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Finnland - Magmatite - Obbnaes-Rapakivi

Der kleine Obbnäs-Pluton südwestlich von Helsinki ist eine Satellitenintrusion zum Wiborg-Batholith. Er zeigt im Anstehenden einen porphyrischen Hornblende-Biotitgranit mit stellenweise zwischengeschaltetem, überwiegend gleichkörnigem Granodiorit. Alter: 1640 Mio. Jahre.



Obbnäs-Granit, S Kyrkslätt, leg. Düne 2003, Geozentrum Hannover auf der Karte: im Bereich von Nr. 1


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		Obbnäs-Granit, Straße von Kyrkslätt nach Obnäs leg. K.-D. Meyer 1965, Geozentrum Hannover auf der Karte: Nr. 1

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	Obbnäs-Granit, Obbnäs, westl. Helsinki, leg. geol. Kommission Finnland BGR Berlin Etikett auf der Karte: südlich von Nr. 1

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Obbnäs-Rapakivigranit, Steinbruch W der Straße nach Obbnäsudden, 800 m S des Abzweigs nach Kantvik-Hamn (Industriegebiet), Blatt Siuntio, Sgl. R. Zwirner 2009, auf der Karte: ca. Nr. 1

	2. Handstück von derselben Lokalität

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Obbnäs-Rapakivigranit, Pippursvägen, 900 m östlich der Hauptstraße (1191), Straßenanschnitt, S Kyrkslätt, Blatt Siuntio, 3.7.2009, auf der Karte: ca. Nr. 1

