Ignimbrites to Batholiths

Abstract:

Ignimbrites sample large magma reservoirs in the Earth’s upper crust, sometimes digging deep enough to link the volcanic realm with the plutonic world. Integrating textural, petrological, geochemical, and geochronological information on such deposits with geophysical signals (gravimetry, seismology, MT surveys) suggest incremental growth and evolution of subvolcanic magmas bodies that are dominated by high crystallinity mush zones, but remain in parts, and during periods of relatively high magma fluxes, sufficiently liquid to erupt. The eruptible upper portions are either extracted melt from the mush, or rejuvenated fractions of it, and constitute only a small volumetric fraction of the vertically extensive mushy batholithic magma body. The high-flux, ignimbrite flare-up phases are typically preceded by waxing magmatism that prime the crust to hold large upper crustal silicic reservoirs where the most evolved magmas, rhyolites, are likely to be generated. Ages and compositional complementarities between the silicic volcanic and plutonic lithologies suggest that they are intimately linked, although plutonic masses will be inherently more complex and longer-lived than any volcanic systems spawned from them. Gas exsolution within such mushy reservoirs, and accumulation of the low density bubbles in the most melt-rich parts of the system, will also enhance eruptibility, emphasizing some of the observed chemical differences between evolved plutonic and volcanic rocks (e.g., typically higher Rb/Sr ratios in volcanic units).