Microstructural Evidence for the Fluid Dynamical Behaviour in Vertically and Laterally Propagated Dykes of the British and Irish Paleogene Igneous Province

Abstract

Abstract A detailed study of dykes from the British and Irish Paleogene Igneous Province, with the Birdsboro dyke of the Newark Basin, USA, as a comparison, shows that microstructures vary systematically as a function of distance from the parent igneous complex. Proximal dykes (both those that propagated vertically and the proximal exposures of laterally propagated dykes) have invariant plagioclase grain shape and a generally coarse grain size, consistent with sustained convection during solidification. In contrast, distal exposures of laterally propagated dykes are generally finer grained and have a systematic spatial variation of plagioclase grain shape indicative of in situ nucleation and growth on inwards-propagating solidification fronts. These differences are argued to be a consequence of a systematic progression in the level of exposure along the dyke swarm, due to differential uplift and erosion associated with underplating of the NW parts of the British Isles. Convection in vertically propagated dykes is likely to occur throughout the full height of the intrusion, but is confined to the wider regions of laterally propagating dykes, with little or no convection in their narrower upper regions. There are generally no marginal reversals in the variation of plagioclase shape in dykes, indicative of relatively rapid emplacement of vertical intrusions compared to sills of comparable thickness. Comparison of plagioclase grain shape in dykes with invariant grain shape with that in bodies for which solidification is inferred to have occurred in marginal solidification fronts may be used to constrain the average width of the convective system. The Birdsboro dyke is anomalously fine grained compared to other dykes with invariant plagioclase grain shape, indicating that a coarse grain size is not a universal characteristic of convecting dykes: the relatively fine grain size is suggestive of a high crystal load, perhaps due to it forming part of an extensive sill complex.