Northern Ireland - Sperrin Mountains

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The Sperrin Mountains are located in the north-west of Northern Ireland in counties Londonderry and Tyrone. The mountains form north-east to south-west aligned ridges that include the main Sperrin Ridge with the highest peak of Sawel Mountain at 678 m. No sedimentary bedrock lies on top of the basement rocks in this area.

Basement rocks

The Sperrin Mountains are made of the oldest rocks in the Province comprising metamorphic rocks 650 to 570 million years old resulting from the alteration of sedimentary and volcanic rocks. The sedimentary layers originally accumulated as layers of sandstone, mudstone and some limestone in an ancient ocean. The lavas were erupted onto the ocean floor and form part of the succession of layers. As the ocean closed adjacent continents collided and the oceanic rocks underwent deep burial and contortion under high temperatures and pressures which altered or metamorphosed the entire rock package. This produced schist and other metamorphic rock types that were then pushed up over younger rocks at the margin of this ocean. The fractured contact between these rock masses is known as the Omagh Fault. The younger rocks, 480 to 465 million years old, are located the south-east of the Omagh Fault and are composed of various igneous rock types such as granite and gabbro. Contained within the metamorphic rocks of the Sperrin Mountains are minerals veins and other deposits rich in gold, silver, copper, lead and zinc.