En geokemisk studie av bergartsvariationen på Bullberget i västra Dalarna

Sammanfattning: The Bullberget hill is situated in the Lima parish of the Dalarna province in western Sweden. The major rock forming the hill is Öje basalt. This is cut by a dolerite sill belonging to the same generation as the Åsby dolerite, which is common in the province. The sill displays a continuos transition from a SiO2-content of 54% to 73%. No lithological boundaries have been found between the various rock types, all changes seem to be gradual. The rock samples have been studied at two prior occasions, which has resulted in two different interpretations to explain the chemical diversification. The first interpretation suggests crystal fractionation. The second interpretation rejects this explanation and suggests wallrock assimilation as the cause of the variation. In order to determine the liability of this conclusion and confirm what process is responsible for the variation new geochemical analyses of trace elements and Sm-Nd isotopes have been carried out. These analyses show that the trace elements as well as the eNd-values follow linear trends between two end members. Data conform well to the two types of plots used, linear companion plots and mixing hyperbolas. The arithmetic means of R2 for these plots are 0.88 and 0.92, respectively, after due transformations to adjust for spurious correlation and non-normally distributed errors. The eNd and Eu-anomaly change from 1.05 and 0.83 to -2.23 and 0.38 respectively, from the mafic to the felsic end member of the data series. The REE plot shows little variation among the samples although a group consisting of the most felsic samples can be identified among the LREE. Together theses results clearly show that two separate chemical reservoirs, having different times of separation from the mantle, have been mixed but not homogenised. A microscopical study of the material reveals an abundance of granophyr as well as wide-spread recrystallisation of most phases. This supports the assumption of two not fully homogenised parent materials. Furthermore it shows that cooling has been very rapid, which is consistent with the idea of a large mass of wall-rock being assimilated and through heat exchange partially melted by the magma.