Geometry, scaling relationships and emplacement dynamics of a ca. 6 Ma shallow felsic sill complex, Calamita Peninsula, Elba Island, Italy

An interconnected system of 5.5 to 5.9 Ma leucogranitic sills is well exposed on the eastern side of the Calamita Peninsula, Elba Island, Northern Tyrrhenian Sea, Italy. The sill complex intrudes the well-foliated Calamita schists, the lowermost thrust unit on eastern Elba. The sills are spatially confined to the high metamorphic grade hornfelsic thermal aureole of the ca. 5.9 Ma Porto Azzuro pluton, to which they may be petrogenetically related. Sill contacts and wall-rock foliations are folded about a gently north plunging open antiform cored by the Porto Azzuro pluton and its thermal aureole. The sills are consistently oriented at shallow oblique angles to the wall-rock foliation, which after removal of post-intrusion folding suggests emplacement in a subhorizontal shear regime. Thickness (T) and length (L) measurements collected from 70 sills range in tip to tip dimensions from L = 1 to 170 m with corresponding maximum openings T = 0.07 to 7 m. Where not modified by syn- and post-emplacement deformation sills are parallel sided for most of their length with convex-inward tapering tips. Reduced major axis regression of the sill dimensional data on logT vs. log L plots defines a power-law scaling relationship T ~ L^0.9. This scaling is difficult to reconcile with Linear Elastic Fracture Mechanics and crack-lubrication theory, which predicts a scaling T ~ L^0.5 for pressurized sills in fracture toughness-dominated propagation regimes or T ~ L^0.25 for viscosity-dominated regimes. If we consider, however, that our two-dimensional measurements are random samples (chords) through disk-shaped sills, then theory predicts that field data should fall between T ~ L^0.5 and L for the toughness-dominated regime or T ~ L^0.25 and L^1.25 for the viscosity-dominated regime. For the Elba data, unrealistically high magma viscosities and volumetric flow rates are required to fit the viscosity dominated regime. However, the fracture-toughness dominated regime brackets the data well if field-scale fracture toughness values exceed laboratory values by one to two orders of magnitude. Such high effective fracture toughness values are feasible if the well-known fracture size and ambient stress dependence of fracture toughness is strong enough, if there is extensive interaction and branching between growing sills, or if magma freezing at sill extremities leads to shielding of the crack tip region.