Textural Equilibrium in Magmatic Layers of the Lough Fee Ultramafic Intrusion, NW Connemara, Ireland: Implications for Adcumulus Mineral Growth

Abstract:Small-scale magmatic layers in a minor ultramafic intrusion at Lough Fee, NW Connemara, Ireland, exhibit superb examples of primary adcumulate textures. Two of these monomineralic layers are quantitatively analysed to establish their degree of textural equilibrium. A series of apparent dihedral angles is measured from threegrain junctions, and cumulative frequency curves are plotted for each layer. The curves are compared to theoretical curves for equilibrated and unequilibrated rocks to determine the degree of textural equilibrium of each layer. The results show apparent dihedral angle values that closely approximate those of a system in local textural equilibrium. The importance of adcumulus growth in the development of magmatic layering and the implications of this process for layer formation are briefly discussed with reference to the tectonic setting of the intrusion, and it is concluded that magmatic layering may have developed in a syn-tectonic intrusion.