NEOGENE VOLCANISM AT THE NORTHEASTERN TRANS-MEXICAN VOLCANIC BELT: EVIDENCE FOR ARC-WIDE FELSIC-BIMODAL VOLCANISM RELATED TO SLAB ROLLBACK

We reconstructed the Late Miocene to Quaternary evolution of a wide region located behind the active front of the eastern Trans-Mexican Volcanic Belt using a large collection of samples located into a geochronologically controlled stratigraphy. The region runs from the Pachuca-Tulancingo area (Hidalgo state) in the south to the Huejutla- Sierra de Tantima area (northern Veracruz state). In this region, volcanic activity shows a progressive trenchward migration, which is interpreted to reflect the rollback of the subducting plate after the detachment of the lower part of the slab at ~7.5 Ma, following a prolonged episode of flat subduction.

The first and northernmost volcanic episode consists of OIB-like basalts with ages between ~7.5 and ~5.15 Ma and geochemical evidence for an origin in the lithospheric mantle, a signature that weakens with time while crustal contribution become progressively dominant. At ~4.4 Ma, massive regional rhyolitic ignimbrite and large peraluminous rhyolite domes erupted. This episode was followed by a mid Pliocene (~4.08-2.14 Ma) succession of mafic to intermediate, partly shoshonitic, lavas, which are locally covered by late Pliocene (2.14 Ma), peralkaline rhyolite lavas and Quaternary intraplate basalts (1.6-1.3 Ma). The isotopic and elemental composition of the early Pliocene to Quaternary lavas indicates negligible contributions of subducted components, and a dominant role of crustal assimilation and magma mixing processes in the genesis of the early Pliocene rocks, which continuously diminished afterwards.

The concurrent inception of silicic volcanism with the beginning of the trenchward migration of the arc points to a causal link between the two phenomena. Trenchward migration of the volcanic front suggests that, after the detachment event, the slab started to rollback and increase its dip. In this scenario, silicic volcanism may have originated by partial melting of the lower crust due to its progressive exposure to asthenospheric mantle as the slab retreated. Large amounts of silicic magma can form by partial melting of lower crust previously hydrated by infiltration of slab-derived fluids during flat subduction. A similar succession of volcanic events can be traced towards the volcanic front as rollback advances.