U-Pb-Hf isotopes and trace elements of metasomatic zircon delimit the evolution of neoproterozoic Capane ophiolite in the southern Brasiliano Orogen

摘要

The Capane ophiolite is a fragment of oceanic lithosphere obducted into the Ediacaran Porongos fold and thrust belt, southern Brasiliano Orogen. A studied rodingite blackwall contained in serpentinite has metasomatic zircon that displays multiple U-Pb ages from Tonian to Cryogenian (793 +/- 0.9, 757 +/- 2.1, 715 +/- 2.2Ma). The ages are interpreted as corresponding to multiple alteration events in the mantle. Multiple U-Pb-Hf isotopes and trace element analyses on the same crystals by laser ablation were controlled by backscatered electron images. Hf isotopes indicate zircon origin from a depleted mantle (epsilon Hf=+15 to +10.7), and trace elements point to an oceanic origin. The Capane ophiolite thus marks the evolution of the Adamastor ocean during the Tonian and Cryogenian, a significant result for the reconstruction of Rodinia and Gondwana supercontinents.