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

Abstract

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