Mechanisms of Magma Disaggregation in a Cooler Host: Volcanic, Plutonic, and Theoretical Considerations

Abstract

In the Siða-Fljotshverfi District of south Iceland, Pleistocene basaltic lava forms flame-like apophyses, dikes, and disaggregation structures that invade overlying hyaloclastite. Apophyses as well as underlying lavas are cube-jointed, indicating rapid cooling due to formation in a wet environment and suggesting that hyaloclastite and lava were emplaced almost concurrently, while hyaloclastite was wet and weak. Thermal modeling suggests that influx of heat from the underlying lava resulted in increased fluid pressure in the hyaloclastite matrix. Fracturing of the chilled rind that had formed atop the lava permitted injection of lava into the overlying hyaloclastite. Diffusion of pressure away from the injection site dragged the matrix apart, facilitating propagation of lava upward to form the apophyses.

Most of Aztec Wash, a mid-sized Miocene pluton in the Colorado River Extensional Corridor, is characterized by mafic, intermediate, and felsic rocks that are interspersed on very small to large scale. Field relations suggest that this heterogeneous assemblage is the product of repeated injection of basaltic magma into a variably crystal-rich magma chamber and subsequent mechanical and chemical interactions. The most pristine manifestations of the intruding mafic magma are sheets and discrete pillow-like masses of basaltic material hosted by granite. The petrographic and textural similarity between sheets and pillows suggests that the latter formed from the former by disaggregation while still molten.

The goal of this study is to understand how mafic disaggregation into a cooler host works in these two specific settings, with potential application to magmatic systems as a whole.