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and Tonkin,
1994
), its high Königsberger ratio and aniso-
tropic magnetic properties (see
Sections 3.2.3.4
and
3.2.3.7
)
being re
ected in the magnetic characteristics of the rocks.
10
-2
Magnetite
zone
(Py + Mg)
Pyrrhotite
zone
(Po)
(Py + Po)
3.9.5.3
Contact metamorphism
Magnetic anomalies associated with thermal/contact
are the result of hot igneous
fluids penetrating the colder
country rock and, where the rocks are chemically reactive,
producing usually a zone of reduced magnetism. Examples
due to both magnetite and pyrrhotite have been reported
erals are formed, their remanent magnetism may be
oriented differently from that of the source intrusion
giving the metamorphosed zone a distinctive contrasting
magnetic signature. Some examples of the magnetic
responses of the rock surrounding intrusions are described
in
Section 3.11.3
.
Figure 3.50
shows the variations in magnetic properties
through a doleritic dyke and adjacent metabasalt country
rocks from the Abitibi area, Ontario, Canada. The baked-
contact zone has higher levels of remanent magnetism and
a higher Königsberger ratio than either the dyke or the
country rocks, but induced magnetism is highest in the
central part of the dyke. The majority of the remanent
magnetic vectors are sub-parallel to the present-day
field,
so the overall magnetism is higher, roughly equal to the
combined strengths of J
Induced
and J
Remanent
. Depending on
the survey height, the responses from the contact zones
could appear as a single anomaly, which would then prob-
ably be erroneously correlated with the dyke itself.
magnetic mineralogy in a contact aureole in eastern Cali-
fornia. They explain the creation and destruction of mag-
netite as a consequence of changing physical and chemical
conditions and their effects on iron
10
-3
10
-4
10
-5
Mg
Po
Py Po
Mg Py
10
-6
Increasing metamorphic grade
Figure 3.49
Variations in magnetic susceptibility with metamorphic
grade in a black shale sequence from the Swiss Alps. The black
circles represent the susceptibility due to ferromagnetic minerals;
the blue squares represent the contribution from diamagnetic and
paramagnetic minerals. Redrawn, with permission, from
Rochette (
1987
).
metamorphism does not diffuse away, unless accessed by
mobile reducing
fluids, so it buffers further reaction to
maintain haematite stability. Consequently, magnetite-
and haematite-rich bands are preserved. In general, iron
oxides in metasediments are predominantly magnetic, in
contrast to the predominance of the more oxidised hae-
matitic forms in unmetamorphosed sediments. This
reflects their approach toward chemical equilibrium.
Carbon is a reducing agent with its effect increasing with
temperature but limited by pressure, so graphitic schists
formed from carbonaceous shales are deficient in magnet-
ite. Several other factors can favour the formation of mag-
netite during metamorphism, i.e. low silicate content, low
titanium content, low pressure and dehydration both
affecting oxidation and reduction, excess aluminium
favouring the formation of muscovite mica and magnetite
rather than biotite, high temperatures in biotite rocks,
equilibrium to low temperatures in titaniferous rocks,
and absence of carbon. Pyrrhotite is the most common
magnetic mineral in reduced graphitic metasediments, e.g.
graphitic schists, especially above greenschist facies (Clark
titanium oxides and
iron magnesium silicates. A metasedimentary succession
(lower greenschist facies) has experienced prograde contact
metamorphism in the aureole of a large pluton, with
some areas subsequently experiencing retrograde meta-
morphism. Three zones are recognised in the aureole based
on silicate and magnetic mineralogy (
Fig. 3.51a
). Suscepti-
bility is low outside the aureole, but a zone characterised by
andalusite and cordierite, and a mineralogical transition
zone, have high magnetism. The zone closest to the intru-
sion, characterised by cordierite and K-feldspar, has low
susceptibility. The conversion of limonite, rutile +/- haem-
atite to magnetite and ilmenite
-
-
haematite causes the
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