Pulling apart the Mid to Late Cenozoic magmatic record of the Gulf of California: is there a Comondú Arc?
The composition of the lithosphere can be fundamentally altered by long-lived subduction processes such that subduction-modified lithosphere can survive for hundreds of millions of years. Incorrect petrotectonic interpretations result when spatial–temporal–compositional trends of, and source contributions to, magmatism are not properly considered. Western Mexico has had protracted Cenozoic magmatism developed mostly in-board of active oceanic plate subduction beneath western North America. A broad range of igneous compositions from basalt to high-silica rhyolite were erupted with intermediate to silicic compositions in particular, showing calc-alkaline and other typical subduction-related geochemical signatures. A major Oligocene rhyolitic ignimbrite ‘flare-up’ (>300 000 km3) switched to a bimodal volcanic phase in the Early Miocene (c. 100 000 km3), associated with distributed extension and opening of numerous graben. Extension became more focused c. 18 Ma resulting in localized volcanic activity along the future site of the Gulf of California. This localized volcanism (known as the Comondú ‘arc’) was dominantly effusive and andesite–dacite in composition. Past tectonic interpretations of Comondú-age volcanism may have been incorrect as these regional temporal–compositional changes are alternatively interpreted as a result of increased mixing of mantle-derived basaltic and crust-derived rhyolitic magmas in an active rift environment rather than fluid flux melting of the mantle wedge above the subducting Guadalupe Plate.