THE VOLCANO-PLUTON CONNECTION: A PROGRESS REPORT BASED ON INTEGRATED RESEARCH OF VOLCANIC AND PLUTONIC SYSTEMS

Despite the evident similarities and analogies between granites and rhyolites, the complementary nature of the volcanic and plutonic records complicates understanding the connections in detail. We study volcanic and plutonic deposits as integrated magmatic systems that feed eruptions. We report on progress on some important questions:

• Contrasting timescales operate in silicic systems. Zircon records crystallization on timescales of 100s ka to ~2 Ma. The longest timescales are found in granites, due to the more protracted evolution of plutonic systems. This suggests a window of high productivity over which eruptible magma is formed and ejected to the surface. Quartz crystallization timescales are much shorter – decades to a few millennia – and reveal that eruptible magma bodies are unstable and prone to eruption. Rhyolites record moments of high melt abundance, while granites record the background activity during the evolution of magmatic systems.
• The fraction of magmas that erupt to the surface is highly variable. The volume of plutonic rocks in the Highland Range-Searchlight system (Nevada) is much greater than that observed in the associated volcanic sequences, as usually expected. Yet, supereruptions deposits leave a more equivocal record. The volume of crystal-poor, eruptible material in the Peach Spring Tuff (SW USA) is comparable or superior to that of erupted crystal-rich cumulates; granites in the region are not associated with the PST system. We infer that most of the PST system erupted, leaving a small volume of plutonics behind.
• Significant differentiation takes place in the shallow crust. While it is difficult to identify granitic cumulates based on textural and geochemical signatures, thermodynamic considerations unambiguously demonstrate that segregation of high-silica rhyolites takes place in the shallow crust. The cumulates formed during such differentiation events resemble granites found in shallow crustal levels. The shallow crust is the fertile ground for the formation of high-silica rhyolites.

Much is yet to be learned from the study of silicic systems. We need to develop approaches to better track the physical processes whereby eruptible magmas are extracted from mushes, and the conditions that lead to such extraction.