The formation and fate of large oceanic igneous provinces

Large igneous provinces are conspicuous features of late Phanerozoic geology, and include continental flood basalts, rifted continental margin volcanic sequences and oceanic plateaus. The latter are formed in an environment which typically recycles back into the mantle on a time scale of < 200 m.y., but because comparisons have recently been made between oceanic plateaus and Precambrian greenstone belt sequences, new questions arise about their formation, their fate and their preservation. Here we review some critical aspects of three oceanic plateaus, Ontong Java, Kerguelen and the Caribbean/Colombian obducted plateau, and comment on their make-up and the factors governing their preservation, with particular relevance to ancient terranes. Many large igneous provinces can be linked to mantle plumes. Where plumes ascend beneath spreading ridges, their energy is transformed into a large melt volume, producing over-thickened plateau crust. Where the spreading rate is low in relation to magma supply, the plateau may become subaerial (e.g. Iceland), but with fast spreading the plateau remains submarine. Thicker lithosphere may result in plume incubation before magma extrusion, and there are many intermediate situations where plumes could readily break through thin lithosphere (oceanic or continental). Because magma supply exceeds extension rate, plateaus may be characterised by thick sequences of flows and sills rather than the sheeted dykes typical of Phanerozoic ophiolites. Precambrian greenstones could represent imbricated oceanic plateaus, or plumes penetrating thin continental lithosphere. The initial high temperature and the buoyant nature of the depleted refractory keel of plateaus contributes to their preservation relative to normal oceanic crust. When they collide with active margins they choke the subduction zone, causing subduction ''flip'' or ''backstep'' and the development of extensive calc-alkaline are volcanism on top of the plateau sequences. However, after much greater than 100 m.y. they are potentially negatively buoyant, so if fluids become available to promote transformation of the deeper zones to eclogite, they may be able to spontaneously subduct.