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Abstract
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 Newry igneous complex 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 anisotropy of magnetic susceptibility (AMS) fabrics show that inflation due to magma overpressure also generated significant space. Strong 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 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. This study suggests that simplistic emplacement models should largely be abandoned in favor of holistic models incorporating the multiple interdependent processes operating during magma ascent and emplacement.
Highlights
Emplacement refers to the way in which an ig neous rock comes to be accommodated in its final crustal position
The results suggest that magnetite is the dom inant magnetic fabric–carrying mineral through out the Newry igneous complex, together with possible minor amounts of titanomagnetite, bio tite, and hornblende
The following four conclusions are drawn from the current study: (1) This work supports the proposal by Beamish et al (2010) that intrusion of the Newry igneous complex was facilitated by pullapart tectonics
Summary
Emplacement refers to the way in which an ig neous rock comes to be accommodated in its final crustal position (i.e., put in place; Pitcher, 1997). This is intimately connected to ascent, which is the process through which magma rises within the crust or uppermost mantle to reach its em placement site (Jacques and Reavy, 1994). A primary dis tinction is often made between magmatic (i.e., magma pressure–related) and tectonic controls on emplacement, there may be a large amount of overlap between these influences (Hutton, 1988; Brun et al, 1990; Stevenson, 2009; de Saint Blanquat et al, 2011). Tectonic controls on emplacement are often associated with shear zones or faults and can be de rived through the study of regional structures (Hutton, 1982; Guineberteau et al, 1987; Brun et al, 1990; D’lemos et al, 1992; Petford et al, 2000; Rosenberg, 2004; Rabillard et al, 2014; McCarthy et al, 2015; Lancaster et al, 2017)
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