Geology of Guernsey

Guernsey has a geological history stretching further back into the past than most of Europe. The majority of rock exposures on the Island may be found along the coastlines, with inland exposures scarce and usually highly weathered. There is a broad geological division between the north and south of the Island. The Southern Metamorphic Complex is elevated above the geologically younger, lower lying Northern Igneous Complex. Guernsey has experienced a complex geological evolution (especially the rocks of the southern complex) with multiple phases of intrusion and deformation recognisable.

The southern part is largely composed of Icart Gneiss. The Icart Gneiss is an augen gneiss of granitic composition containing potassium feldspar. This was formed from a granite dated at 2,061 million years ago using U-Pb dating on zircon grains. A foliated Perelle quartz diorite (also called Perelle Gneiss), occurs in the centre and west of the island. This is a calc-alkaline tonalitic rock. The foliation was formed at around 600 million years ago during the Cadomian Orogeny. Rafts of metamorphosed sediments, older than the Foliated Perelle Diorite are embedded between them.

The Pleinmont Formation consists of metamorphosed sediments is of unknown age, although it has been proposed that these belong to the Brioverian group that outcrop on nearby Jersey It is named after Pleinmont Point on the south west tip of the Island.

The older rocks were deformed during the Cadomian Orogeny. The Icart Gneisses formed the basement to an outboard terrane that was subsequently accreted to Armorica.

The Cobo Granite was formed at approximately 570 million years ago, named after Cobo Bay on the mid west coast. The north end of the island is an unfoliated calc-alkaline pluton of the Bordeaux Diorite Complex consisting of diorite, tonalite, and granodiorite. This is also dated at 570 million years ago.

...
Wikipedia