Ritva Karhunen - arkisto.gsf.fi - Geologian tutkimuskeskus

Suomen geologinen kartta 1 : 100 000, Kallioperäkarttojen selitykset, lehdet 1041 ja 1043
Iniön ja Turun kartta-alueiden kallioperä – Berggrunden inom Iniö och Åbo kartblad
gneisses are usually fine-grained, banded gneisses; the banding is considered to represent
original layering as the result of deposition in shallow water.
Amphibolites, hornblende gneisses and biotite-hornblende gneisses
Mafic supracrustal rocks form long, narrow belts especially in the western and
southern areas of the Iniö map-sheet, often intercalated with thicker beds of mica
gneiss and quartz-feldspar gneiss. The amphibolites and hornblende gneisses are usually
fine- to medium-grained, even-grained and often banded. In the Turku map-sheet
area larger continuous amphibolite zones have been found at the northwestern margin
of the map-sheet, southeast of Turku and around Parainen. Well-preserved primary
structures, such as pillow-lavas and pillow-breccias, indicate the volcanic origin
of the amphibolites. In contrast the hornblende gneisses and the biotite-hornblende
gneisses may contain considerable amounts of sedimentary material.
Pillow lavas and pillow lava breccias
Pillow lava and pillow breccia structures have been observed in some well-exposed
areas in the Iniö map-sheet area, eg. at Nötö and Baggholma (1041 02C) islands
in the province of Åland, in the island of Iniö (1041 08B), and in small islands
belonging to the Velkua commune (1041 11B and 11C). Clearly recognizable pillow
lava structures are quite rare in the Turku map-sheet area. A few exposures have
though been found southwest (1043 09A and 09D) of Turku.
A spectacular volcanic succession, ranging from pillow lavas to amphibolites can
be seen in some of the small islands of Velkua in the easternmost part of the Iniö
map-sheet area (1041 11A and 11B). The volcanic succession starts with well-preserved,
pillow lava on top of mica gneisses. The pillow lava consists of isolated,
often elongated and somewhat deformed pillows. The pillow lavas are followed in
succession by pillow breccias and fragmented amphibolitic lavas which represent
the topmost part of the subaquatic lava sequence. Small exposures of agglomerate
are found as well. The uppermost parts of this sequence are represented by amphibolitic
lavas. The cavities between the pillows are often filled with crystalline calcitic
limestone. The pillows consist of hornblende and plagioclase.
Limestone
Layers of crystalline limestone are commonly intercalated with mica-poor gneisses,
sometimes also with mica gneisses and amphibolites. The width of the layers varies
from a few tens of centimetres up to a few metres. The limestone layers are intensely
folded and usually more strongly weathered than the surrounding gneisses.
In addition to the main mineral calcite the limestone also consists varying amounts
of quartz, plagioclase, hornblende, diopside, titanite and epidote.
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