Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 147

strength of remanent magnetism with possibly a slight
increase in induced magnetism. When greenschist facies
is attained, both forms of magnetism are reduced com-
pared with the protolith, especially the remanent magnet-
ism. The decrease in strength of remanent magnetism in
basalts during low-grade metamorphism is probably due to
oxidation of iron-rich titanomagnetites. Once greenschist
facies are reached, induced magnetism has also been sig-
nificantly reduced.
In gabbro the fine magnetic grains within silicates may
be protected by their silicate hosts from low- to medium-
grade metamorphism, so gabbro can retain its magnetic
properties. Magnetite in felsic plutons appears to be more
resistant to metamorphic destruction than felsic and mafic
volcanic rocks at greenschist and also amphibolite grade.
A 3 km thick sequence of basaltic

mangerites returned both susceptibility and remanent
magnetism to amphibolite-facies levels. Petrologically this
is due to magnetite, ilmenite and ilmenite haematite being

replaced by silicates, titanite and haematite. A further
factor at granulite facies grade is that temperatures are
sufficient for stability of the complete titanomagnetite
solid-solution series. Exsolved titanomagnetites can there-
fore recombine, which may result in a loss of magnetism
due to the creation of less iron-rich members.
High-pressure granulites and eclogites tend to be
paramagnetic, with magnetite breaking down at 1000

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2000 MPa (Clark, 1997 ) , and the iron being taken up by
garnet and clinopyroxene. Decompression of high-
pressure granulite during rapid uplift can produce

ne-

grained magnetite by breakdown of garnet and

flows exposed in
eastern Iceland provides an opportunity to monitor mag-
netic properties as a function of hydrothermal alteration in
the zeolite and prehnite

clinopyroxene.

3.9.5.2 Metamorphism of sedimentary rocks
The magnetic properties of metamorphosed sediments
depend largely on the nature of the original sediment. An
iron-poor protolith, such as a mature sandstone or pure
carbonate, cannot produce signi cant quantities of mag-
netic minerals. This means that only metamorphosed argil-
laceous rocks are likely to be magnetic. When iron is
present, the oxidation state is important, in particular the
ratio of ferric to ferrous iron. Low ratios favour the forma-
tion of iron silicates, intermediate values favour magnetite,
and high ratios favour haematite and ilmenite. Reduced
sediments devoid of Fe 3+ will generally produce a non-
magnetic metasediment. This may result in variations in
magnetic properties reflecting original facies variations.
The magnetic susceptibility of the shales is due to iron-
bearing silicates, magnetite and pyrrhotite. Rochette ( 1987 )
reports on data from the Swiss Alps ( Fig. 3.49 ) . The overall
susceptibility of the rocks depends on all the minerals
making up the rocks, but only the ferromagnetic mineral-
ogy is affected by changes in metamorphic grade. As meta-
morphic grade increases, initially susceptibility decreases
owing to destruction of detrital magnetite to create mainly
pyrite, then it sharply increases as pyrite is converted into
pyrrhotite. These reactions are caused by increasingly
reducing conditions resulting from the maturation of
organic matter within the shales.
Sediments in which most of the iron is in iron oxides,
such as banded iron formations and haematite-bearing
sandstones, retain their unaltered assemblages up to the
highest metamorphic grades. This occurs because oxygen
released from the

pumpellyite facies ( Fig. 3.48b ) .
There is a signi cant scatter in the data but it is clear that
an increase in susceptibility occurs into about the middle of
the zeolite facies. As metamorphic grade increases, the
strengths of both the induced and remanent magnetisms
decrease

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pumpellyite conditions are reached. From the figure inset,
a decrease in the strength of remanent magnetism with
increasing metamorphic grade is clear.
As demonstrated by Bourne et al.( 1993 ) and Hageskov
( 1984 ), the transition from greenschist to amphibolite
facies has little influence on magnetism, other than slightly
reducing already low levels of magnetism. In general,
amphibolite tends to be weakly magnetic if it contains
chlorite and/or biotite, but hornblende-bearing varieties
are much more magnetic.
The effects of the amphibolite

signi cantly, especially when prehnite

granulite transition are
described by Olesen et al.( 1991 ) and Schlinger ( 1985 )
using data from an acid to intermediate gneissic terrain
in northwestern Norway, and shown in Fig. 3.48c . Much of
the scatter is due to varying lithotypes, although there is no
compositional difference between the lower- and higher-
grade equivalents. The transition from amphibolite facies
to granulite facies is associated with substantial increase in
the strengths of both the induced and remanent magnet-
isms, owing primarily to the growth of metamorphic mag-
netite. The metamorphism is interpreted as breaking down
hydrous iron-bearing silicates (biotite and amphibole)
with the iron taken up by iron

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titanium oxides. In this
area, retrogressive metamorphism of the granulite-facies

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oxidation of magnetite during