SIMS U-Pb, Sm-Nd isotope and geochemical study of an arkosite-amphibolite suite, Peräpohja Schist Belt: Evidence for ca. 1.98 Ga A-type felsic magmatism in northern Finland

Abstract

In the northern and north-eastern part of the Perpohja Schist Belt, northern Finland, an extensive supracrustal rock unit has been identified which is composed of alternating amphibolitic and arkositic components. The amphibolites form layers whose thickness varies from one millimeter to some tens of meters, being most often a few tens of centimeters. They represent mafic tuff beds deposited concurrently with more abundant arkositic rocks. Most of the arkosites have a modal and major and trace element composition similar to that of A 2 -type granites. For example,they exhibit high LREE/HREE, negative Eu anomalies, and flat HREE and are moderately enriched in Nb,Zr, and Y. The genesis of the arkosites is enigmatic as they show features supporting either a volcaniclastic or an epiclastic origin. In the latter case,they were derived via erosion of a source dominated by A 2type granitic rocks.