A comparison of the geochronology and geochemistry of plagioclase-dominated granitoids across a major terrane boundary in the SW Balitic Shield

Abstract

To make reconstructions of the Proterozoic palaeogeography meaningful, the terranes making up the individual shields must first be identified and the timing of terrane accretion determined. New chemical and isotopic data confirm the terrane-separating character of a major shear zone, the Mylonite Zone, in the Southwestern Scandinavian Gneiss Complex. These data relate to plagioclase-dominated granitoids to the east of the Mylonite Zone. Two zircon upper-intercept age determinations result in 1674 +24 −19 and 1688 +10 −10 Ma. Two separate crystals from the 1688-Ma rock give two single-zircon evaporation ages: one crystal gives 1646 ± 172 Ma (2σ) and the other 1654 ± 46 Ma (2s). Earlier 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 Mylonite Zone are typical calc-alkaline rocks with a pronounced increase in the content of mafic minerals concurrent with a decrease in quartz. The trend of the eastern rocks is different, mainly involving changes in the quartz/total feldspar ratio. At the same SiO 2 content, the eastern rocks are lower in MgO and CaO and higher in (Na 2O + K 2O) and especially in Ba than their western counterparts. Also other elements like Ni show differences between the two suites. We conclude that these similar looking rocks belong to two different suites that are different in age and have different chemical signatures, probably caused by different source-rock chemistries. ϵ Nd1.68-values in the eastern suite range from + 2.0 to 2.5, suggesting that assimilation of large amounts of old continental rocks did not take place. Results from Sr-isotope determinations support this suggestion. The present results suggest, together with older data, that no granitoid rocks older than approximately 0.9 Ga are common to the two terranes separated by the Mylonite Zone. There is thus no compelling evidence suggesting that the two terranes formed one entity before the Sveconorwegian (Grenvillian) orogeny.