Slieve Croob
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Geology & landscape
Geological Feature: Rock Outcrop · Geological Period: Silurian · Rock Type: Granite
Landscape processes: Volcanic / Igneous
Description
Slieve Croob Caledonian Igneous Slieve Croob is of importance because of its geology. It is one of a series of sites that describes the Newry Igneous Complex NIC, one of a number of granitic complexes emplaced within the Southern Uplands and Midland Valley Terranes in Ireland and Scotland during Caledonian times. This period was characterised by mountain building events following the closure of the Iapetus proto-Atlantic Ocean. The NIC intrudes older folded Silurian rocks and extends over an area of about 45km2, from Slieve Croob in the north-east to Forkhill in south Armagh. The NIC comprises a series of overlapping plutons consisting of a variety of acidic, intermediate and ultramafic rocks. U-Pb zircon dating method indicates that the age of the NIC is 414 to 407Ma million years old. The north-east or the Rathfriland Pluton is the oldest at 413 to 411Ma, the central or Newry pluton is 411 to 410Ma, and the south-west or Cloghoge Pluton is the youngest at 407Ma. The NIC featured in past international debates concerning the origin of granitic rocks; whether such rocks were de-rived ultimately from a magma source or whether they represent the end point of an alteration process affecting pre-existing rocks. The NIC continues to be of considerable importance for research. The rocks exposed at Slieve Croob are metamorphosed Silurian country rock. These are biotite and cordierite hornfels and mobilised sediments. The granodiorite is generally fine-grained and massive, weathering in an irregular manner giving rise to a furrowed appearance. Textures vary in detail from those resembling fine grained biotite granite to a hornfels rich in biotite. These rocks contain cordierite, a common mineral in hornfelsed turbidite rocks. Where this mineral has been weathered out the rock takes on a pitted appearance. To the east of the outcrop at Slieve Croob further exposures of metamorphosed country rock and granodiorite of the north-east pluton 413 to 411Ma are found, indicating the approximate position of the contact at this site. At Slieve Croob, heat generated from the northeast pluton of the NIC has contact metamorphosed sedimentary greywackes and shales forming a rock type known as hornfels. The metamorphic effects of the intrusion occur in a zone around the granodiorite up to 1km wide. Close to the contact the sediments have been mobilised and intruded by melt and recrystallised giving rise to fused mobilised sediment. Slieve Croob offers important exposures of hornfels and fused sediments that together form part of the metamorphosed country rock on the edge of the northeast pluton of the NIC. It also hosts representative granodiorite of the northeast pluton. https://www.daera-ni.gov.uk/publications/slieve-croob-assi