30
Geological Survey of Finland, Bulletin 395
Tapani Mutanen
with the UC sequence, which will be described
later in connection with the UC layer (see Fig.
10).
Leucocratic layers become thinner and more
sparse upwards from 10 – 15 m above the UC,
vanishing altogether about 40 m above it. The
interval between the UC succession and mag-
netite gabbro consists mainly of cotectic py-
roxene-plagioclase cumulates that are macro-
scopically monotonous except for size layer-
ing. In this innocuous gabbro a 14-m-thick in-
terval enriched in PGE was revealed by sys-
tematic Pd assays. The overlying cumulates up
to the magnetite gabbro are variably anoma-
lous in PGE.
Near the top of the UZ is a 100-m-thick se-
quence of plagioclase-rich cumulates (mott-
led anorthosites, gabbro-anorthosites and
anorthosite gabbro), with gabbro interlayers.
Ilmenite may have crystallized as a cumulus
mineral, possibly due to silicic-aluminous
contamination. The sequence contains a 40-m-
thick bipartite PGE-enriched interval.
The sudden arrival of cumulus titanomag-
netite marks the sharp basal contact of the
magnetite gabbro. At this phase contact there
are marked anomalies of P
2
O
5
(up to 0.3%), Zr
(210 ppm), Cr (376 ppm), La (45 ppm) and Ce
(76 ppm), which suggest that the arrival of
magnetite was promoted by convective over-
turn and associated contamination of the mag-
ma. The high Cr reflects the high D
Cr
sp/liq
, but
the source of Cr may be Cr-enriched refractory
residue matter of melted crustal rocks. Geo-
chemical anomalies of Zr, P and REE seem to
be characteristic of reversals analysed in detail
(see Lambert & Simmons, 1984; Halkoaho,
1994) and I take the liberty to interpret them
with the CC model.
The magnetite-gabbro unit consists of two
magnetite-rich layers (with V > 0.1%) separat-
ed by a layer poor in magnetite. The lowermost
6 m consists of plagioclase-magnetite cumu-
late with a V content of 0.4%. Magnetite has
partly altered into silicates (hornblende, bi-
otite) in metamorphic reactions.
Titanomagnetite was evidently the sole cu-
mulus Fe-Ti oxide in magnetite gabbro. With
decreasing temperature ilmenite exsolved from
titanomagnetite first by granule oxidation-ex-
solution, later by lamellar exsolution (il-
menomagnetite). Titanomagnetite crystallized
as a cumulus phase up to the end with plagi-
oclase and pyroxenes. These were probably
later joined by fayalite. Apatite joins the cu-
mulus assemblage at about 150–200 m below
the granophyre.
Thin (1–4 cm) anorthosite layers occur in
the lower part of the unit. In the magnetite-
poor upper part there are seven layers of plagi-
oclase-rich cumulates (mottled anorthosite,
anorthosite gabbro), from 1 m to 7.5 m thick,
and more than ten ultramafic layers (now horn-
blendite) 0.2 – 1.7 m thick. In these rocks there
is a positive correlation between plagioclase
content and P-Zr-Ce-La.
The uppermost mafic cumulates are ferro-
gabbros (SiO
2
< 55%, FeO(tot) 17–25%, CaO/
Na
2
O < 2), apatite-ferrogabbros (apatite 2–5%)
and apatite-ferrodiorite.
Granophyre is the uppermost magmatic
unit of the intrusion. The basal 20 m is ferro-
granophyre (FeO 11 – 12.5%, SiO
2
60 – 65%),
which grades into underlying ferrodiorite and
overlying acid granophyre (SiO
2
70 – 75%).
The total thickness of the granophyre is > 260
m. Plagioclase was the first major silicate to
crystallize from the melt. The contact relations
with the older acid volcanites of the roof are
not known.
Microgabbros are encountered from the
base of the intrusion up to the UZ. They in-
clude rocks of the lower chilled margin, layers
(?) and autoliths in the basal LZ, and discon-
tinuous layers and autoliths in the upper MZ
and in the UC succession. Microgabbros in the
cumulate succession are almost always associ-
ated with anorthositic cumulates. In the UC se-
quence microgabbros occur as irregular layers
below, in and immediately above the UC layer,
and as autoliths above the UC (see Appendix
2). There are two main types, a fine-grained