Middle Cambrian rift-related volcanism in the Ellsworth Mountains, Antarctica: tectonic implications for the palaeo-Pacific margin of Gondwana

The Ellsworth Mountains of West Antarctica represent part of a displaced te rrane once situated along the palaeo-Pacific margin of Gondwana, prior to s upercontinent break-up, adjacent to South Africa and the Weddell Sea coast of East Antarctica. Middle Cambrian sedimentary rocks of the southern Ellsw orth Mountains host locally thick volcanic and subvolcanic rocks forming fi ve igneous centres. Geochemically, most of the igneous samples are mafic, w ith a subordinate suite of evolved types. The mafic suite is geochemically varied, ranging from MORE (mid-ocean ridge basalt)-like compositions to sho shonitic and lamprophyric (e.g. La-N/Yb-N = 0.95 to 15.2), with epsilon Nd- i values ranging from +5.2 to -2.0, correlating with Ti/Y. They are interpr eted as representing melts derived from more than one mantle source, with t he MORE-Like rocks being derived from a depleted mantle source, and the mor e enriched compositions representing partial melting of lithospheric mantle . Silicic rocks contain melt contributions from Late Proterozoic crust, whi ch is inferred to form the basement of the Ellsworth Mountains. We interpre t these igneous rocks as having been formed in a continental rift environme nt, with MORE-like basalts erupted near the lift axis, and melts from litho spheric mantle emplaced on the rift shoulder. Such an interpretation is con sistent with the sedimentary host-rock palaeogeography and contemporaneous structures. This Middle Cambrian rift event is correlated spatially and tem porally with rift-related sedimentary rocks in South Africa. It is currentl y unclear what rifted off the southern African-Weddell Sea sector of the Go ndwana palaeo-Pacific margin at that time. (C) 1999 Elsevier Science B.V. A ll rights reserved.