Early Tertiary extensional magmatism in southern Mexico and its relationship to exhumation of the Xolapa complex and detachment of the Chortis block

The Xolapa complex of southern Mexico is a crustal block, the age and nature of which are still unresolved. Its origin and tectonic evolution enclose critical clues about the geodynamic evolution of southern Mexico and the detachment and translation of the Chortis block of Central America and, hence, about the evolution of southern North America at the end of Mesozoic and the genesis of the Caribbean plate. To gain insights on these issues, detailed field observations, petrography, and whole-rock (major- and trace-element) geochemistry analyses, complemented with new geochronological (U-Pb and Ar-Ar) data, and whole-rock isotopic (Nd, Sr, Pb, and Os) determinations, were performed on deformed to undeformed plutons and dikes from the northern Xolapa complex. In addition to the Acapulco and Xaltianguis-Tierra Colorada magmatic events of ca. 54-50 Ma and 35-34 Ma, respectively, 10 distinctive magmatic pulses were recognized in the zone with ages ranging from ca. 61 Ma to ca. 32 Ma. Our geochronological data constrain the ages of the end of deformation and migmatization in the zone at ca. 60 Ma and ca. 55 Ma, respectively. Early Tertiary magmatism was dominantly mafic or felsic, and at least three bimodal magmatic suites were identified. Their geochemistry and isotopic compositions are compatible with emplacement in a continental back-arc or in a postcollisional, extensional setting and provide information on the participation of mantle and continental crust in magma genesis. Extension produced the exhumation of the northern Xolapa complex and the detachment and migration of the Chortis block at ca. 59 Ma. Extensional magmatism transitioned to a compressional regime related to subduction at the end of the Eocene.