En jämförelse mellan Jorandomens tennanomala graniters och revsundsgranitens (Västerbotten) mineralogiska och petrografiska karaktär

Summary: The Joran-dome is one of the rounded granite structures found in the Precambrian of northern Sweden. It is situated near Storuman in the county of Västerbotten. It is most easily identified as a positive γ-radiation anomaly due to increased U-Th-K contents. According to a zircon U-Pb dating the age of the granites in the Joran dome is 1744 +/- 35 Ma. The country rock consists of Revsund granite. In the investigated area it is a gray, coarse-grained, porphyritic rock that is homogeneous in vast areas. Major minerals are quartz, microcline, plagioclase, biotite, and hornblende. Its Rb-Sr age is 1747 +/- 40 Ma. The Joran dome is a complex granite intrusion and consists of several granite types. The oldest rock is a medium-grained porphyritic granite almost only found in the northern part of the dome. In the southern part somewhat younger, medium-and fine-grained granites as well as a granite-porphyry appear as late differentiation products. All rocks are peraluminous, felsic, albite-microcline granites with low K/Rb-ratios. The dark mica is presumably a Li-biotite and/or a Li-siderphyllite. Fluorite, zircon, ilmenite, monazite, bastnaesite and topaz are typical accessories. Secondery sphene, apatit and allanite are present. In metasomatically altered parts native silver, argentite and cassiterite occur. Cassiterite only occurs together with a dark mica (Lisiderophyllite). The Joran-dome granites have been altered by late-and post-magmatic alkalimetasomatism. Secondary albite formation partly in the form av replacement pertites occurs in connection with sodium-metasomatism. Potassium-metasomatism results in microclinization of plagioclase. The post-magmatic fluids are in part the source of cassiterite and other typomorphic minerals related to tingranites. The Joran dome granites are lower than the Revsund granite in CaO, MgO, FeO, TiO2, Ba and Sr but higher in SiO2, K2O, Li, Rb, Be and F. The medium-grained, porphyritic granite in the Joran-dome occupies an intermediate position between younger Joran-dome granites and the Revsund granite. This might be due to metasomatic alterations caused by fluids escaping from the consolidating medium-grained granite type.