Abstract
Understanding magma transport in sheet intrusions is crucial to interpreting volcanic unrest. Studies of dyke emplacement and geometry focus predominantly on low-viscosity, mafic dykes. Here, we present an in-depth study of two high-viscosity dykes (106 Pa·s) in the Chachahuén volcano, Argentina, the Great Dyke and the Sosa Dyke. To quantify dyke geometries, magma flow indicators, and magma viscosity, we combine photogrammetry, microstructural analysis, igneous petrology, Fourier-Transform-Infrared-Spectroscopy, and Anisotropy of Magnetic Susceptibility (AMS). Our results show that the dykes consist of 3 to 8 mappable segments up to 2 km long. Segments often end in a bifurcation, and segment tips are predominantly oval, but elliptical tips occur in the outermost segments of the Great Dyke. Furthermore, variations in host rocks have no observable impact on dyke geometry. AMS fabrics and other flow indicators in the Sosa Dyke show lateral magma flow in contrast to the vertical flow suggested by the segment geometries. A comparison with segment geometries of low-viscosity dykes shows that our high-viscosity dykes follow the same geometrical trend. In fact, the data compilation supports that dyke segment and tip geometries reflect different stages in dyke emplacement, questioning the current usage for final sheet geometries as proxies for emplacement mechanism.
Highlights
Dykes and sills,vertical andhorizontal sheet intrusions, respectively, are unarguably some of the most fundamental magma transport pathways in the Earth’s crust [1,2,3] and references therein
Our field observations on the geometry of the two exceptionally well exposed magmatic sheets: the Sosa Dyke and the Great Dyke, in the Miocene Chachahuén volcano in Argentina combined with magma flow indicators within the dykes, and their calculated magma viscosity lead us to conclude that:
The segment geometries observed in this study are related to the emplacement of the dykes, rather than the result of post-emplacement tectonic deformation; Anisotropy of Magnetic Susceptibility (AMS) and curved fractures record a dominant lateral magma flow direction in the Sosa Dyke towards the East, contradicting expected vertical magma flow derived from dyke segment geometry
Summary
Dykes and sills, (sub-)vertical and (sub-)horizontal sheet intrusions, respectively, are unarguably some of the most fundamental magma transport pathways in the Earth’s crust [1,2,3] and references therein. The low values of thickness-to-length aspect ratios of sheet intrusions have been interpreted as indicators of emplacement as Mode I tensile fractures [2,17,18,19,20] In agreement with this model, field observations of sharp intrusion tip geometries have been mostly described for low-viscosity magma [18,21,22,23]. Recent studies of low-viscosity magma sheet intrusions observed blunt tip shapes, suggesting the need to consider alternative emplacement mechanisms for mafic sheet intrusions as well [21,28,29]. Better constraining the emplacement mechanisms of felsic sheet intrusions requires additional data on (1) magma flow proxies within the intrusion, such as the orientation of crystals, e.g., ferro magnetic grains, within the intrusions [33,34,35], and (2) outside the intrusion, such as deformation structures in the host rock [26,27,36,37]
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