Age relations of Sveconorwegian granitoid rocks in the Stora Le-Marstrand belt, Orust area, Sweden

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Abstract
Two generations of Sveconorwegian granitoid rocks are recognized in the Orust-Skaftölandet sector of the Stora Le-Marstrand belt of SW Sweden. The earlier is a suite of large pegmatites containing quartz, plagioclase, K-feldspar, muscovite, minor biotite and garnet. These bodies cut the D3-deformed Orust dykes which have a metamorphic age of 1060 Ma. They are in turn deformed by F4 folds. Rb-Sr ages calculated from three mineral pairs from two of these pegmatite bodies arc close to a mean value of 950 Ma which is interpreted as the emplacement age of the pegmatites. The later generation of Sveconorwegian granitoid is represented by the 891 Ma Bohus granite pluton and associated minor pegmatites. In the islands west of Skaftölandet the Bohus granite contact clearly cuts the Orust dykes and also elements of the post-dyke (Sveconorwegian) deformations, D3 and D4 (and probably D6). The granite contact itself is undeformed although the granite elsewhere exhibits minor shears. These granitoid ages place close constraints on the timing of Sveconorwegian deformations in this area.

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