140
Geological Survey of Finland, Bulletin 395
Tapani Mutanen
The basal microgabbro grades into a pyrox-
ene cumulate and pyroxene-olivine cumulate
rich in intercumulus plagioclase. This transi-
tional zone, with MgO 10.8 – 17.0% and Al
2
O
3
7.4 – 5.0%, is ca 40 m thick. Chlorapatite and
euhedral zircon (with 1920 ppm U in zircon
fraction, 2600–5300 ppm U in electron micro-
probe analysis of the skeletal zircon in Fig.
46f) occur in this rock. Sulphide-mantled
euhedral orthopyroxene (Fig. 77) is common
in the basal ultramafic zone.
The main part of the ultramafic zone con-
sists of olivine-clinopyroxene-orthopyroxene-
magnetite cumulates. Chromite occurs only in
clinopyroxene, as tiny euhedral to subhedral
inclusions.
The H
2
O-rich intercumulus liquid crystal-
lized largely to primary hornblende and bi-
otite. Orthocumulate textures are best dis-
played where the intercumulus consists of sul-
phides and plagioclase. Because of the original
“bastard” nature of the cumulates, and reflect-
ing it, H
2
O was inhomogeneously distributed
in the intercumulus liquid. Thus, the degree of
replacement of clinopyroxene by brown horn-
blende commonly varies in a single clinopy-
roxene grain, the part against plagioclase being
fresh. In general, in samples and in parts of
thin sections that are rich in intercumulus pla-
gioclase clinopyroxene crystals are fresh, not
spattered by inclusions of brown hornblende.
The amount of olivine generally varies in
the range 15 – 25 vol%. Unless encased in sul-
phides, orthopyroxene occurs as anhedral to
oikocrystic grains and contains resorbed oliv-
ine and clinopyroxene inclusions (Fig. 77).
This indicates a reaction relation of olivine
with orthopyroxene and calcic clinopyroxene.
The outlines of orthopyroxene oikocrysts are
sometimes euhedral, suggesting settling of
composite olivine(-clinopyroxene)-orthopy-
roxene cumulus grains (e.g., Jackson, 1961).
Stress-lamellae sometimes occur in olivine.
On the other hand, as in the peridotitic cu-
mulates of Koitelainen intrusion, clinopyrox-
ene commonly occurs as roundish inclusions in
olivine, suggesting that the magma began crys-
tallizing in the clinopyroxene field and turned,
possibly due to added H
2
O, into the olivine
field. On the other hand, clinopyroxene con-
tains olivine inclusions. All this can be trans-
lated into a cumulus path cpx -> ol -> kpx ->
(peritectic reaction of ol, and apparent reaction
of kpx) -> opx. Euhedral plagioclase inclu-
sions are found in orthopyroxene oikocrysts,
suggesting that it was an early cumulus phase.
Of course, before the intervention of contami-
nation, plagioclase and pyroxenes crystallized
together from the original basaltic parent mag-
ma (Table 6).
Along the deepest DDH slight downward
trends can be discerned: the proportion of
clinopyroxene and its replacement by primary
hornblende increase and the amounts of or-
thopyroxene and primary OH minerals de-
crease. Nevertheless, primary biotite-
phlogopite is common in the Main Ore at the
bottom of the westernmost DDH (Appendix 5).
The clinopyroxene/orthopyroxene ratio var-
ies in ultramafic cumulates; false ores are nor-
mally richer in orthopyroxene, whereas in the
Ni-PGE ore type the orthopyroxene content is
very low. Chlorapatite often occurs as big
grains, not unlike cumulus crystals.
Cumulus magnetite occurs as solitary euhe-
dral crystals. The original magnetite was a
mixed Fe-Al-Cr-Ti-Mg spinel, which exsolved
into a complex fractal lattice-lace symplectite
of spinel, ilmenite and magnetite. In spot anal-
yses the Cr
2
O
3
content varies between 2.45 and
21%, and that of Al
2
O
3
between 0.75 and
8.35%. The exsolved “pure” spinel contains
(wt%): 15.98 Al
2
O
3
, 6.70 MgO, 34.47 Cr
2
O
3
,
35.01 FeO(tot), 3.21 TiO
2
, 0.49 MnO, 0.64
ZnO, 0.25 V
2
O
3
. The rocks that contain more
primary OH minerals (hornblende, biotite-
phlogopite) also contain more primary magnet-
ite, while those rich in intercumulus plagiocla-
Petrography of ultramafic cumulates