The Rhyolite Factory: Insights from rhyolite-MELTS geobarometry of plutonic rocks and associated volcanics

Guilherme Augusto Rosa Gualda , Calvin F Miller, Blake M Wallrich

Magmatic systems feed eruptions to the surface; lead to the formation of ore deposits; provide energy for geothermal systems; and are key to Earth’s differentiation. While it is commonly accepted that silicic magmatic systems span much of the crust, little direct evidence is available for their vertical continuity (or lack thereof), or for the distribution of melt within them.

We focus on Miocene plutonic and volcanic units exposed in the Colorado Extensional Corridor, SW USA. Plutonic units (Searchlight Pluton–SLP, Aztec Wash Pluton–AWP, and Spirit Mountain Batholith–SMB) consist primarily of coarse-grained granitoids rich in feldspar that can be credibly considered cumulates. Marginal facies and fine-grained dikes and sills are interpreted to record melt compositions that fed the plutons. Leucogranite dikes and roof units were extracted from the crystallizing plutons. The nearby Upper Highland Range Volcanics record compositions extracted from the SLP system.

We use whole-rock compositions of granitoids and rhyolites to calculate extraction pressures, and glass compositions from volcanic rocks to calculate pre-eruptive storage pressures using rhyolite-MELTS. We seek pressures consistent with assemblages containing quartz+2 feldspars±magnetite±ilmenite (Q2F or Q2FMI assemblages). We use the calculated pressures to assess the distribution of magma in silicic magmatic crustal columns.

The dataset reveals three main clusters of compositions and pressures: 72-74.5 wt.% SiO2, 450-370 MPa (Q2F extraction); 75.5-77 wt.% SiO2, 300-185 MPa (Q2FMI extraction and pre-eruptive storage); 77.5-78 wt.% SiO2, 180-120 MPa (Q2FMI extraction and pre-eruptive storage). Compositions attributed to cumulates (based on texture, major and trace-element compositions) do not typically yield extraction pressures, suggesting that rhyolite-MELTS can generally distinguish magmatic from cumulatic compositions.

Our data show that magma distribution spanned from the middle crust to the surface, with well-defined gaps in pressure between the three groups. Magma mushes were located in the middle crust (~400 MPa, ~15 km depth), from which magmas that fed the shallow plutonic units were derived – there is no exposed record of these magma mushes, and they are inferred from extraction pressures for the less evolved fine-grained rocks. We infer two sets of shallower mush bodies that fed eruptions to the surface. The leucogranite roof zones represent bodies of melt-dominated magma that failed to erupt and instead solidified in the subsurface. Magma distribution is vertically discretized, rather than continuous as shown in most models – there are specific horizons within the crust where magma accumulation is favored, while much of the crust remains melt-free.