Cenozoic Ignimbrites, Source Calderas, Relict Magma Chambers, and Tectonic Settings: Perspectives from Cordilleran North America (Invited)

In the early 1960s, new concepts and innovative techniques coalesced spectacularly to improve understanding of Tertiary pyroclastic volcanism in North America. Spotty recognition of welded tuff, among rocks mostly described as silicic lava flows, exploded with identification of individual ignimbrite sheets, some having volumes >103 km3 and extending >100 km from source calderas. R.l. Smith, during study of the Bandelier Tuff in New Mexico, documented complexities of welding and crystallization zones that provided a genetic framework (cooling units) for ignimbrite studies (even while confusion continues in some areas where talus and vegetation obscure bench-forming contact zones between densely welded cliffs). Also in the 1960s, application of isotopic age determinations (initially K-Ar, now largely superceded by 40Ar/39Ar laser fusion) and precise paleomagnetic pole directions became key tools for correlating ignimbrites, deciphering eruptive histories, and determining volcano-tectonic patterns. Dated ignimbrites provide unique stratigraphic markers within volcanic field, as well as datums for regional structures and the shifting patterns of volcanism related to global plate motions--another happy coincidence in the 1960s as plate-tectonic models were formulated. Tertiary ignimbrite flare-ups along the Cordilleran margin increasingly are recognized as coinciding with inception of regional extension, especially during transitions from episodes of low-angle convergence. Many large caldera sources for the Tertiary ignimbrites have now been identified, in place of prior vague concepts of “volcano-tectonic depressions”, especially as the contrasts between thin outflow and thickly ponded intracaldera ignimbrite with interleaved collapse breccia became appreciated. Multi-km-thick fills in many calderas document that collapse begins early during large ignimbrite eruptions, and downsag inception was succeeded by breakage along ring faults. Resurgent uplift has been identified at many ignimbrite calderas, building on the pioneering observations of van Bemmelen at Lake Toba, Indonesia. Still many Tertiary caldera systems remain poorly understood where buried beneath younger rocks, others completely eroded to levels of subvolcanic granitic plutons. Links between silicic volcanism and batholith formation in continental crust continue a major research focus; improved petrologic, isotopic, and geophysical techniques are helping evaluate compositional and age relations between extrusive and intrusive components, as well as present-day intrusion geometry relative to times of peak volcanism. Ignimbrites that preserve quenched compositional gradients, commonly from rhyolite upward into crystal-rich dacite, were early recognized as special opportunities for magma-chamber studies, especially as analytical methods improved (XRF and INAA rock chemistry, microprobe mineral compositions, radiogenic and stable isotope geochemistry). These demonstrated the importance of mafic magma from the mantle, melting/assimilation in the lower crust, and mixing of diverse magmas during rise and eruption, even as recent studies by electron and/or ion probe documented complex crystal cargos (mixed phenocrysts, xenocrysts, and antecrysts).