Abstract
Exhumed magma conduits provide important evidence of the development and evolution of subvolcanic plumbing systems. We use a 5−14-m-thick flow-banded rhyolite dike in Arran (Scotland) to present the first reconstruction of the directions and styles of initial propagation and subsequent magma flow, based on mesoscale kinematic indicators. The dike has concave-inward dike-margin segments with plumose-like structures that record vertical and horizontal propagation of lobes, which inflated and linked to form a through-going sheet. Devitrified rhyolite zones at the dike margins show gentle to open folds. In contrast, glassy central parts of the dike are flow laminated and preserve folded and refolded isoclinal, curvilinear folds and sheath folds that record sustained progressive deformation. The inner interface between the glassy and devitrified facies is abrupt and marked by elongation lineations and mullions. In the dike center, fold axes plunge 27°NE along the dike, and parallel to elongation lineations. Combined with shear sense indicators (σ- and δ-objects, sheared vesicles, and asymmetric folds), these features indicate that magma flow was obliquely upward, to the southwest, and locally ≤60° to the propagation direction of the dike. The distribution of structures within the rhyolite indicates local accretion of the (now) devitrified material to the margins, with localization of flow into the center of the dike. We find that the initial magma flow direction was controlled by fracture propagation and interaction, with the subsequent flow record controlled by accretion and flow localization in the conduit. This study demonstrates that analysis of mesoscopic structural and kinematic features (several of which have not previously been reported from dikes) is a powerful tool that can be used to reconstruct the complex evolution of conduit initiation and magma flow processes.
Highlights
Exposed igneous dikes are exhumed frozen conduits along which magma flowed, and in some cases, fed explosive or effusive volcanic eruptions
Thin sections highlight several axial planar fabrics, including preferential microlite alignment (Fig. 8D), and two scales of fractures: (1) mm233 length and μm-aperture fractures that are preferentially developed in devitrified laminations (Fig. 8B) and form close-spaced networks; and (2) cm-length fractures with apertures up to ~0.1 mm (Fig. 8b,C) that are spaced at the mm-scale
Axial planar fabrics have not been reported in glassy dikes hitherto, an example has been reported in a glassy rhyolite lava and inferred to have formed by mechanical rotation of phenocrysts
Summary
Exposed igneous dikes are exhumed frozen conduits along which magma flowed, and in some cases, fed explosive or effusive volcanic eruptions. Measured fabrics record the last increments of strain as magma shear at that local site ceased, whereas initial conduit propagation by hydrofracturing (e.g., Baer 1995) ahead of the dike tip may be independent of magma transport direction in the formed conduit, it is important to constrain the temporal evolution of flow within the system. Preserved mesoscopic structures and fabrics highlight how changes to magma properties cause the conduit geometry to evolve during transport and arrest. Mesoscopic features, such as dikewall plumose structures, s - and d - objects, verging folds, sheath folds, sheared vesicles, and mullion structures, are common in silicic plumbing systems, and can readily be employed to 70 reconstruct processes in intrusions, eruptive conduits, and lavas
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