Determining the dynamics of magma flow within intrusions: Insights from field and petrographic studies of dikes and sills of the Inner Hebrides, Scotland.

Abstract:

Investigating the factors that influence magma flow dynamics within intrusions is important for understanding how magma migrates through the crust and erupts at the surface. Studying extinct volcanoes where erosion has exposed their plumbing systems provides insights into the final stages of magma movement. Fieldwork was conducted on the Isles of Mull and Skye, Western Scotland to study how magma flow is preserved within intrusions. On the Isle of Mull, 2-8m thick tholeiitic dolerite sills of the Loch Scridain Sill Complex (c.58Ma), crop out along the northern and southern coasts of the Ross of Mull, intruding into Paleogene lavas (c.60Ma) and Moine Supergroup Psammites respectively. Large columnar jointing is present perpendicular to the sill margins and is cross cut by arcuate fracture planes with parallel vesicle concentrations. Raised lineations were found on planar surfaces striking in similar directions to the arcuate fractures. These lineations have a range in strike from 148° to 172° and are inferred to be magma flow indicators. On the Isle of Skye, fieldwork was conducted in a disused quarry at Invertote where stacked olivine dolerite sills from the Little Minch Sill Complex (c.60Ma) intrude into Jurassic sediments. The sills are 0.5 to 8m thick, their contacts being defined by contrasts in jointing frequency, crystal layering, occurrence of gabbro lenses and variations in mineral proportion, size and composition. A series of five distinct sills crop out, with two N-S striking dikes cross-cutting the site. Pahoehoe ropes were found preserved on exposed contacts of one dike, which helped to infer the magma flow direction. Detailed sampling was carried out and petrographic thin sections analysed to describe the crystalline fabrics across the length and breadth of the intrusions. The results of the petrographic study are compared with magnetic fabrics from AMS and AARM analysis techniques to provide an independent quantification of flow trajectories.