Short-lived magmatic activity in an anorogenic subvolcanic complex: 40Ar/39Ar and ion microprobe U^Pb zircon dating of the Erongo, Damaraland, Namibia

The Erongo subvolcanic center in Namibia is the largest composite, bimodal complex in the Mesozoic Etendeka igneous province of Namibia. This study of 40Ar/39Ar and high spatial resolution U^Pb zircon dating demonstrates that emplacement of the various igneous units at Erongo took place within a time span equivalent to or shorter than geochronologically resolvable age differences (ca. 2 Ma), and at the peak of regional flood-basalt activity in the Etendeka^Parana¤ province. The Erongo complex comprises a series of felsic volcanic and intrusive units that overlie or intrude basaltic lavas attributed to the Etendeka Group. The stratigraphically oldest rhyodacite (Erongorus unit) yielded U^Pb zircon ages of 131.8 ; 1.1 Ma (1c). The overlying Ombu rhyodacite was previously dated at 135.0 ; 1.6 Ma and our new ages for a resurgent intrusion of the same magma type (Ombu granodiorite) in the vent area are 132.6 ; 1.1 Ma (40Ar/39Ar) and 132.3; 1.9 Ma (U^Pb zircon) respectively. The youngest felsic magmas at Erongo are the high-silica Ekuta rhyolite and compositionally equivalent Erongo granite. These units yielded U^Pb zircon ages of 131.9 ; 2.9 and 130.3 ; 1.4 Ma, respectively. Concordant 40Ar/39Ar biotite ages from two samples of the granite average 132.2 ; 0.8 Ma. The final stage of magmatism at Erongo involved emplacement of basic alkaline plugs and dikes. Phlogopite and kaersutite from foidite plugs of this series yielded 40Ar/39Ar ages of 130.8; 1.0 and 132.0 ; 1.0 Ma. In a regional context, these age results indicate that silicic magmatism in the largest complexes of the Damaraland (Erongo, Brandberg, Paresis, Messum) began simultaneously with the peak of flood-basalt effusion at about 132 Ma throughout the Etendeka province and ceased by about 130 Ma. The silicic magmas are hybrid, with varying contributions of crustal and mantle-derived melts, and the age constraints suggest that crustal melting was caused by a short-lived thermal pulse related to the main flood-basalt event. Basic magmatism in the Damaraland complexes continued sporadically thereafter to about 123 Ma, but lacked the power to create further crustal melting. @ 2003 Elsevier B.V. All rights reserved.