A COMPARISON OF THE GEOCHRONOLOGY AND GEOCHEMISTRY OF PLAGIOCLASE-DOMINATED GRANITOIDS ACROSS A MAJOR TERRANE BOUNDARY IN THE SW BALTIC SHIELD

To make reconstructions of the Proterozoic palaeogeography meaningful, the terranes making up the individual shields must first be identifie d and the timing of terrane accretion determined. New chemical and iso topic data confirm the terrane-separating character of a major shear z one, the Mylonite Zone, in the Southwestern Scandinavian Gneiss Comple x. These data relate to plagioclase-dominated granitoids to the east o f the Mylonite Zone. Two zircon upper-intercept age determinations res ult in 1674(-19)(+24) and 1688(-10)(+10) Ma. Two separate crystals fro m the 1688-Ma rock give two single-zircon evaporation ages: one crysta l gives 1646 +/- 172 Ma (2 sigma) and the other 1654 +/- 46 Ma (2s). E arlier investigations have pointed to vast amounts of similar looking granitoids to the west of the Mylonite Zone. However, these rocks are younger, approximately 1.60 Ga. The granitoids to the west of the Mylo nite Zone are typical calc-alkaline rocks with a pronounced increase i n the content of mafic minerals concurrent with a decrease in quartz.T he trend of the eastern rocks is different, mainly involving changes i n the quartz/total feldspar ratio. At the same SiO2 content, the easte rn rocks are lower in mgO and CaO and higher in (Na2O+K2O) and expecia lly in Ba than their western counterparts. also other elements like Ni show differences between chemical signatures, probably caused by diff erent source-rock chemistries. epsilon(Nd1.68)-values in the eastern s uite range form + 2.0 to 2.5, suggesting that assimilation of large am ounts of old continental rocks did not take place. Results from Sr-iso tope determinations support this suggestion. The present results sugge st, together with older data, that no granitoid rocks older than appro minately 0.9 Ga are common to the two terranes separated by the Myloni te Zone. There is thus no compelling evidence suggesting that the two terranes formed one entity before the Sveconorwegian (Grenvillian) gro cery.