GEODYNAMIC SIGNIFICANCE OF LARGE VOLUME SILICIC VOLCANISM IN THE TRANS-MEXICAN VOLCANIC BELT

A large volume of silicic lavas and tuffs were emplaced in the western and eastern Trans-Mexican Volcanic Belt (TMVB) after a late Miocene episode of mafic volcanism related to slab detachment. New and published geologic, geochronologic and geochemical data shed light to the geodynamic mechanism responsible for this uncommon volcanism in an arc setting. West of 103°W silicic volcanism consists of large dome complexes and less ignimbrites (~1,200 km³) emplaced in a position similar to the previous mafic episode. East of 101° W dome complexes and large volume dacitic to rhyolitic ignimbrites (>50 km³ each) are found immediately to the south of the mafic episode. Similar features shared by both regions include: 1) silicic rocks are initially the dominant or sole type of volcanism; 2) since the end of Miocene mafic lavas start to be emplaced together with the silicic rocks or mingled in some ignimbrites; 3) silicic volcanism migrated southward (>200 km in the east and ~100 km in the west). The two regions also show some differences: 1) in the west pre-Pleistocene volcanism is essentially bimodal and some of the mafic lavas show an intraplate signature; 2) Sr and Nd isotope data indicate much less involvement of the crust in the west than in the east; 3) intra-arc extensional faulting since the end of Miocene is more prominent in the west.

Thermal modeling of the present subduction system shows temperatures high enough to produce partial melting at the base of the crust, also consistent with a low resistivity layer imaged by a magnetotelluric study in the lower crust. We proposed that silicic volcanism in the TMVB is the manifestation of the progressive ascent of hot sub-slab asthenosphere induced by slab detachment and enhanced by slab rollback since the late Miocene. This overarching mechanism promoted variable degree of crustal melting, modulated by the different nature and thickness of the crust in the western and eastern TMVB. Thinner and younger crust in the west together with higher rate of extensional faulting hindered the formation of large magma chambers (less ignimbrites) while imparted less radiogenic signature to the rhyolites. Thick Precambrian and Paleozoic crust and less extension in the east promote ponding of mantle melts in the crust, formation of large magma chambers, magma hybridization and a more radiogenic signature.