The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry

Abstract

KOSUNEN, PAULA 1999. The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry. Bulletin of the Geological Society of Finland 71, Part 2,.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 evengrained 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 (An 1541 ), 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 FeO tot /[FeO tot +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, TiO z , MgO, and FeO, and depleted in Si0 2 and K 2 0 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.

The Paleoproterozoic Nattanen-type granites in northern Finland and vicinity - a postcollisional oxidized A-type suite

“…Comparison fields after representative (SiO 2 > 65 wt. %) analyses of rapakivi granites from Rämö (1991), Rieder et al (1996) and Kosunen (1999) (from Wiborg, Bodom, and Obbnäs) and representative (SiO 2 > 65 wt. %) shoshonitic rocks after Rutanen et al (1997) (from southern Finland and Russian Karelia).…”

Anorogenic magmatic rocks in the Estonian crystalline basement

“…In the ternary Rb-Ba-Sr diagram the rocks are comparable to the less differentiated granitoid phases from the Wiborg and Laitila batholiths (Fig. 3) and the Obbnäs granite of southern Finland (Kosunen 1999). The granodiorite of the Märjamaa stock and the biotite granite of the Neeme stock are richer in Sr, Ti, and P (Petersell & Kirs 1992).…”

“…Molybdenite and galena are met locally. A small but interesting difference to the Finnish rapakivi granites is in Ti minerals: in the Finnish rapakivi granites the typical accessory Ti minerals are ilmenite and anatase, titanite is known only from the Obbnäs granite in the southern coast of Finland (Kosunen 1999).…”

The Veikkola granite area in southern Finland: Emplacement of a 1.83-1.82 Ga plutonic sequence in an extensional regime

“…2) is of ~1.64 Ga age (Kosunen, 1999). A swarm of WNW trending diabase dikes are seen on aeromagnetic map as straight positive anomalies, and on a few outcrops as steeply dipping dikes 20 -100 cm in width.…”