Heavy-mineral and garnet compositions of stream sediments and HP–UHP basement rocks from the Western Gneiss Region, SW Norway

We present heavy-mineral and garnet geochemical data of recent stream sediments and HP?UHP bedrock from the Flatraket and Ulvesund plutonic bodies and from the island of Runde, Western Gneiss Region, SW Norway, to test to what extent the heavy minerals and the garnet geochemistry in stream sediments reflect the geological situation in the source area. The heavy-mineral assemblages of the stream sediments contain garnet, green calcic amphibole and epidote-group minerals, which reflect greenschist-facies and higher grade metamorphism in the source area. The geochemical data of garnets point to high-grade metamorphic conditions. Overall, the heavy-mineral and garnet geochemical data reflect the geological situation in the source area, which confirms the application and the importance of heavy minerals in sedimentary provenance analysis. Geochemical data of heavy minerals usually show a wider distribution in the sediments than the data of heavy minerals measured in the bedrock. However, our results demonstrate that this is not always the case. Some garnets measured in the bedrock, especially lower grade and ultrahighgrade metamorphic garnets, are only of secondary importance or they are lacking in the sediments. This probably results either from larger grain sizes of garnets in some source rocks than in the studied grain-size window of the sediments, or because those garnets are diluted by a high input of garnets from source rocks containing higher amounts of garnets.

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