THE OLDEST ADAKITES OF THE FENNOSCANDIAN SHIELD

Abstract

Adakites are andesidacite–rhyolite rocks of trondhjemitic affinity characterized by high Na 2 O, Al 2 O 3 , Ba, and Sr contents and high Sr/Y and (La/Yb) n ratios [1, 2]. Adakitic rocks are only formed in a subduction setting [3]. They are typical components of Phanerozoic rock associations of active continental margins and island arcs. In the Archean, adakite findings are rare and mainly date to 2.7–2.8 Ga. This work presents the results of geochemical and Sm–Nd isotopic study of adakites that were found in the oldest (3.05–2.95 Ga) Fennoscandian basalt–andes-ite–dacite–rhyolite (BADR) island-arc association of the Upper Archean Vedlozero–Segozero greenstone belt.