72
Geological Survey of Finland, Bulletin 395
Tapani Mutanen
Emelyanenko, 1980; Lorand & Ceuleneer,
1989) and from ilmenite (Weiblen, cited by
Roedder, 1979). Alkalic inclusions are known
to occur in chromite both in tholeiitic intru-
sions (McDonald, 1965; Irvine, 1974; Alapieti.
1982; Alapieti et al., 1989) and in ophiolitic
chromites (e.g., Talkington et al., 1984). The
chromites and chromitites of Akanvaara and
Koitelainen represent an extreme case of alkali
enrichment: both the melt inclusions and the
matrix are very potassic. The subject is dis-
cussed in more detail later.
Ascending the stratigraphy we find right
above the peridotites a peculiar and seemingly
incompatible association of olivine pyroxen-
ites, sulphide-disseminated pyroxene cumu-
lates and pyroxenites rich in quartz, potassium
feldspar, biotite and fluorapatite. An unknown
Ti-Th-Fe(-REE-P) silicate with inclusions of
perrierite(?) occurs in the intercumulus of a
sulphide-disseminated pyroxene cumulate. The
sulphides are very low in Ni and show only
slightly elevated PGE-Au concentrations (see
Fig. 27c); in fact, they are quite similar to the
false ore sulphides of Keivitsa. The pyroxene
cumulates are closely associated with, and
grade into, alkali-rich quartz gabbros contain-
ing inverted pigeonite, cumulus magnetite and,
interestingly, solitary chromite euhedra. The
latter rocks are often fine-grained, resembling
chilled microgabbros.
Here we find the unholy cohabitation of
magnesian olivine and intercumulus quartz
(0.1 mm apart at the closest), encountered later
again above the Lower Chromitite layers and
also in the Merensky Reef, Bushveld (Mu-
tanen, 1989b), and recently at Keivitsa, too.
This disequilibrium assemblage, which was
brought about by magma currents from differ-
ent parts of the chamber, was incompletely
mingled before settling. Disequilibrium phen-
ocryst assemblages of quartz and olivine are
known from mixed lavas (Sakuyama, 1981). A
peculiar feature found in mixed lavas is the re-
verse peritectic resoption of orthopyroxene to
olivine and augite (Kuno, 1950). Similar re-
sorption of orthopyroxene (to clinopyroxene)
is common in pyroxene cumulates in the upper
LZ.
Olivine and orthopyroxene often occur in
these rocks as separate crystals, apparently
without a reaction relationship, suggesting co-
precipitation of the minerals. But the pressure
needed, 5.4 kb, (Boyd et al., 1964), corre-
sponding to a depth of ca 18 km, is certainly
too high for the basal part of the Koitelainen
intrusion, and also for the Keivitsa intrusion,
where olivine and orthopyroxene co-exist sim-
ilarly. I think that this is another case of bas-
tard cumulate assemblage, of minerals bred
separately but later mingled together (see also
Jackson, 1961).
The unexpectedly early appearance of in-
verted pigeonite may be due to chilling (Wager
& Brown, 1968), but here it suggest contami-
nation by exotic iron, such as has been noted
or suspected elsewhere, for instance, in Dore
Lake Complex (Allard, 1986), Stillwater
(Barker, 1975; Page, 1979; Raedeke & McCal-
lum, 1984), Duluth Complex (Tyson & Chang,
1978) and Platreef area, Bushveld (Buchanan
& Rouse, 1984). The occurrence of cumulus
magnetite indicates that the contaminant in-
creased the oxidation state of the local system.
This local system is spatially intimately as-
sociated with the quartz monzonite diapirs (see
Fig. 21). The quartz monzonite magma stocks
(diapirs) intruding the LZ cumulates are mani-
festations of large-scale early melting of the
floor rocks (labelled monzonite diapirs on the
geological map, Appendix 3; for quartz mon-
zonite compositions, see anal. 5–11, Table 3).
Their U-Pb zircon age, 2434 Ma (Olavi Kou-
vo, letter, May 19th, 1994), is the same as the
age of the intrusion. The quartz monzonite
contains finer-grained autoliths of about the
same composition. Dendritic growth of plagi-
oclase and olivine(?) suggests superheating
and subsequent supercooling of the quartz
monzonite magma. Against the overlying cu-
mulates the stocks have a blanket of chilled
rocks (internal chill) with abundant sulphides