Part of an ophiolite sequence and pre-Jurassic continental basement rocks are remnants of the composite floor of the Cretaceous back-arc basin system on South Georgia. The partially preserved ophiolite consists of pillow lavas and breccias, inter-bedded tuffs, basic and silicic dykes and occasional sills. It is intruded by a composite gabbro-plagiogranite pluton and smaller gabbroic plutons. The basement gneisses are intruded by layered gabbros, diorites, granitic rocks and numerous basic dykes. A migmatitic aureole surrounds the plutonic rocks, which are fragmented and net-veined by remobilized crustal rocks.
The mafic rocks of the ophiolite sequence and basement complex are geochemically similar and both formed by differentiation of a mantle-derived, sub-alkali tholeiitic magma in a former back-arc basin tectonic setting. The plagiogranite and silicic dykes and lavas of the ophiolite and some of the silicic rocks within the basement formed by differentiation of the basic magma. The remainder of the granitic rocks within the basement are geochemically distinct, comprising remobilized basement rocks and a calc-alkaline pluton of a remnant magmatic arc.
The back-arc basin formed by emplacement of the tholeiitic magma within pre-existing basement rocks. Up to 80% extension of the continental crust occurred prior to rifting and development of the back-arc basin.