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Geological Survey of Finland, Bulletin 395
Tapani Mutanen
The first high-aluminous schists in the roof
to melt were boron-rich layers (see Mutanen,
1989b, p. 44). The addition of boron lowers
the eutectic of the water-saturated granite
composition by 125°C (Chorlton & Martin,
1978). Thus, depending on the vertical distri-
bution of the lowest-melting point sediment
compositions, incipient roof melting may have
occurred simultaneously at many levels and
the melt fraction at those levels may have been
very large (Holtz & Johannes, 1991). Even be-
fore the experimental study of Chorlton and
Martin (1978) the fluxing action of boron was
noted in the field (e.g., Ashworth, 1976).
By its very nature, boron is concentrated in
residual liquid; indeed, high boron concentra-
tions have been noted in the granophyre of the
Dillsburg sill (Hotz, 1953). The occurrence
and effects of boron in mafic magmas deserve
more studies (I have not found any). High bo-
ron concentrations (up to 1000 ppm) have been
reported from the Varena Ti-Fe deposit (Kazi-
mieras, 1996). Tourmaline, apparently as a
primary constituent, abounds in many parts of
the Koivusaarenneva Ti-Fe-V deposit, western
Finland (my own observations, 1972).
I must remind my readers here that the melt-
ing in the roof advanced upwards, after the
complete solidification of the mafic (but still
hot) intrusion (Kadik, 1970). The melted, bo-
ron-rich layers high above the granophyre and
also the Cr-enriched upper margin of the grano-
phyre of the Koitelainen intrusion actually
represent this belated melting.
Sandstones and shales generally contain ex-
cess silica and alumina in relation to a eutectic
granite melt. The discussion that follows deals
mainly with pelitic rocks, these being the most
common metasediments associated with lay-
ered intrusions.
The refractory residue material after frac-
tional melting is impoverished in silica and en-
riched in Al, Mg and Fe (Bowen, 1928; Wyllie
& Tuttle, 1961; Smith, 1969); also, mg#, Cr,
Ni, Co and V increase in the residue (Vielzeuf
& Holloway, 1988; Evans, 1964). Thus the
contamination of magma by residue phases
may give rise to reversals that mimic the effect
of new, primitive magma pulses. Moreover,
the ferric-ferrous ratio increases in the residue
(Vielzeuf & Holloway, 1988), which, when
brought to mafic magma, would promote the
crystallization of chromite and magnetite. The
effect of melting and contamination by pelitic
material adds Si, Al, Cr and increases the fer-
ric-ferrous ratio of the magma, all of which are
good for the crystallization of chromite (Ir-
vine, 1977b).
The refractory phases were dense minerals,
and eventually they got through the grano-
phyre melt to the mafic main magma. Even
plagioclase settled through granophyre magma
(Roobol, 1974). Participation of refractory
phases in the magma contamination is inevita-
ble, and although solitary grains are doomed,
by reaction, to annihilation, the results are ob-
vious (Bowen, 1928). Pelites, containing typi-
cal refractory minerals of the fractional melt-
ing (sillimanite, mullite, cordierite, spinel, co-
rundum, staurolite), have been found pre-
served as big xenoliths in most layered intru-
sions (e.g., Hall & Nel, 1926; van Zyl, 1950;
Worst, 1960; Cameron & Desborough, 1969;
Willemse & Viljoen, 1970; Kozlov, 1973; Mo-
lyneux, 1974; Hor et al., 1975; Myers, 1975
Page, 1977, 1979; Buchanan & Rouse, 1984;
see also Sharkov & Sidorenko, 1980, Fig. 40).
In the Lukkulaisvaara intrusion rocks rich in
staurolite and corundum are associated with
crustal metals (Pb, Mo, Re, Ag, Sn) and PGE
(see Barkov et al., 1996a); these I interpret as
representing an aluminous refractory contami-
nant. Similarly, the peculiar Fe-rich, alumi-
nous layer at an important reversal boundary in
the Penikat intrusion (see Halkoaho et al.,
1990) I interpret as representing a cumulate of
undigested detritus of xenolithic pelitic materi-
al from the original roof. This conjecture is
supported by the coincident P-Zr anomaly (see
Halkoaho, 1994).
Although the occurrence of aluminous re-
fractory xenolith material is sometimes accred-