Als Gesteinsbeschreibung ein Ausschnitt aus dem Begleittext zum 33. IGC-Kongress "The Nordic Countries" 2008, Exkursion 16 (Paula Kosunen), S. 9-13: "...The Obbnäs rapakivi granite pluton lies on the southern coast of Finland, 35 km west-southwest of the capital Helsinki. The pluton encompasses an area of 50 km 2, with a maximum length of 15 and width of 6 km, and forms a large part of the Upinniemi (Obbnäs) and Porkkala peninsulas. Sederholm described the pluton in his 1926 treatise “On Migmatites and Associated Pre-Cambrian Rocks of Southwestern Finland, Part II”, where he coined the term “Obbnäs granite”. He observed that the Obbnäs granite and a similar pluton found further northeast, Bodom, were clearly younger than the surrounding Svecofennian rocks and wrote: “The Bodom, as well as the Obbnäs granite, has in the most typical way penetrated the older rock masses, punching holes in them. There are no primary gradations between them and the older granitic formations. ...And if the evidence gained at all the contacts, especially at those of the Obbnäs area, are taken into consideration, it seems indubitable that these granites are decidedly younger than the surrounding rocks” (Sederholm, 1926 p. 101). He did not, however, associate the Obbnäs granite with the classic rapakivi plutons of southeastern Finland, but considered it to “belong to a group of late pre-Cambrian granites intermediate in age between the Hangö type of granite and the rapakivi granites”, with no clear genetic connection to either group (Sederholm, 1926, p. 93 and 116). Although later U-Pb -dating revealed the Obbnäs granite to be of the same age as the rapakivi granites of southeastern Finland (1645 ± 5 Ma; Vaasjoki, 1977), Sederholm’s age estimate reflects his accurate recognition of features that make the Obbnäs pluton different from the other Finnish rapakivi granites. The Obbnäs pluton is almost entirely composed of porphyritic hornblende-biotite granite, which is distinctly more homogeneous than the older Svecofennian granites in the area and, as Sederholm observed, clearly transects the older lithologies. The granite is coarse-grained with alkali feldspar megacrysts from 2 to 5 cm in diameter, and becomes gradually more mafic toward the southwest. This is accompanied by changes in the appearance of the rock: the red color turns to reddish-brown, ovoidal megacrysts become more abundant and more commonly mantled by plagioclase, and the groundmass becomes finer-grained. Some euhedral plagioclase grains are present in the southern part of the pluton. Zircon, apatite, and ilmenite are common; metamict allanite, interstitial fluorite, and rare ilmenomagnetite are also present. The Obbnäs granite also contains primary titanite, which is lacking in the other Finnish rapakivi granites (Laitakari et al., 1996). Unlike most rapakivi granites, the Obbnäs pluton commonly displays a distinct fabric alignment. Near the contacts, a magmatic foliation is commonly evident in the alignment of alkali feldspar megacrysts and examples of both ductile and brittle deformation are locally seen, especially in the northwestern part of the pluton. The presence of cataclastic deformation was one of the reasons why Sederholm regarded the Obbnäs granite as “somewhat older than the rapakivi, because the latter is almost entirely devoid of cataclastic phenomena”. He did, however, write: “Later the writer found that most of these cataclastic phenomena were due to movements which accompanied, or in any case had not been much later than the eruption of the same granite” (Sederholm, 1926 p. 113) and speculated that this may relate to depth of emplacement and that deeper down, the rapakivi granites might assume similar characteristics. Also, most (if not all) of the deformation present in the northwest can be attributed to movements within the adjacent Porkkala-Mäntsälä shear-zone during and after crystallization of the granite (Elminen, 1999). ... Description The southern margin of the Obbnäs pluton is beautifully exposed in the coastal outcrops at Obbnäsudden, the southernmost tip of the Obbnäs peninsula. Several interesting features can be seen on the glacially-polished surfaces, including various Svecofennian xenoliths in different stages of assimilation and magma mingling and mixing features. Most of the area is composed of granitic to granodioritic rocks with variable amounts of alkali feldspar megacrysts. These “porphyritic granodiorites” form large, irregular pillows within the “proper” Obbnäs granite, and have characteristics typical of hybrid rocks resulting from magma mixing and mingling. Besides alkali feldspar megacrysts (2 to 5 cm in diameter), the “porphyritic granodiorites” contain quartz phenocrysts (diameter up to 1 cm) and microphenocrysts of plagioclase. The alkali feldspar and quartz crystals are commonly rounded and slightly corroded. Needle-like apatite is present, and the larger quartz crystals are locally rimmed by dark amphibole. The “porphyritic granodiorites” can be separated into two main varieties based on the amount of alkali feldspar phenocrysts: a densely-porphyritic type and a sparsely-porphyritic type. Contacts between these two types are usually distinct but not sharp. In addition to the “porphyritic granodiorites”, a rather mafic, even-grained granodiorite is found on the small island just off Obbnäsudden, where it is in distinct, gently undulating contact with the “porphyritic granodiorites”. Smaller (diameter usually 10-40 cm), intermediate magmatic microgranular enclaves with variable amounts of incorporated alkali feldspar megacrysts are also found within the porphyritic hornblende-biotite granite in the southern part of the pluton. Both the “proper” Obbnäs granite and the hybrid rocks contain a large number of variably sized Svecofennian xenoliths, mostly migmatites, rather mafic gneisses and older granite (Hangö-Ingå granite). Some of the fragments are unchanged, but most are in different stages of assimilation into the surrounding granite/granodiorite. Sederholm studied the various assimilation phenomena in the area and dedicated an entire chapter to Obbnäsudden. He wrote: “The rock of the fragments in part retains its former composition, in part it is more or less strongly changed, passing by gradations into massive rocks of dioritic composition”, “In the neighbourhood of a rather basic migmatite, the granite passes by gradations into a dioritic rock which still shows the great porphyritic crystals characteristic of the Obbnäs granite” and “…the existence of a great number of fragments…more or less changed by the influence of the granite, and on the other hand a basic facies of the granitic magma, makes it probable that there is a genetical connection between them” (Sederholm, 1926, p. 103-104). The most intensively assimilated gneiss fragments are commonly surrounded by zones rich in dark minerals and it seems the “sparsely-porphyritic” type of granodiorite is in many places associated with them, suggesting that assimilation has played a role in its formation. It seems that the two processes, magma mingling and mixing and assimilation, have operated together in Obbnäsudden to create the various types of rocks observed in the area, and that further, more detailed work would be required to shed light on their relative roles in the whole process..."

Literatur:
33 IGC excursion "The Nordic Countries" No 16, August 16 – 21, 2008: 100 years of migmatite - In Sederholms footsteps. Als PDF-Datei: http://www.iugs.org/33igc/fileshare/filArkivRoot/coco/FieldGuides/No%2016%20Sederholms%20foot%20steps.pdf
Kosunen P. 2004: Petrogenesis of mid-Proterozoic A-type granites: Case studies from Fennoscandia (Finland) and Laurentia. University of Helsinki (dissertation)
Sederholm J. J. 1926: On Migmatites and Associated Pre-Cambrian Rocks of Southwestern Finland.
II. The Region around Barösundsfjärd W of Helsingfors and Neighboughring Areas. Bull. Comm. géol. Finl. 77. 1926.

	zur Übersicht Rapakivi des Wiborg-Batholiths zur Übersichtskarte Wiborg-Batholith zur Übersicht finnische Rapakivi-Plutone