A refined model of incremental emplacement based on structural evidence from the granodioritic Newry igneous complex, Northern Ireland

Although many intrusions are now known to have been incrementally emplaced, the mechanisms through which this takes place are generally poorly understood. The Newry igneous complex was incrementally emplaced within the Southern Uplands-Down-Longford terrane of Northern Ireland during late Caledonian sinistral transtension. This study uses a variety of new and existing data and techniques to provide a fuller and firmer understanding of incremental emplacement than has previously been available; addressing both deep crustal processes and those operating within the emplacement site. Host rock orientations suggest that some of the accommodation space for the NIC was generated due to pull-apart tectonics operating within the Southern Uplands-Down-Longford terrane. Local host rock deflections, concentric igneous foliations and concentric linear AMS fabrics show that inflation due to magma overpressure also generated significant space. Strong anisotropy of magnetic susceptibility (AMS) fabrics close to the boundaries of some magma pulses in turn suggest that inflation was accomplished by injection of individual magma pulses and was thus incremental. The dome-like orientations of mineral foliations within plutons and the truncation of steep local host rock tracts by the Newry igneous complex imply that the complex consists of four laccolithic bodies. On a larger scale it is suggested that the deep-seated Argyll and Newry Lineaments represent faults that allowed magma generated at depth to ascend to the crustal level of the Southern Uplands-Down-Longford terrane tract boundaries. It is also inferred that sinistral movement along the Argyll and Newry lineaments may have produced the releasing bend within the Southern Uplands-Down-Longford terrane. Higher in the crust reduced confining pressure resulted in tectonic opening along this releasing bend. This local stress field induced horizontal magma flow and emplacement of the Newry igneous complex as laccolithic bodies. The study suggests that simplistic emplacement models should largely be abandoned in favour of holistic models incorporating the multiple interdependent processes operating during magma ascent and emplacement.