Partial melting of basement metasedimentary rocks from the central Xolapa Complex, southern Mexico

The Xolapa Complex in the Amuzgos region is made up of a poly-lithological metasedimentary sequence mainly consisting of garnet-sillimanite paragneisses, garnet mica schists, mafic meta-graywackes and marbles interlayered with mafic and quartz-feldspatic orthogneisses. These units as a whole constitute a highly deformed metamorphic basement, within which structural and mineralogical characteristics typical for high metamorphic grade (D1) are recognized. These features precede the development of a complex migmatitic structure which produced a wide range of metatexitic and diatexitic morphologies. The interaction between a generalized partial melting process and a syn-anatectic heterogeneous deformation process (D2) partially destroyed the internal pre-anatectic structure of the metamorphic complex. This metamorphic-migmatitic basement is affected by at least two post-anatectic ductile-fragile deformation stages (D3-D4) that produced the dislocation of D1-D2 structures, generating folds, back-thrusts and regional shear zones. Pre-anatectic paragenesis and deformation structures in lithological relicts define a prograde metamorphic path with a minimum pressure of 9 kbar and temperature >600 °C, which represents the relatively oldest recognizable crystallization stage. Subsequently, the formation of neosomes defines a new metamorphic stage at suprasolidus conditions at 750-850 °C and 7-8 kbar. The latest period of this stage evolved with the development of replacement paragenesis that indicate back reaction and rapid decompression under conditions close to the solidus at 600-700 °C and 4-4.5 kbar. Finally, the generalized presence of low temperature mineral paragenesis and deformation structures indicate the heterogeneous development of a retrograde stage at green-schist facies metamorphic conditions. The progressive consumption of residual and peritectic mineral phases in the presence of melt during decompression, controls the occurrence of cordierite and hercynite. These phases are tracers of a late migmatitic evolution, related to the highest point of anatectic segregation-accumulation and to the development of the diatexitic structure. Integration of geological and thermobarometric information reveal a fragment of Xolapa Complex's tectono-thermal history, from its thermal peak to the beginning of its exhumation.