Browsing by Subject "petrography"

This MSc thesis is built on drill core and outcrops data at the Haukivuori area where fluorite bearing granitoids cut sharply the country rocks (mafic volcanic rocks). This thesis presents new whole-rock geochemical and zircon uranium-lead age data for Haukivuori granitoids in southeast Finland, which provide insights into the distribution of post-collisional granitoids in this region. The purpose of this thesis is to classify a granitoids in Haukivuori and to determine the conditions of their formation. Tectonic evolution of the central and southern Finland is also reviewed and the relationships between Haukivuori whole-rock geochemical data and already published whole-rock data from southern Fin-land post-collisional granitoids are discussed. In the present thesis, the focus is on the 1.815–1.77 billion years post-collisional granitic magmatism across the southern Finland. Haukivuori granitoids modal compositions vary from quartz-monzonite through granite to granodiorite. The contents of quartz, K-feldspar and plagioclase remain consistent all in all, covering about 95 % of the mineral assemblage. The accessories are biotite, muscovite, fluorite, calcite, apatite and zircon as well as oxides. Granitoids show high-K calc-alkaline to shoshonitic affinities and are metaluminous to weakly peraluminous with enrichment in light rare earth elements (example lanthanum normalized to chondrites shows ratios between 93 to 4263) and granitoids lack significant europium anomalies. Granitoids show enrichment in large-ion lithophile elements such as barium (1359–10000 ppm) and strontium (827–8318 ppm), and they display negative anomalies on chondrite normalized spider diagrams in high field strength elements such as niobium, tantalum, zirconium, and titanium. Concordia-intercept age from zircons uranium-lead data of 1794 ± 13 million years is the best crystallization age estimate for the Haukivuori granitoids. Haukivuori granitoid’s age, their undeformed nature, and the fact that granitoids cut country rocks clearly put granitoids into the post-collisional group. Thus, Haukivuori granitoids can be classified as post-collisional granitoids. Haukivuori granitoids display all features of typical high Barium-Strontium granitoids. Thus, granitoids are interpreted to represent a high-level expression of the mantle magmatism that was derived from depleted mantle source which was enriched during an earlier subduction episode.

The Bjurböle meteorite, which fell near Porvoo in 1899, is categorised as a fall. The Bjurböle meteorite is classified as an L/LL4 ordinary chondrite. Chondrites are undifferentiated meteorites that contain notable amounts of non-volatile elements of the early solar system. Chondrites usually contain small spherical igneous chondrules and they are classified into three groups: carbonaceous (C), enstatite (E) and ordinary chondrites (O). Ordinary chondrites are further classified into groups H, L and LL according to their metallic iron content. Chondrites are classified into petrologic types 1 – 6 based on their metamorphic grade so that Type 3 is the least metamorphosed and Type 6 is the most strongly metamorphosed, whereas Type 1 and Type 2, on the other hand correspond to meteorites that are, respectively, strongly and mildly affected by low-temperature aqueous alteration processes. The chemical equilibration of chondrites is controlled by the degree of metamorphism they go through. The more equilibrated a chondrite is, the more simplified its mineral assemble is and the more homogenised compositions the minerals have. Chondrules contain mostly olivine, pyroxene and interchondrule matrix. They are classified into Type I and Type II by the compositions of their olivine and pyroxene. Chondrule types can be further divided into A, B and AB by their olivine content. In addition, chondrules can be classified according to their texture into porphyritic, barred, radial, granular, cryptocrystalline, and metallic chondrules. The aim of this thesis is to inspect the variation in olivine composition in Bjurböle meteorite chondrules, to document the petrography of the Bjurböle meteorite and to find connections between petrographic discoveries and olivine composition. Furthermore, in this study I will discuss about the classification of the Bjurböle meteorite and the formation of chondrules in the Bjurböle meteorite. For this study, I examined 11 thin sections and three epoxy buttons that have 34 chondrules in total. Thin sections were inspected by microscope and six of the uncoated thin sections were also inspected under cold-cathode luminescence. The epoxy buttons were inspected with a scanning electron microscope, scanning electron microscope cathodoluminescence and scanning electron microscope energy-dispersive X-ray spectroscopy. Most of the inspected chondrules in the Bjurböle meteorite samples have very uniform olivine composition, as reported in earlier studies. Most of the olivines are Fo75–78, but there is also a bimodal peak at Fo89–92. The E1 chondrules (small) have very uniform olivine compositions of approximately Fo76–77. The E2 chondrules (medium) have also olivines of approximately Fo76–77, but some of the olivines reach Fo80–83. The E3 chondrules (large) have the broadest olivine composition variation at Fo74–93. However, most of the E3 olivines are Fo76–79. The E3-2 chondrule, in particular, has a broad olivine composition variation and its Fo-value increases up to 93 in the core of the chondrule. All chondrules with broader olivine variation have barred texture and are relatively large. Cathodoluminescence did not provide substantial data for the study. Other links between olivine compositions and petrography were not observed and, for example, proximity to metallic phases did not seem to have any effect on the olivine composition, and the exceptionally Mg-rich (Fo90) olivines may represent primary compositions. E3-6 and E3-7 chondrules are abundant in sulphide and metallic phases. The E3-7 chondrule resembles something that might be called micropallasite. Based on porosity, magnetic susceptibility, metamorphic signs, and the Fe variation in olivine and mineral assemblages, the Bjurböle meteorite fits the classification of an L/LL4 ordinary chondrite. Chondrules of the Bjurböle meteorite also have diverse forming histories.