Episodic silicic volcanism in Patagonia and the Antarctic Peninsula: Chronology of magmatism associated with the break-up of Gondwana

Abstract

New SHRIMP U-Pb zircon, Rb-Sr whole-rock, and 40Ar-39Ar data are presented for the Jurassic silicic volcanic rocks and related granitoids of Patagonia and the Antarctica Peninsula. U-Pb is the only reliable method for dating crystallization in these rocks; Rb-Sr is prone to hydrothermal resetting and Ar-Ar is additionally affected by initial excess 40Ar. Volcanism spanned more than 30 My, but three episodes are defined on the basis of peak activity: V1 (188-178 Ma), V2 (172-162 Ma) and V3 (157-153 Ma). The first essentially coincides with the Karoo-Ferrar masfic magmatism of South Africa, Antarctica and Tasmania. The silicic products of V1 are lower-crustal melts that have incorporated upper-crustal material. The geochemistry of V2 and V3 ignimbrites is more characteristic of destructive plate margins, but the presence of inherited zircon still points to a crustal source. The pattern of volcanism corresponds in space and in time to migration away from the Karoo mantle plume towards the proto-Pacific margin of Gondwana during rifting and break-up. The heat required to initiate bulk crustal fusion may have been supplied by the spreading plume-head, but thinning of the crust during continental dispersion would also have facilitated anatexis.