Syn-deformational migmatites and magmatic-arc metamorphism in the Xolapa Complex, southern Mexico

The Xolapa Complex (XC) is the largest plutonic and metamorphic mid-crustal basement unit in Mexico and represents an ancient continental magmatic-arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high-grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz +/- Opx and Grt + Opx + Pl + Qtz +/- Cpx pre-migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre-, syn- and post-migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2-D3 phases. Late post-migmatitic ductile-brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E-W left-lateral mylonitic shear zones (D5). Biotite-breakdown melting in felsic rocks and amphibole-breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900 degrees C and pressures ranging from &rt;= 6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P-T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time-marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north-east-verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous - Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic-arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico.