Enigmatic Relationship Between Slicic Volcanic and Plutonic Rocks

The accumulation of sizeable volumes of magma in the upper crust may produce plutons and/or result in supereruptions. Geophysical observations provide constraints on the rates, volumes, and melt distributions in magmatic systems, but they suffer from limited resolution and inherent nonuniqueness. Different, yet complementary, geophysical approaches must be combined with petrological, laboratory, and geochemical measurements. We summarize the results from such a combined approach from the central Andes. Taking a global perspective on large silicic systems reveals that several have >10% partial melt over large areas (10s of km2), and there may be localized zones with 50% or more.

Crustal magmatic systems are giant heat engines, fed from below by pulses of hot magma, and depleted by loss of heat to their surroundings via conduction or convection. Heat loss drives crystallization and degassing, which change the physical state of the system from relatively low-viscosity, eruptible melt, to high-viscosity, immobile, partially molten rock. We explore the temporal evolution of incrementally grown magmatic systems using numerical models of heat transfer. We show that their physical characteristics depend on magma emplacement rates and that the majority of a magma system’s lifetime is spent in a highly crystalline state. We speculate about what we can, and cannot, learn about magmatic systems from their volcanic output.

A rich history of experimental petrology has revealed the paths by which silicic igneous rocks follow mineral–melt equilibria during differentiation. Subdividing these rocks by ‘molar Al versus Ca + Na + K’ illustrates first-order differences in mineralogy and gives insight into formation mechanisms. Peraluminous magmas, formed by partial melting of sediments, largely owe their attributes and compositions to melting reactions in the protoliths, whereas most metaluminous felsic magmas record both continental and mantle inputs. Peralkaline rhyolites are mainly derived from either protracted crystallization or small degrees of partial melting of basalt, with only a marginal crustal contribution. Most silicic magmas hold 3–7 wt% H2Omelt, which is inversely correlated with pre-eruptive temperature (700 °C to >950 °C) but unrelated to their reduced/oxidized state.

Silicic volcanic systems provide timed snapshots at the Earth’s surface of the magmatic processes that also build complementary plutons in the crust. Links between these two realms are considered here using three Quaternary (<2.6 Ma) examples from New Zealand and the USA. In these systems, magmatic processes can be timed and the changes in magmatic conditions can be followed through the sequence of quenched volcanic eruption products. Before an eruption, magma accumulation processes can occur on timescales as short as decades, and whole magma systems can be rebuilt in millennia. Silicic volcanic processes, in general, act on timescales that are too rapid to be effectively measured in the exposed plutonic record.

Beneath volcanoes are magmas that never erupt but that become frozen into feldspar- and quartz-rich rocks broadly called granite. Where the crystallized magmas form bodies with distinctive textures, they are grouped into named units—plutons. The rate (pace) at which magmas accumulate into plutons is fundamental to understanding both how room is made for the magmas and how unerupted and erupted magmas are connected. Dating plutonic rocks suggests that plutons accumulate slowly. Although the pace of magma accumulation does not preclude direct connections between plutons and small volcanic eruptions, it appears to be far too slow to support connections between most plutons and supereruptions.

The relationships between silicic volcanic and plutonic rocks have long puzzled geologists because the rich set of observations from petrology, geochronology, thermal modeling, geophysical techniques, and geochemistry have led to contradictory interpretations. Although compositional evolutionary trends leading to granite and rhyolite are congruent, it is not clear if rhyolites are formed by the extraction of melt from shallow crystal mushes that otherwise solidify to form granite plutons, or are derived from a greater depth in parallel with granite plutons, or are formed by processes separate from those which form granite plutons. Finding a consistent explanation for the silicic volcanic–plutonic relationship bears on important Earth science questions, including, “How is silicic continental crust formed?” and, “Can we predict supereruptions?”