En petrografisk och geokemisk undersökning av inneslutningar i Nordingrågraniten

Sammanfattning: Fine grained inclusions in the 1578 Ma Nordingrå rapakivi massif have been studied with respect to petrography, geochemistry and zircon age in order to see whether they belong to the same intrusion event as the granite, and whether they are different in mineral compositions or geochemical characteristics compared to the host granite. Such differences could indicate that fractionation processes have played an important role in the evolution of the Nordingrå granite magma or that another magma with a composition different from the granite has intruded the surrounding rocks. The Nordingrå massif is in all evidence anorogenic in relation to the Svecofennian orogeny. It is bimodal and consists of leucogabbro, anorthosite, gabbro and granite which have intruded rocks (metagreywackes and I-type granites) of Svecofennian age (>1.95-1.8 Ga). The inclusions are brownish red and fine grained (often very fine grained). All of them, except for the most fine grained varieties, contain some larger phenocrysts of potash feldspar and quartz. The granites at the two localities differ slightly from each other in their chemical composition. The first granite contains less silica, slightly more plagioclase and has more pyroxene among its mafic minerals than the granite at the second locality, which contains more amphibole and less pyroxene. These reations are also mirrored in the inclusions. However, the inclusions have different compositions to the granites; they all have higher contents of FeOtot, MgO, TiO2, K2O, P2O5 and lower contents of SiO2 compared with the granites. One of the hypotheses discussed in the paper is that a heterogenous quartz-syenitic magma could have been derived from different amounts of partial melting of an Archean rock in the lower crust. This is indicated by significantly lower, but also more varying, eNd-values in analysed inclusions than samples from the granite. However, difficulties in separating pure material from the inclusions makes the chemical analyses a bit unsure. Two analysed zircons from the inclusions and one from the granite all result in an age of 1578 Ma, the same age as obtained from a previous U/Pb-dating (Welin and Lundqvist 1984). The intrusion age of the massif thus seems to be verified.