Evidence for an age progression along the Tristan-Gough volcanic track from new 40Ar/39Ar ages on phenocryst phases

Age progressive hotspot chains are commonly believed to form by decompression melting as the lithosphere moves over a mantle plume. The Tristan-Gough volcanic track in the South Atlantic, composed of the Walvis aseismic ridge and the associated Guyot Province, extends from the Etendeka continental flood basalts (CFBs) in Namibia to the volcanically active islands of Tristan da Cunha and Gough. Here we present new laser step-heating 40Ar/39Ar ages of mineral separates from samples from the Tristan-Gough volcanic track and the Rio Grande Rise. The 40Ar/39Ar ages have primarily been determined on feldspar crystals, but also on biotite crystals (one sample) and whole rock chips (one sample). Our data indicate a younging age progression from the Etendeka CFBs to Tristan da Cunha and Gough. The ages range from 114 Ma at the northeastern end to 58-72 Ma for DSDP Sites 525A and 528 at the southwestern end of the Walvis Ridge, to 27-49 Ma for the Guyot Province and to 80-87 Ma for the Rio Grande Rise, which is believed to represent the counterpart of the Walvis Ridge on the South American Plate. We also found anomalously young (late-stage) volcanism on the Walvis Ridge (similar to 55 Ma) and the Rio Grande Rise (similar to 46 Ma), which, like late-stage volcanism on other hotspot tracks, represents more silica-undersaturated compositions than the main shield stage tholeiitic volcanism. Combining our new ages with published ages for the Etendeka CFBs yields a general volcanic migration rate of similar to 30 mm/a for the Tristan-Gough track assuming a constant velocity of the African Plate. Our new geochronological constraints for an age progression provide further support for a mantle plume origin of the Tristan-Gough track. Finally our new age data imply a faster volcanic migration rate for this hotspot track than previously published rates, which needs to be factored into plate tectonic reconstruction models. (C) 2012 Elsevier B.V. All rights reserved.