EXTENSIONAL TECTONICS TRIGGERING PARTIAL MELTING OF SHALLOW INTRUSIONS IN THE BOLAñOS GRABEN, SOUTHERN SIERRA MADRE OCCIDENTAL, MEXICO

It has been proposed that large silicic eruptions tapping semi-molten batholiths may be triggered by concurrent extensional faulting, and would produce crystal-rich, fissure-fed ignimbrites and “graben-calderas” (Gottsman et al., 2009, EPSL). The 140 km long, N-S trending Bolaños graben, in the southeastern part of the Sierra Madre Occidental silicic large igneous province, has been suggested as an example of these volcano-tectonic structures. Here we present field, U-Pb zircon geochronologic and chemical data from the central Bolaños graben that are at odds with this ‘graben-caldera’ model. At Bolaños a lower package of sandstones, basaltic lavas and Late Oligocene ignimbrites are overlain by the distinctive ~25 Ma (Ar-Ar) Alacrán ignimbrite and numerous rhyolite domes dated at ~25 to ~23 Ma, which are capped by ~21 and ~19 Ma basalts and ignimbrites from sources away from the graben. The Alacrán eruption and rhyolite dome emplacement occurred along N-S faults during the initiation of graben formation. However, several aspects contradict eruption(s) from a pre-existing molten or semi-molten batholith. The Alacrán ignimbrite is the best candidate for a graben-caldera-related ignimbrite but is very crystal poor (<5%) and the low zircon yields, high antecrystic zircon content (clustering at ~29 Ma) and Zr dissolution modeling indicate rapid magma generation and eruption with magma residence times <30,000 yrs. Furthermore, rhyolite domes emplaced in the graben and associated in space and time with the Alacrán have different age systematics and zircon compositions, precluding a common batholithic chamber beneath the Bolaños graben. The repeated and widespread eruption of basalts through extensional faults is also hard to reconcile with a semi-molten chamber, which should have acted as a density barrier to ascending mafic magmas. We favor a model in which early Miocene extension allowed basalts to intrude at shallow depths and erupt, encouraging the partial melting of Oligocene plutons, and rapid removal of rhyolite melts without the necessary formation of batholiths.