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Central and southern Mexico represents a strategic place to understand the dynamics of Pangaea break-up and its influences on the evolution of the Pacific margin of North America. Lower–Middle Jurassic volcano-sedimentary successions, and scarce magmatic rocks, crop out discontinuously across this region and have been interpreted either as a vestige of a continental arc or as several deposits of syn-rift magmatism. At present, their origin is controversial. Available geochemical data on these igneous rocks suggest that they represent almost pure crustal melts produced in a rift environment rather than in an arc. In fact, the studied rocks exhibit the high silica contents and moderate to strong peraluminous character typical of sediment melts. The enriched isotopic composition (high 86Sr/87Sr and low 143Nd/144Nd) and the age distributions of inherited zircon grains readily identify the widespread Upper Triassic metasedimentary sequences presently exposed in southwestern and central Mexico as the most likely crustal source of these Jurassic igneous rocks. Accordingly, we argue that these Early–Middle Jurassic magmas originated in a syn-rift igneous province associated with extensional-driven crustal attenuation in the context of Pangaea fragmentation. Our findings also constrain post-Pangaea subduction initiation to be younger than Middle Jurassic time in central and southern Mexico.

The biological and sedimentological surface-processes recorded in the compositions of seafloor sediments are transferred to the Earth's interior at convergent margins. These materials can be either recycled into the deep mantle forming long-term compositional heterogeneities (Stracke et al., 2005), or be reworked shallowly into arc magmas giving shape to the continental masses (Behn et al., 2011). While it has been long recognized that subducted sediments exert an important control in the compositions of island arcs (Plank, 2005), less is known about the geochemical imprint of the oceanic realm in thick-crusted continental arc sections. This is because subducting sediments (i.e., GLOSS; Plank, 2013) are compositionally very similar to the overriding continental crust with which ascending magmas may interact, and because forearc subduction erosion may introduce a volumetrically higher crustal influx at most continental margins that could overwhelm the oceanic inputs (Straub et al., 2020).