Silicate melt inclusions from a mildly peralkaline granite in the Oslo paleorift, Norway T. H. HANSTEEN Mineralogical-GeologicalMuseum, Sars Gate 1, N-0562 Oslo 5, Norway* AND W. }. LUSTENHOUWER Netherlands Organization for ScientificResearch (NWO), the Free University, N-1007 MC Amsterdam, The Netherlands Abstract The mildy peralkaline Eikeren-Skrim granite belongs to the Permian magmatic province of the Oslo rift, south-east Norway. Euhedral quartz crystals from the abundant miarolitic cavities contain primary inclusions of partly crytallized silicate melts and coexisting primary, aqueous fluid inclusions. Micro- thermometric measurements give maximum estimates for the granite solidus of 685-705 ~ Quenched silicate melt inclusions are not peralkaline, have normative Or/Ab weight ratios of 1.15-1.44 (compared to 0.49--0.80 in whole-rock samples) and F and C1 contents of 0.1 and 0.21-0.65 wt. %, respectively. Coexisting magmatic fluid inclusions are highly enriched in Na, C1, S and to some extent K. These chemical characteristics are the results of late-magmatic melt-mineral-fluid interaction in the miarolitic cavities. KEYWORDS: silicate melt inclusions, granite, peralkaline, Oslo rift, Norway. Introduction A considerable amount of experimental data exists on solidus and liquidus temperatures of water and halogen-bearing granitic systems (e.g. Tuttle and Bowen, 1958; Bailey, 1977; Manning, 1981). The results show that addition of volatiles to the dry systems decreases liquidus and solidus temperatures, and also changes the compositions of minimum melts. There are, however, potential problems with transferring experimental results to complex natural rock systems. (1) Whole-rock samples do not represent the bulk chemical sys- tem because volatiles have exsolved and escaped during the late-stage evolution. (2) Most whole-rock samples contain more than one generation of magmatic minerals. It may thus be difficult to assess the composition of the late- magmatic residual liquids, which in turn is crucial for prediction of solidus temperatures (e.g. Bailey, 1977). The problems of finding the composition and * Present address: Nordic Volcanological Institute, University of Iceland, Geoscience Building, 101 Reyk- javik, Iceland. Mineralogical Magazine, June 1990, Vol. 54, pp. 195-205 9 Copyright the Mineralogical Society melting behaviour of late magmatic residual liquids can be overcome by studying samples of silicate melts trapped as inclusions in late-magma- tic quartz crystals from miarolitic cavities. Pro- vided that the inclusions behaved as closed systems since the time of trapping, they are unbiased random samples of the late-magmatic liquids. In this paper, microthermometric measur- ements and microprobe analyses of late-magmatic silicate melt inclusions provide complementary date on late-stage processes in the midly peralka- line Eikeren-Skrim granite complex ('ekerite' in BrOgger's (1906) terminology) (ESG; Fig. 1). Geology and petrography The Oslo region alkaline province comprises Permo-Carboniferous lavas and Permian monzo- nitic to syenitic and grantic intrusions. (Br0gger, 1890, 1906; Ramberg and Larsen, 1978). Igneous activity in the rift covers the time span 300--240 Ma (Sundvoll, 1978; Rasmussen et al., 1988; Neu- mann et al., 1988). Subvolcanic rocks can be grouped as (a) older and younger biotite granites, (b) intermediate saturated to understurated rocks (mainly augite monzonite-augite syenite and