THE PETROGENESIS OF EARLY–MIDDLE JURASSICMAGMATISM IN SOUTHERN AND CENTRAL MEXICO ANDITS ROLE DURING THE BREAK-UP OF WESTERN PANGAEA

Central and southern Mexico represents a critical location to investigate the dynamics of Pangaea break-up and its influence on the evolution of the Pacific margin of North America. However, the origin of the Early-Middle Jurassic magmatic rocks and volcano-sedimentary successions presently distributed across this region is highly debated. Some authors proposed that they represent a vestige of a continental arc, while others interpreted them as several deposits of syn-rift magmatism.

In this contribution, we present a reassessment of the petrogenesis of Early-Middle Jurassic magmatic rocks from southern and central Mexico based on available geochemical data. Our results indicate that these rocks likely represent almost pure crustal melts associated with a rift environment. In fact, they exhibit high silica contents, low concentration of MgO, FeO and CaO, and a moderate to strong peraluminous character which are typical of sediment melts. Moreover, the modest HREE fractionation, the low Sr and Ca contents and the high K₂O/Na₂O ratio of these rocks suggest that melting took place in a relatively shallow environment (<0.8–1.0 GPa) under fluid-absent conditions. Interestingly, the age distributions of their inherited zircon grains identify the widespread Upper Triassic metasedimentary sequences presently exposed in southwestern and central Mexico as a potential crustal source of these Jurassic igneous rocks.

Accordingly, we argue that these Early-Middle Jurassic magmas likely originated in a syn-rift igneous province associated with the continental attenuation that preceded the opening of the Gulf of Mexico, rather than in a continental arc. This rifting may not necessarily be related to the Pacific tectonic evolution of North America, but rather be a consequence of Pangaea fragmentation. Within this interpretation, we propose the existence of a Triassic–Middle Jurassic transform fault, here named the Trans-America-transform boundary, that connected the North and South American subduction zones during Early-Middle Jurassic time. This imply that post-Pangaea subduction initiation beneath southern and central Mexico must be younger than Middle Jurassic time, possibly triggered by changes in plate motions resulting from the opening of the GM, in the context of Pangaea fragmentation.