ORBICULAR GABBRO OCCURRENCE FROM OHTOLA, VIRRAT

ORBICULAR GABBRO OCCURRENCE FROM OHTOLA, VIRRAT
Paula Raivio
Fig. 80. Orbicular gabbro from Ohtola, V i a t . Note the fragments of broken orbicules in the matri
The orbicules are 3-6 cm in diarneter. Photograph: P. Raivio.
First boulders of orbicular rock in Ohtola, Virrat,
was found by Pekka Ohtola in 1981. In the regional
geological studies a fan consisting of several hundred orbicular rock boulders was located and later
in the surnmer of 1982 the orbicular rock was exposed by digging in two test pits near the proximal
end of the boulder fans (Hirvas & Huhta 1982).
The Ohtola orbicular rock has previously been
described by the present author (Raivio 1985a, b, c).
The mineralogy of the rock corresponds to that of
gabbroic rocks and its chemical analyses (Table 7)
also plot in the gabbro field in the R,R, classification diagram (Fig. 7 1).
On ihe basis of the distribution of the boulders,
the length of the orbicular rock occurrence is estimated to be about 800 m, while the width probably
ranges between 0 and 20 m. The dyke-like occurrence is mainly surrounded by granite.
There are two main types of orbicules in Ohtola
called proto-orbicules and multishelled orbicules
(Figs 80 and 81). The proto-orbicular type is present
in the central and eastern part of the occurrence and
the multishelled type that often heterogeneous and
contain numerous xenoliths in the western part of the
occurrence. Coarse-grained hornblende gabbro is
also found as boulders and probably represents related, though non-orbicular rock types.
The size of the orbicules varies from a few up to
about ten centimetres. Generally the proto-orbicules
are smaller than the multishelled ones. The smallest
"orbicules" are actually hornblende phenocrysts and
the Iarger ones resemble ovoids.
The proto-orbicules consist mainly of hornblende,
which is arranged radially around the central core,
although not so strikingly as in the multishelled type.
Fig. 81. Heterogeneous inclusion-type orbicular gabbro from Ohtola.
The rock contains mafic fragments (autoliths), poorly developed protoorbicules, single-shelled and multishelied orbicules (0 5-8 cm). The core
of the orbicule in the upper right comer is a fragment of an orbicule.
Photograph: P. Raivio.
n e radial hornblende crystal prisms cross the thin
concentric plagioclase subshells in some proto-orbicules. These orbicules are a kind of transitional type
between the proto-orbicular and multishelled types.
In the multishelled orbicules the alternation of the
concentric hornblende and plagioclase shells is the
most conspicuous feature. The hornblende crystals
are ananged radially in the hornblende shells (thickness 1-20 mm), but plagioclase is granular in the
plagioclase-rich shells (thickness 1-3 mm). Often
the hornblende radii have grown uninterrupted from
the core outwards, although many discontinuous plagioclase sheiis cross the radii. Where the plagioclase shells are thicker they usually interrupt the growth
of the inner hornblende shell. Approximately twothirds of a multishelled orbicule is hornblende, with
lesser amounts of plagioclase (An,,-An,,) and biotite.
Both in the proto-orbicular and multisheiled type
a few grains form the nuclei of the cores. The main
constituents in the cores are hornblende, plagioclase
(An,-An,,), biotite and often also quartz. In some
orbicules a single hornblende crystal forms the core,
especially in the proto-orbicular type. Locally large
fine-grained mafic autoliths or xenoliths (10-20 cm
long) are also comrnon among the orbicules. The inclusions are often surrounded by a centimetre-thick
shell of hornblende sirnilar to the shells of a "proper" orbicule. In a few cases, fragments of broken orbicules were observed as nucleus in the orbicules.
The matrix between the orbicules consists of fmegrained plagioclase (An,,-An,,), hornblende, biotite
and quartz. Idiomorphic hornblende phenocrysts (up
to 1.5 cm in size) are common in the matrix of the
proto-orbicular type.
ORBICULAR GABRO OCCURRENCE FROM LASTUSTENKULMA, LEMPAALA
Seppo 1. Lahti
Fig. 82. Weathered surface of orbicular gabbro from Lastustenkulma, Lempäala. The orbicules (02-3 cm)
are single-shelied. There are locally shelled gabbroic inclusions that are angular in size. Photograph:
S.I. Lahti.
Geologist Seppo Turkka informed the author of
the occurrence of the orbicular rock boulders he had
discovered during geological field work in 1980s in
Lastustenkulma, Lempäälä. On the basis of his ob-
servation, the author studied the boulders with field
assistant Tuomo Turunen (GSF) and discovered in
surnrner 2000 a set of new boulders and a small outcrop of orbicular rock (Fig. 83) close to the south-