Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 146

a) b)

Remanent magnetism (A/m)

10 2 10 2

1 .0 10.0

0.0

No zeolites

0.5

Chabazite
Analcite

Increasing
metamorphic
grade

10 1 10 1

1.0

Mesolite

1.5

10 0 10 0

Laumontite

2.0

10 -1 10 -1

Increasing
metamorphic
grade

Increasing
metamorphic
grade

10 -2 10 -2

Sea-floor basalts (Atlantic) Basalts (Iceland)

10 -3 10 -3

10 -3 10 -2 10 -1 10 0 10 1 10 -3 10 -2 10 -1 10 0 10 1

Induced magnetism (A/m) Induced magnetism (A/m)

Slightly
altered
Strongly altered
(zeolite facies)

Moderately altered
(zeolite facies)

Zeolite facies
(lower)

Zeolite facies
(medium)

Fresh

Greenschist
facies

Zeolite facies
(laumontite zone)

Prehnite-pumpellyite
facies

c) d)

10 0

Fresh

Remanent magnetism
dominant
( Q > 1)

Granulite
facies

10 -1

Zeolite
facies

Increasing
metamorphic
grade

Prehnite-pumpellyite
facies

Amphibolite
facies

10 -2

Eclogite
facies

High-grade terrain
(Norway)

Induced magnetism
dominant
( Q < 1)

Greenschist
facies

10 -3

Induced magnetism

10 -3 10 -2 10 -1 10 0 10 1

Induced magnetism (A/m)

Amphibolite
facies

Transition
zone

Granulite
facies

Figure 3.48 Examples of the effects of metamorphism on rock magnetism. (a) Variations in the strength of magnetism for sea-floor
basalts from the Atlantic Ocean showing the changes in magnetism as metamorphic grade increases to greenschist facies. Based on data in
Fox and Opdyke ( 1973 ). (b) Variation in the strength of magnetism as a function of grade in the zeolite to prehnite - pumpellyite facies
from a 3 km thick pile of basaltic lavas in eastern Iceland. Based on data in Bleil et al.( 1982 ) , Mehegan and Robinson ( 1982 ), and
Robinson et al.( 1982 ). Inset: Variation in the strength of remanent magnetism versus depth/metamorphic grade. Metamorphic grade
varies from zero through various zones of the zeolite facies. The vertical errors bars reflect uncertainty in estimated depth. Based on diagrams
in Wood and Gibson ( 1976 ). (c) Changes in magnetism of acid to intermediate gneisses associated with the transition from amphibolite
to granulite facies. Based on data in Olesen et al.( 1991 ). (d) Schematic illustration of the effects of metamorphism on the magnetim of
ma c rocks.