Åland-Inseln  -  Magmatite  -  postorogene Granite

Vier intrusive Granitstöcke des südwestfinnischen Schärengartens (Åva, Mosshaga, Seglinge und Lemland) sind postorogene Intrusionen, sie werden auf 1815-1770 Mio. Jahre datiert (Lindberg 1988), sind somit jünger als die Grundgebirgsgneise und Granodiorite des Svecofenniums, aber älter als die Rapakivi-Batholithe. Diese vier kleineren, klar umrissenen Massive liegen größtenteils unter Wasser und treten nur in wenigen Inseln zutage (siehe Kartenskizze). Sie enthalten rötliche, ungleichkörnige oder porphyrische Granite.
 
Handstücke aus dem Anstehenden, Geozentrum Hannover, Sgl. v. d. Heide und R. Hanning:
 
Lemland-Granit Åva-Granit   Kökar-Granit, spätsvecofennisch
 
Zitat aus Haapala u. Ehlers 1989, S. 5:
Postorogenic intrusions
"The postorogenic intrusions, which are often small ring-shaped complexes comprising lamprophyres, monzodiorites, monzonites, granodiorites and granites, have been described by Sederholm (1934), Kaitaro (1953), Ehlers and Bergman (1984), Bergman (1986) and Hubbard & Branigan (1987).
In SW Finland there are four intrusions: Lemland, Mosshaga, Seglinge and Ava (Fig. 2). The intrusions have ages around 1.79 Ga (Welin et al. 1983, Patchett and Kouvo 1986, Suominen 1987) and thus predate the rapakivis of the same area by some 0.2 Ga. Most of the complexes are rounded, 4-5 kilometres in diameter and show distinct phases of mingling and mixing of coeval mafic and granitic magmas (Hubbard and Branigan 1987, Lindberg and Eklund 1988). There are signs of rapid emplacement at high crustal levels (ring structures, gently inward dipping or steep cone sheets and breccia sheets with angular xenoliths of the local wall rock, occasional tuffisites).
The postorogenic ring complexes are cut by a regional swarm of diabase and quartz porphyq dykes of rapakivi age (Ehlers and Ehlers 1977, Ehlers and Bergman 1984).
The postorogenic granites are coarse, porphyritic, red rocks with a foliation defined by tabular microcline megacrysts. The foliation is mostly due to a flow fabric that is distorted around xenoliths and irregularities in the wall-rock (Hubbard and Branigan 1987). The mineralogy of the postorogenic granites is characterized by rotated and sometimes fractured
porphyroclasts of microcline. The quartz is strained and recrystallised, sometimes the plagioclase is strained too. Hornblende, biotite, sphene, magnetite and apatite form elongated aggregates. The granodioritic - monzonitic
varieties are less porphyritic, and the megacrysts are only weakly
oriented (Seglinge).
The granodiorites are medium grained with a mottled appearance caused by clusters of biotite and hornblende together with sphene-rimmed magnetite. The progression to monzodiorite is marked by an increase in hornblende
and biotite at the expense of quartz and microcline (Hubbard and Branigan 1987). Contrary to most rapakivi granites, the postorogenic granites are rich in sphene and magnetite and can be identified on aeromagnetic maps
as strongly magnetic areas."
 
 
       
Literatur:
Branigan N. 1988: Small-scale granitic diapirs, Åland, SW Finland. GFF Volume 110, Issue 3, 1988
Ehlers C. et al. 2004: Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland. Bulletin of the Geological Society of Finland 76.1-2 (2004): 63-91
Eklund O. et al. 1998: 1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield. Lithos
V. 45, Bde 1–4, Dec. 1998, S. 87–108
Haapala I. u. Ehlers C. 1989: Rapakivi granites and postorogenic granites of southwestern Finland. Excursion A1, University of Helsinki
Hausen H. 1964: Geologisk beskrivning över landskapet Åland. Mariehamn 1964
Lindberg B., Eklund O. 1988: Interactions between basaltic and granitic magmas in a Svecofennian postorogenic granitoid intrusion, Åland, southwest Finland. Lithos V. 22, Bd. 1, Sep. 1988, S. 13–23
Welin E. et al. 1983: Age differences between Rb-Sr whole rock and U-Pb zircon ages of syn- and postorogenic Svecokarelian granitoids in Sottunga, SW Finland. Lithos V.16, Bd 4, Oct. 1983, S. 297–305
       
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