Interpretation

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(based on much work by several geologists
and the whole of the Ox Mountains Granodiorite)

The Ox Mountains Granodiorite is complex and reveals various features that formed at different times. It thus helps us to understand the history of its formation and what happened to it subsequently.

Crystallization of the Ox Mountains Granodiorite from molten magma occurred approximately 415 million years ago. Melting at the base of the Earth’s crust at that time coincided with major crustal sub-horizontal shearing (of left-lateral sense) that was accompanied by compression. The Ox Mountains Granodiorite was injected as a sequence of essentially liquid sheets into the Ox Mountains Shear Zone, which afforded crustal weakness and space. Sheeting was on all scales. Shearing began before intrusion of the granitic body and remained active during and after its full crystallization.

The individual crystals are often large because they crystallized slowly. At a few localities, but not here, they show signs of flow-alignment in the still liquid magma, though more generally their alignment in the foliation is a product of deformation after crystallization was about completed.

Compositional (colour) banding can be used to trace the sequence of injection of granitic rock types within the body as a whole. Early granite (seen elsewhere) was followed by granodiorite and tonalite whose mutual time relationships are not clear. These were all followed by more granite. Orientation of the largely sub-parallel compositional banding was controlled by the overall forces active in the Ox Mountains Shear Zone during the intrusion of successive molten sheets. These forces were NW-SE compression accompanied by left-lateral sub-horizontal shearing. Steeply dipping compositional banding with NE-SW trend is generally the case, though at Pontoon Bridge the dip is moderate to the NW. Put simply, flattening with stretching caused by shearing produced the principal post-crystallization features.

The foliation (planar mineral orientation with lineation) is also a consequence of the same deformation, but of the rock formed largely after crystallization of the rock when it was in a solid state. The lineation is sub-horizontal and represents the principal stretching direction within the shear zone. At Pontoon Bridge it plunges gently to the SW.

Pegmatite veins formed at several times. Some are strongly foliated and others are essentially weakly to non-foliated and therefore very late, though still genetically related to the Ox Mountains Granodiorite.

In a broader geographical context, intrusion of the Ox Mountains Granodiorite and its deformation during and after its intrusion were consequences of final closure of an ocean (Iapetus Ocean, named after the father of Atlantis) that had previously separated the northern and southern parts of what eventually became Ireland.

SHORE OF LOUGH CULLEN

A visit to nearby lakeside outcrops of the Ox Mountains Granodiorite will show further complexity within it, particularly with regard to lithological banding and ductile shearing.


C. Barry Long, September 2000