Abstract
The Late Caledonian Newry Igneous Complex (NIC), Northern Ireland, comprises three largely granodioritic plutons, together with an intermediate–ultramafic body at its northeastern end. New whole-rock geochemical data, petrological classifications, and published data, including recent Tellus aeromagnetic and radiometric results, have been used to establish 15 distinct zones across the four bodies of the NIC. These become broadly younger to the southwest of the complex and toward the centres of individual plutons. In places, zones are defined by both current compositional data (geochemistry and petrology) and Tellus results. This is particularly clear at the eastern edge of the NIC, where a thorium-elevated airborne radiometric signature occurs alongside distinct concentrations of various elements from geochemistry. However, in the northeastern-most pluton of the NIC, a prominent ring-shaped aeromagnetic anomaly occurs independent of any observed surface compositional variation, and thus the zones in this area are defined by aeromagnetic data only. The origins of this and other aeromagnetic anomalies are as yet undetermined, although in places, these closely correspond to facies at the surface. The derived zonation for the NIC supports incremental emplacement of the complex as separate, distinct magma pulses. Each pulse is thought to have originated from the same fractionally crystallising source that periodically underwent mixing with more basic magma.
Highlights
The zonation of an igneous body represents the systematic change in composition, structure or geophysical properties throughout the body (Richey, 1928; Schetselaar et al, 2000, Cooper et al, in press)
Within the Seeconnell Complex, mean concentrations of the radiometric elements potassium (K20) and uranium (U) are relatively high (3.98 wt% and 3.5 ppm respectively) and thorium (Th) is moderate (12.4 ppm) in relation to other parts of the Newry Igneous Complex (NIC) (Fig. 4). This is consistent with the interpretation of the Seeconnell Complex as a zone of ‘mixed’ radiometric signature from airborne data reported by Cooper et al
Geochemistry reveals low SiO2 and high Fe2O3(t) in this area relative to the inner and southwestern parts of the pluton (Fig. 5). These results support the compositional distinction of the eastern Rathfriland pluton rim indicated by the airborne radiometric data of Cooper et al and suggest that the area is more basic than the inner and southwest parts of the pluton
Summary
The zonation of an igneous body represents the systematic change in composition, structure or geophysical properties throughout the body (Richey, 1928; Schetselaar et al, 2000, Cooper et al, in press). Zonation can be gradational (Bowen, 1919; Exley, 1996) or expressed through more abrupt lithological changes (Richey, 1928; Pitcher, 1997; Hecht and Vigneresse, 1999; Kryza et al, 2014). In the latter case, the pluton can often be subdivided into distinct zones, which may reflect incremental emplacement via separate magma pulses. Schetselaar et al (2000) use aeromagnetic results to identify a previously undetected unit boundary within the Western Slave Granitoid in northeast Alberta
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