Irregular contact aureoles in limestone: Implications for the thermal consequences of dike emplacement in echelon fracture arrays, transient porosity, and kinetic isograds, Killala dike swarm, Ireland

Date of Completion

January 1996






Small basaltic dikes of the Killala dike swarm have irregular contact aureoles. Contact aureoles of dikes 1.5 to 2 m thick are discontinuous and include only the grossularite zone. The grossularite zone adjacent to dikes greater than 4 m thick is continuous, but may change thickness along the length of the dike at dike-contact offsets and may differ in thickness on opposite sides of the dike. The wollastonite zone is present only locally along the contacts of dikes 4 to 5 m thick. Typical methods of modeling contact aureoles including: thermal modeling, analysis of bulk composition, analysis of differential fluid flow, and non-dilational emplacement processes only partly explain the observed irregularities in the metamorphic zones along and across the dikes.^ The absence of a grossularite zone in the aureoles of dikes less than 1 m thick, its discontinuous development in the aureoles of dikes 1 to 2 m thick, and the location of the grossularite-in and smectite-out isograds closer to the dikes than predicted by thermal history modeling, argue for kinetic inhibition of the grossularite-producing reaction during the short time during which the aureole was heated above 350$\sp\circ$C.^ The absence of a wollastonite zone in the aureoles of dikes less than 5 m thick, and its discontinuous development in the aureole of a 5 m thick dike, may be explained by reaction overstepping. If reaction overstepping is high, the path of fluid-rock system on T-X$\rm\sb{CO\sb2}$ diagram intersects the wollastonite-producing reaction. If overstepping is low, the path of fluid-rock system never intersects the wollastonite-producing reaction. The amount of overstepping is influenced by the amount of fluid flow. Presence of pore-lining cements in preserved fossils in the inner grossularite- and wollastonite zones implies a pathway for enhanced fluid flow was present in the inner contact aureole during metamorphism.^ Reaction overstepping in combination with non-dilational process during dike emplacement may explain the development of irregular contact aureoles. This implies that the typical assumptions used to evaluate contact aureoles may not be valid for contact aureoles associated with small dikes. ^