Rifting of a tethyan continent — Rare earth evidence of an accreting plate margin

Rifting of a continent in the Tethys ocean was associated with two forms of volcanism initially identified by Hynes (1972). An early light rare earth element (LREE)-enriched magma accompanied rifting of the continental crust and subsidence of a marginal carbonate platform. The early basalts are high K₂O, nepheline-normative basalts, associated with silic igneous rocks, and carrying olivine pseudomorphs. A later or contemporaneous LREE-depleted magma is associated with the active formation of sea floor in a marginal embryo ocean basin. The ophiolite basalts are low K₂O, hypersthene-normative basalts containing feldspar laths and pyroxene subhedra. Similar transitions or changes in extrusives are evident in present-day embryo oceans and at the edges of rifted continental margins which surrounded larger ocean basins. Genesis of the tholeiites can be related to 10–30% partial fusion of foliated mantle lherzolites a sample of which adheres to the base of the Othris ophiolite. The alkalic basalts require either a fractionation model, or a more LREE-enriched source perhaps similar to the Ataq lherzolites, since the “tholeiite source lherzolite” can only produce alkalic basalts at low degrees of melting.