The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry
The Obbnäs and Bodom granite plutons of southernmost Finland show the typical petrographic and geochemical features of the Proterozoic rapakivi granites in Finland and elsewhere: they cut sharply across the 1900 Ma Svecofennian metamorphic bedrock and have the geochemical characteristics of subalkal...
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| Format: | Article |
| Language: | English |
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Geological Society of Finland
1999-12-01
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| Series: | Bulletin of the Geological Society of Finland |
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| Online Access: | http://www.geologinenseura.fi/bulletin/Volume71/sgs_bt_071_2_pages_275_304.pdf |
| _version_ | 1818934840377999360 |
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| author | P. Kosunen |
| author_facet | P. Kosunen |
| author_sort | P. Kosunen |
| collection | DOAJ |
| description | The Obbnäs and Bodom granite plutons of southernmost Finland show the typical petrographic and geochemical features of the Proterozoic rapakivi granites in Finland and elsewhere: they cut sharply across the 1900 Ma Svecofennian metamorphic bedrock and have the geochemical characteristics of subalkaline A-type granites. The Bodom pluton is composed of porphyritic granites (hornblende-, hornblende-biotite-, and biotite-bearing varieties) and an even-grained granite that probably represent two separate intrusive phases. This lithologic variation does not occur in the Obbnäs pluton, which is almost entirely composed of porphyritic hornblende-biotite granite that gradually becomes more mafic to the southwest. Three types of hybrid granitoids resulting from magma mingling and mixing occur on the southwestern tip of the Obbnäs peninsula. The Bodom granites are syenogranites, whereas the composition of the Obbnäs granite varies from syeno- to monzogranite. The main silicates of both the Bodom and Obbnäs granites are quartz, microcline, plagioclase (An1541), biotite (siderophyllite), and generally also amphibole (ferropargasite or hastingsite). Plagioclase-mantled alkali feldspar megacrysts are absent or rare. The accessory minerals are fluorite, allanite, zircon, apatite, and iron-titanium oxides; the Obbnäs granite also contains titanite. The Bodom and Obbnäs granites are metaluminous to weakly peraluminous, with average A/CNK of 1.00 and 1.05, respectively, have high Fe/Mg (average FeOtot/[FeOtot+MgO] is 0.94 for the Bodom and 0.87 for the Obbnäs granites), and high Ga/Al (3.78 to 5.22 in Bodom and 2.46 to 4.18 in Obbnäs). The REE contents are high with LREE-enriched chondrite-normalized patterns and moderate (Obbnäs) to relatively strong (Bodom) negative Eu-anomalies. The Obbnäs granite is enriched in CaO, TiO2, MgO, and FeO, and depleted in SiO2 and K2O compared to the Bodom granites. Also, there are differences in the Ba, Rb, and Sr contents of the two plutons (Ba and Sr higher, Rb lower in Obbnäs) that cannot easily be traced to single parental granite magma. These features, together with the presence of titanite in the Obbnäs pluton, suggest different sources for the granites of the two plutons: a potassium feldspar-rich source for Bodom and potassium feldspar-poor for Obbnäs. |
| first_indexed | 2024-12-20T05:10:40Z |
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| institution | Directory Open Access Journal |
| issn | 0367-5211 1799-4632 |
| language | English |
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| spelling | doaj.art-5654d08143fa438da1175bf8d1dfb2de2022-12-21T19:52:16ZengGeological Society of FinlandBulletin of the Geological Society of Finland0367-52111799-46321999-12-0171227530410.17741/bgsf/71.2.005The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistryP. KosunenThe Obbnäs and Bodom granite plutons of southernmost Finland show the typical petrographic and geochemical features of the Proterozoic rapakivi granites in Finland and elsewhere: they cut sharply across the 1900 Ma Svecofennian metamorphic bedrock and have the geochemical characteristics of subalkaline A-type granites. The Bodom pluton is composed of porphyritic granites (hornblende-, hornblende-biotite-, and biotite-bearing varieties) and an even-grained granite that probably represent two separate intrusive phases. This lithologic variation does not occur in the Obbnäs pluton, which is almost entirely composed of porphyritic hornblende-biotite granite that gradually becomes more mafic to the southwest. Three types of hybrid granitoids resulting from magma mingling and mixing occur on the southwestern tip of the Obbnäs peninsula. The Bodom granites are syenogranites, whereas the composition of the Obbnäs granite varies from syeno- to monzogranite. The main silicates of both the Bodom and Obbnäs granites are quartz, microcline, plagioclase (An1541), biotite (siderophyllite), and generally also amphibole (ferropargasite or hastingsite). Plagioclase-mantled alkali feldspar megacrysts are absent or rare. The accessory minerals are fluorite, allanite, zircon, apatite, and iron-titanium oxides; the Obbnäs granite also contains titanite. The Bodom and Obbnäs granites are metaluminous to weakly peraluminous, with average A/CNK of 1.00 and 1.05, respectively, have high Fe/Mg (average FeOtot/[FeOtot+MgO] is 0.94 for the Bodom and 0.87 for the Obbnäs granites), and high Ga/Al (3.78 to 5.22 in Bodom and 2.46 to 4.18 in Obbnäs). The REE contents are high with LREE-enriched chondrite-normalized patterns and moderate (Obbnäs) to relatively strong (Bodom) negative Eu-anomalies. The Obbnäs granite is enriched in CaO, TiO2, MgO, and FeO, and depleted in SiO2 and K2O compared to the Bodom granites. Also, there are differences in the Ba, Rb, and Sr contents of the two plutons (Ba and Sr higher, Rb lower in Obbnäs) that cannot easily be traced to single parental granite magma. These features, together with the presence of titanite in the Obbnäs pluton, suggest different sources for the granites of the two plutons: a potassium feldspar-rich source for Bodom and potassium feldspar-poor for Obbnäs.http://www.geologinenseura.fi/bulletin/Volume71/sgs_bt_071_2_pages_275_304.pdfgranitesrapakiviplutonspetrographygeochemistryProterozoicBodomObbnäsFinland |
| spellingShingle | P. Kosunen The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry Bulletin of the Geological Society of Finland granites rapakivi plutons petrography geochemistry Proterozoic Bodom Obbnäs Finland |
| title | The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry |
| title_full | The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry |
| title_fullStr | The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry |
| title_full_unstemmed | The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry |
| title_short | The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry |
| title_sort | rapakivi granite plutons of bodom and obbnas southern finland petrography and geochemistry |
| topic | granites rapakivi plutons petrography geochemistry Proterozoic Bodom Obbnäs Finland |
| url | http://www.geologinenseura.fi/bulletin/Volume71/sgs_bt_071_2_pages_275_304.pdf |
| work_keys_str_mv | AT pkosunen therapakivigraniteplutonsofbodomandobbnassouthernfinlandpetrographyandgeochemistry AT pkosunen rapakivigraniteplutonsofbodomandobbnassouthernfinlandpetrographyandgeochemistry |