xenolith-type orbicular gabbro boulder from jussinjärvi ... - Arkisto.gsf.fi

XENOLITH-TYPE ORBICULAR GABBRO BOULDER FROM JUSSINJÄRVI,
RAAHE AND ONNETON, HAAPAJARVI
Seppo 1. Lahti & Ilkka Laitakari
fig. 175. Xenolith-type orbicular gaooro wuiuer rrom ~usr>injärvi, Raahe. The cores of the orbicules are
composed of calc silicate rocks and concretions. A sample (30 cm in diarneter) from the Mineralogical
Museum of the Geological Suwey of Finland. Photograph: J. Keskinen.
The orbicular rock erratic from Jussinjärvi, Raahe,
was found in 1955 by Professor Tauno Piirainen
and Dr. Valto Veltheim in connection with ore prospecting
being carried out by the Geological Survey
of Finland (Jussinjärvi boulder no. 80). The boulder,
some 30 cm in diarneter, has been transfened to the
museum of the Geological Survey (Fig. 175).
Most of the orbicules in the boulders are oval or
subangular and from 5 to 10 cm in diameter. There
is also one larger orbicule (18 cm long) in the Jussinjärvi
boulder. The coxe of the orbicules is a xenolith
of a calc silicate rock or caleic conceretion. The
shell around the cores is one to two centimetres thick
and consists of several thin alternating light and dark
subshells. The main mineral in the light subshells is
plagioclase and in the dark subshells hornblende,
chlorite and biotite. Most of the hornblende crystals
are prismatic and radially oriented. Plagioclase is
zoned and invariably slightly altered and in many
cases it too is radially aligned. Accessory rninerals
in the shells include biotite, muscovite, carbonate,
epidote and opaques, with rare titanite.
The matrix in the boulder is medium-grained gabbro,
consisting of hornblende, plagioclase and biotite,
with minor amounts of quartz. The plagioclase
in the matrix is often zoned (Ana-An5,) and twinning
is typical of many hornblende prisms. The accessory
phases are muscovite, chlorite, carbonate,
apatite, titanite and zircon. Some of the orbicules in
the boulders are broken and fragments of shells can
also be observed in the matrix.
The Jussinjärvi orbicular rock is called xenolithtype
orbicular gabbro, because its matrix and shells
are gabbroic in composition and the cores of all the
orbicules are composed of xenoliths. Because the
sample analysed contained much core materia1 (mafic
calc silicates), the chemical analysis of the rock
plot out of the gabbro field in the R,R, classification
diagram (chemical analysis in Table 15 and classification
diagram in Fig. 173).
A quite similar, though even smaller boulder was
also found by Veikko Autio in Onneton, Haapajärvi,
about 90 km SE of the Jussinjärvi locality (Onneton

oulder no. 85). This boulder is roundish, only about
15 cm in diameter, and consists of closely spaced
multishelled orbicules in a homogeneous gabbroic
matrix (Fig. 176). The cores of the orbicules are also
composed of dark-green rounded xenoliths of schist,
most likely fragments of calc silicate rocks.
Because the general appearance of the rocks - the
core material, the shells and matrix - closely resernble
each other in both the Jussinjämi and Onneton
orbicular rocks, the authors believe that both erratics
may be derived from a common, presently unknown
source, although the distance between the localities
is considerable (nearly a hundred kilomehres).
ne roun&sh fom of the Onneton boulder, furthemore,
indicates that it was trmsported a long dstance
by the ice.
Fig. 176. Weathered swface of a xenolith-type orbicular rock h m
Onneton, Haapaj-Wi. The cores of the orbicules are calc-silicate rocks.
A sample (about 20 cm wide) belonging to V. Autio. Photomh: J. Keskinen.
ORBICULAR GABBRO BOULDER FROM KANGASPAANJARVI, REIS JARVI
Seppo 1. Lahti, Ilkka Laitakari and Paula Raivio
Fig. 177. Weathered swface of orbicular gabbro boulder from Kangaspahjrvi, Reisjárvi. The orbicules
are medium-sized (0 5-8 cm), single-shelled and often deformed. A sample from the Mineralogical Museum
of the Geological Survey of Finland. Photograph: J. Keskinen.
Geologist Eero Pehkonen discovered this orbicu- cal Survey of Finland. The chemical analysis indilar
rock in 1958 west of lake Kangaspaanjärvi in cate that the rock is syenodiorite in composition (Ta-
Lehtoranta, near Reisjärvi. The boulder that is ble 15, Fig. 173).
rounded and about 120 x 80 x 70 cm in size was The KangaspWjämi orbicular rock consists of
found during geological fieldwork of the Geologi- very closely spaced orbicules, 4-6 cm in diarneter,