orbicular monzonite boulder from kurki jarvi, vilppula

ORBICULAR MONZONITE BOULDER FROM KURKIJARVI, VILPPULA
Paula Raivio
Fig. 87. Polished sample from an orbicular monzonite boulder from Kurkijärvi, Vilppula. Some of the
orbicules are flattened and broken. A polished specimen from the Mineralogical Museum of the Geological
Survey of Finland. The length of the long orbicule with a black mica-nch core in the upper nght part that
is 15 cm. Photograph: J. Keskinen.
During the constmction of the main road on the
northeastern side of Lake Kurkijärvi (formerly
Löytänajärvi) in Vilppula in 1947, a boulder of orbicular rock was blasted into many pieces. During
fieldwork of the Geological Survey of Finland geologist Olavi Waldén discovered the pieces of the
boulders and informed his superiors. Later the rest
of the orbicular rock boulders were brought to the
Mineralogicai museum of the Geological Survey of
Finland in Espoo.
Simonen (1950) subsequently studied the orbicular rock and published a short mineralogical-geological description of it. He also gave chemical analyses of the matrix, nucleus and zones of the orbicules.
For this article a representative sample of the whole
rock was analysed chemically and the results are given in Table 7. In the R,R,classification diagram, the
bulk rock composition falls into the monzonite field,
quite close to the corner of the monzodiorite, tonalite and quartz monzonite fields (Fig. 7 1).
The general appearance of the rock is rather heterogeneous and the rnineralogy of the rock varies in
different parts of the boulder. The diameters of the
orbicules vary in size from a few centimetres up to
about 15 cm (Fig. 87). The orbicules tend to be ellipsoidal in form, but many of them are more or less
deformed or broken and irregular.
One-third to two-thirds of each orbicule is occupied by a core, most of which consist of coarsegrained plagioclase (An,,-An,,) with some quartz
and biotite (Simonen 1950). In a few orbicules, however, the core has a nucleus of a dark-grey, finegrained schist- or gneiss-like xenolith.
The shell around the orbicules is 1-3 cm thick and
consists mainly of radially oriented plagioclase
(An,,-An,,). Individual plagioclase crystal laths may
extend across the whole shell. Tiny biotite flakes occur as inclusions in the radial plagioclase crystals,
and form several indistinct and irregularly spaced
subshells that nevertheless do not intenupt the
growth of the plagioclase crystal laths. The amount
of biotite increases towards the periphery of the orbicules. Tiny opaque mineral crystals also form irregular thin, less than one millimetre wide subshells
among the radial plagioclase crystals in the outer
part of the shells. The inner part of the shells is quite
free of the dark constituents.
According to Simonen (1950), the matrix between
the orbicules is distinctly migmatitic, and contains
both quartz dioritic and granitic portions. Locally
large microcline porphyroblasts (1-2 cm in diameter) are characteristic in the matrix. Simonen sug-
gests that the primary quartz dioritic (or tonalitic)
matrix was strongly granitized during the crystallization processes of the rock and the secondary microcline has metasomaticaily corroded the primary
quartz dioritic fabric (Simonen 1950). Microcline
porphyroblasts growing across the shell of the orbicules give evidence of the metasomatic processes.
ORBICULAR GRANITE OCCURRENCE FROM KUORENIEMI, VILPPULA
Seppo 1. Lahti
Fig. 88. Weathered surface of orbicular rock from Kuoreniemi, Vilppula. The orbicules (0 5-7 cm) are
poorly developed and they often have a feldspar core. The pen is 10 cm long. Photograph: S.I.Lahti.
During geological mapping for the Geological
Survey of Finland, Liisa Pajari (GSF) discovered an
orbicular rock outcrop (Fig. 88) in Kuoreniemi, Vilppula. Later the present author studied the occurrence
with field assistant Tuomo Turunen (GSF), mapped
the occurrence and drilled some samples for chemical analyses and petrographic determinations. The
orbicular rock and its host rock are granitic, as is
shown by the mineralogy of the rock and by the
chemical analyses plotted in the R,R, classification
diagrarn (see chemical analysis in Table 7 and classification diagram in Fig. 71).
Originally the orbicular rock obviously formed a
narrow dyke or lens in granite, but because of strong
erosion, it is nowadays only a thin layer on the surface of a fifteen metres long and three metres high
steep granite cliff (see Fig. 89). The orbicules can
be recognized as roundish spheres 5-10 cm wide on
the weathered rock surface (Fig. 88). Irregular remnants of rnica schist locally occur in the orbicular
rock, and locally the orbicules grow inside these
dark, fine-grained mica-rich schist or gneiss inclusions.