preceded ascent and eruption. This study shows that detailed xenolith analysis may be used to track magma evolution in a chamber.