Kilbroney River

Also within

Down County Mourne Scenic Area

Habitats

Fast flowing river and stream

Geology & landscape

Geological Period: Paleogene · Rock Type: Granite

Landscape processes: Volcanic / Igneous

Description

Kilbroney River Tertiary igneous The area is of special scientific interest because of its important geology. The rocks here are igneous and are of Palaeogene Tertiary age, some 56 million yeaJ:s old. The river bed and banks expose the two main rock types of the Western Mournes and a gradational contact between them. This is an example of stoping and assimilation of the older granite G4 by the younger granite G5. The granites of the Moumes were intruded as a series of separate pulses of magma. Pre-existing granites were cool enough that subsequent intrusions display chilled margins where they encounter the older rocks. In most cases, these contacts are sharp with no evidence of assimilation of the older material. At Kilbroney River, however, the two granites of the Western Mournes G4-older, G5-younger show a gradational contact. The river bed and banks show all stages in the disintegration of xenoliths of the coarse grained G4 granite in the finer-grained G5 granite. The younger granite contains coarse-grained porphyritic areas along with patches of large quartz and feldspar crystals. These coarse grained areas vary in size from a 10 cm to several metres across. The coarse grained areas are what remain of G4 granite that has been engulfed by the G5 when it was still molten magma. During this process, known as stoping, veins of G5 penetrated along crystal boundaries, separating both individual crystals and composite fragments of G4. Such assimilation is restricted to a narrow zone along the contact and Kilbroney River provides access to a well exposed section. https://www.daera-ni.gov.uk/publications/kilbroney-river-assi

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