Browsing by Author "Niskanen, Maria"

The ultramafic and mafic metapicrites and metagabbros in the Salittu formation (1878–1875 Ma) within the Orijärvi area are situated in the Uusimaa Belt of the Southern Finland arc complex. A granitic pegmatite dike and several felsic dikes have been found from the Salittu quarry in Suomusjärvi in the Salo urban area, SW Finland. Three outcrops of the granitic pegmatite dike and three outcrops of the other felsic dikes, including the granitic neosome and leucogranite dikes, were examined in the field. Representative rock specimens were also taken from loose rock piles at the quarry. The mineral compositions of the dikes and their relative ages were examined by means of petrographic microscopy, x-ray diffraction and electron microprobe analysis. The pegmatite outcrops are structurally complex and zonal features can occasionally be observed together with varying grain size. The rock-forming minerals are quartz, plagioclase, alkali feldspar and the main mica is biotite. Pegmatite minerals such as cordierite, beryl, tourmaline and garnet occur locally. Accessory minerals are apatite, zircon, muscovite, chlorite group minerals, columbite, niobian and tantalian rutile, monazite, ilmenite, hematite, galena, pyrite, chalcopyrite, arsenopyrite, and bismuth. The Salittu granitic pegmatite dike is of the rare element class and of the beryl type and additionally of the beryl-columbite-subtype. In the petrogenetic classification it belongs to the LCT-family (lithium-cesium-tantalum) and its geochemistry is characterized by beryllium, niobium, titanium, and boron. After the formation of the folded granitic neosome dike, a shear zone developed due to the the deformation of the bedrock. The shear zone contributed to the intrusion of the granitic pegmatite dike that crystallized in the ultramafic country rock. A thermal (hydrothermal) pulse enabled interaction between the pegmatite and the surrounding rock and resulted in the formation of a sillimanite-corundum rock. This exceptional mineral assemblage also includes the accessory minerals spinel, paragonite, diaspore, and probably boehmite. After the emplacement of the pegmatite dike, leucogranite dikes formed in the shear zone of the Salittu bedrock. Leucogranite dikes locally contain inclusions of sillimanite-corundum rock.