HR: 14:15h
AN: GP73A-02 INVITED     [Abstracts]
TI: Lateral Emplacement of the Western Mourne Granite, N.Ireland, From AMS Fabric Data
AU: * Bennett, N R
EM: bennett@geology.utoronto.ca
AF: University of Toronto, Department of Geology 22 Russell Street, Toronto, ON M5S 3B1, Canada
AU: Stevenson, C T
EM: c.t.stevenson@bham.ac.uk
AF: University of Birmingham, Earth Sciences School of Geography, Earth and Environmental Sciences Edgbaston, Birmingham, B15 2TT, United Kingdom
AB: Field observations and anisotropy of magnetic susceptibility (AMS) measurements of oriented blocks from the Palaeogene Western Mourne granite pluton indicate the presence of a weak fabric. Visible fabrics, determined from the preferred alignment of feldspar phenocryst long axes in outcrop, trend between NNE and north but it is unclear whether these are gently plunging lineations or the trace of dipping foliations. Texturally the granite shows little evidence for plastic strain suggesting that the observed fabric delineates magma flow. AMS fabrics are dominantly oblate, defining sub-horizontal foliations parallel to gently dipping margins and gently plunging lineations that trend SSW-NNE and diverge northward. These data so far point to an emplacement model for the Western Mourne granite that describes a laccolith, fed laterally from the SSW. This mirrors the NNE directed lateral emplacement of the adjacent Eastern Mourne granite (Stevenson et al. 2007, J Geol Soc Lond, 164, 99-110) suggesting that these two centres share a common feeder zone outside the Mourne area and located to the SSE, coincident with a 50mGal peak gravity anomaly close to the coast (GSNI data). Contemporaneous mafic dykes that outcrop along this stretch of coast exhibit xenoliths of mafic cumulate that, together with the gravity anomaly, suggest there may be an unexposed mafic pluton in this area. Given the genetic links between mafic and felsic magmas in this region, the coincidence of the projected Mourne granite feeder zone and possible buried mafic centre, leads to a model in which the Mourne granites were emplaced NNE and NW as a gently dipping sheet, up dip, from this unexposed mafic centre. This model raises the possibility that the other Palaeogene igneous centres in NE Ireland (Slieve Gullion and Carlingford) may be laterally linked. The main implication of significant lateral movement of magma in the upper crust is that the location of igneous centres in the upper crust or volcanic edifice at the surface may not reflect the point at the base of the crust where magma was generated.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1540 Rock and mineral magnetism
DE: 8035 Pluton emplacement
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2009 Joint Assembly