Geochemistry and petrography of magnesium-rich intrusive rocks in the Rompas-Rajapalot area, Peräpohja Belt, northern Finland
1University of Oulu, Oulu Mining School, Geology
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|Persistent link:|| http://urn.fi/URN:NBN:fi:oulu-201412042100
|Publish Date:|| 2014-12-17
|Thesis type:||Master's thesis
The aim of this Master’s project was to describe high-MgO intrusive rocks from the Rompas-Rajapalot area and to find potential analogues among known Paleoproterozoic mafic-ultramafic intrusions that occur in the Karelian schist belts. The focus area of this study, the Rompas-Rajapalot area, is situated in the Peräpohja Belt, northern Finland. The belt represents ~500-Ma-long geologic history and supracrustal rocks that have been deformed in several tectonic events. The Rompas-Rajapalot area is located at the northern margin of the Peräpohja Belt close to the Central Lapland Granitoid Complex and the contact between these units is thought to be tectonic. The sample material of this study was collected from three drillholes and two outcrops. Thin sections of ultramafic were studied using polarization microscopy and whole-rock major and trace element compositions were determined with X-ray Fluorescence Spectroscopy (XRF) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The primary mineral assemblage has been completely altered in all samples, and the secondary minerals consist mainly of amphiboles (actinolite and tremolite), chlorite, biotite, plagioclase and epidote. Accessory opaque minerals include ilmenite, magnetite, pyrite and pyrrhotite. Some samples reveal clear “porphyritic-looking” texture, in which large clinopyroxene oikocrysts enclose cumulus olivine grains. The presence of metamorphic olivine indicates peak metamorphic conditions of amphibolite facies. Whole-rock geochemistry shows that the parental magma had Al2O3/TiO2 of ~5.5, Ti/V of ~30, and high LREE/HREE. This together with petrographical similarities suggests that the closest analogues for the studied high-MgO rocks are ultramafic cumulates from the 2.22 Ga Gabbro-Wehrlite Association intrusions. This conclusion has stratigraphic implications that calls for a reappraisal of the structure of the northern part of the Peräpohja Belt.
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