Mantle wedge perturbation induced by slab detachment and the Mio-Pliocene bimodal volcanism in the Trans-Mexican Volcanic Belt

The trench-oblique orientation, the coexistence of geochemical diverse lavas (OIB, CAB, Adakites etc.), and the absence of seismicity beneath the Trans-Mexican Volcanic Belt (TMVB) prompted several workers to formulate genetic models at variance with a classic subduction scenario, including the presence of a mantle plume beneath central Mexico. Based on a careful analysis of its geologic and geochemical evolution we consider, in turn, that this volcanic chain is a continental arc whose complexity is due to the thermal and mechanical perturbation of the mantle wedge imposed by plate history and modulated by crustal thickness, composition and structures. The TMVB began in Middle Miocene as a WNW trending arc of andesitic-dacitic polygenetic volcanoes. This relatively normal situation changed in Late Miocene, when mafic plateaus, cinder cones and fissural lava flows were emplaced to the north of the present TMVB with a clear eastward migrating pattern from ~11.5 and 6.5 Ma. This mafic pulse has been related to the eastward propagation of a slab detachment episode, in the southern Gulf of California, which produced a transient thermal anomaly in the mantle (Ferrari, 2004, Geology). Following this episode, volcanism strongly decreased and becomes more evolved. Dacitic to rhyolitic domes and ignimbrites were emplaced in a belt located just to the south of the previous episode between 7.5 and ~3.0 Ma. Dome complexes dominate the western half of the TMVB, whereas caldera-forming ignimbrites are common to the east. The geochemical character of this bimodal volcanism was analyzed using new Sr and Nd isotope and our database of chemical data (~3,000 samples). The mafic pulse has a basaltic composition [average SiO2 = 50.7±4.0(1s)] readily distinguishable from the following volcanism. Most basalts have a subduction signature, although they show no systematic correlation with distance from the trench. East of Long. 99° W, however, they are Ne-normative and display much lower to none influence of subducted sediments and fluids (lower Ba/Nb, La/Nb and Th/Nb). This geochemical boundary separates the region of Oligo-Miocene subduction metasomatism related to the Sierra Madre Occidental (to the west) from the region where the mantle was unaffected by subduction since the Permian. For the western TMVB rhyolites, the available isotope data (87Sr/86Sr = 0.70396-0.70597; ɛNd = 4.07-5.01) point to a mantle origin with variable crust assimilation. This suggests that the latest Miocene switch of volcanism toward more silicic composition was the effect of the decrease in subduction rate of the Rivera plate (DeMets & Traylen 2000), an expected consequence of the loss of slab pull after slab detachment. Decrease in convergence reduced flux of the mantle and amount of melting, so the magma started to pond in the crust and underwent fractional crystallization and variable assimilation. In the eastern half of the TMVB both basalts and rhyolites show the highest signature of crustal contamination in the 87Sr/86Sr vs. 143Nd/144Nd plane. This region corresponds to the area where crust is thicker and extension was much less intense than in the west or absent. Here partial melting of the crust may play an important role in generating the dacitic to rhyolitic magmas, likely as a consequence of the rollback of the slab that exposed the base of the upper plate to hotter asthenosphere.