Early cretaceous basalt and picrite dykes of the Southern Etendeka Region, NW Namibia : windows into the role of the Tristan Mantle Plume in Paraná–Etendeka magmatism.

摘要: Abundant dykes in the southern Etendeka region, NW Namibia, mostly contain 8–20% MgO. Almost all can be allocated to previously described Early Cretaceous magma types. Horingbaai-type basalts–picrites occur up 120 km inland. Some have superficially mid-ocean ridge basalt (MORB)-like compositions: (La/Nb)n 1·0; (Sm/Lu)n 1·5; Nd >8; initial 87Sr/86Sr 0·7032. Others show major- and trace-element, Sr–Nd–Pb isotopic evidence of contamination during upwelling by a few per cent K-feldspar-rich upper crust. Extremely magnesian olivine macrocrysts (Fo91–93·3) some Horingbaai picrites indicate that komatiitic (MgO 24%) liquids were associated with this suite, although they too dense reach crustal levels currently exposed. Ferropicrite resemble closely nearby ferropicrite basal lavas, total iron (as Fe2O3) >MgO; 1·0–1·5; 2·5–7; 2–3, except samples geochemical Archaean–Proterozoic lower A low-Ti geochemically lavas main succession, 2·0–2·5; = 0 -8; >0·707. Dykes north Huab river define fourth type, Nil Desperandum (ND), may fed part sill complex. The dyke ages are constrained their field relationships. consanguineous erupted at 133 132 Ma, respectively. Both ND cut Ma swarm forsteritic is 131 Brandberg plutonic Forward inverse modelling genesis ferropicrites picrite–komatiites gives two temporal ‘windows’ into physicochemical conditions within head impacting Tristan mantle plume. At lithosphere was >100 thick only incipient melting predominantly Fe-rich peridotite streaks occurred rising plume (Tp 1500°C). By 2 my later, pre-Atlantic extension had reduced thickness beneath adjacent there intense 1500–1700°C) even depleted head.