JOURNAL
OF GEOPHYSICAL
RESEARCH, VOL. 102, NO. B12, PAGES 27,271-27,283, DECEMBER
10, 1997
Paleomagnetismand paleothermometryof the SydneyBasin
1. Thermoviscousand chemical overprinting of the Milton Monzonite
D.J.Dunlop,
• P.W.Schmidt,
• 0.0zdemir,
• andD.A.Clark:
Abstract. The Early Triassic(N245 Ma) Milton Monzoniteof the SydneyBasin,Australia,
hasfour distinctcomponents
of naturalremanentmagnetization
(NRM) with only slightly
overlappingrangesof unblockingtemperatures.The low-temperature
(LT) component,
the
first to be thermallydemagnetized,
is thoughtto be a Late Cretaceous(= 100 Ma)
thermoviscous
overprintacquiredin slowcoolingduringuplift. The high-temperature
(HT)
component,
the secondto be demagnetized,
is probablythe primarythermoremanent
magnetization(TRM) of the Milton intrusionbut couldpossiblybe a Jurassicoverprint.LT
andHT are usuallycarriedby magnetiteandoccasionallyby pyrrhotite. Samplesfrom nine
siteshave a furtherNRM componentwhich unblocksat highertemperatures
thanHT but
below the magnetiteCurietemperatureof 580øC. This componentis arguedto be a chemical
remanentmagnetization
(CRM) becauseof its discreterangeof highunblocking
temperatures,
abovethoseof the thermalcomponents
HT andLT, andis calledCRM1.
CRM 1 hasalmostthe samedirectionasLT andis likely carriedby authigenicmagnetite
producedduringuplift N100 Ma. Samplesfromfive siteshavea fourthNRM component,
with a directionresemblingthatof HT but carriedby hematite.This fourthcomponentcould
be a primaryTRM but is morelikely a CRM andis thereforecalledCRM2. The HT-CRM2
meandirectionis D= 50 ø, I= 75.5ø, defininga paleopoleat 16øS, 172øE. The HT-CRM2
paleopolefallsnear150 Ma on theAustralianapparentpolarwanderpathbut is a
considerable
distancefrom paleopolesof PermianandEarly Triassicage. Thereis no known
tectonicor otherremagnetizingeventin the SydneyBasinaround150 Ma. For thisreason,
we proposethattheHT-CRM2 paleopoledefinesa newTriassicsegmentof theAustralian
polarwanderpath. The LT-CRM 1 meandirectionis D= 348o,I= -79 ø, with a paleopole
falling at 56øS, 158øE,near 100 Ma on thepolarwanderpath. This ageis consistent
with
uplift andcoolingrelatedto initial rifting of the TasmanSea.
1. Introduction
The depositionaland tectonichistoryof the SydneyBasin
is relatively simple [Conoily and Ferm, 1971; Mayne et al.,
The Milton Monzoniteof the SydneyBasinin southeastern
1974;Herbert, 1980]. The basinbeganto subsidein the Early
Australia has a whole rock K/Ar date of •245 Ma (J. R.
Permian, resulting in deposition of several kilometers of
Richards,as cited by Joplin [1968]; Facer and Carr [1979]).
Deposition
in theLatePermianandEarly
Theintrusion
cropsoutovera 50 km2areaaroundthetownof marinesediments.
Triassicwas in marginalto fluvial environments,with numerMilton in coastalNew SouthWales,• 175km SSWof Sydney
(Figure 1). It intrudesflat-lyingmid-PermianUpperMarine
Seriessedimentaryrockswhich havenot beensignificantly
tilted duringsubsidence
andsubsequent
uplift of the Sydney
Basin [Packham,1969; Mayne et al., 1974; Branaganet al.,
1976]. The Milton Monzonitehas beenpreviouslystudied
paleomagneficallyby Schmidtand Embleton [1981]. Their
data were usedby Middleton and Schmidt[1982] to deduce
paleotemperatures
reachedduring basin subsidence,but the
paleomagneticallyinferred temperatureswere anomalously
high comparedto thoseestimatedfrom measuresof organic
diagenesis.
Thisdiscrepancy
wastheincentivefor ourrestudy
of the Milton
Monzonite.
•Geophysics
Laboratory,
Department
of Physics,
Universityof
Toronto, Toronto, Canada.
2Division
of Exploration
andMining,CSIRO,NorthRyde,New
South Wales,Australia.
Copyright1997 by theAmericanGeophysical
Union.
Papernumber97JB02479.
0148-0227/97/97JB-02479509.00
ous coal swamps. The Greta Coal Measuresdate from this
time. Althoughminor depositioncontinuedinto the Jurassic
andtherewassporadicvolcanismandintrusiveactivityin the
Triassic and Jurassic[Wellmanand McDougall, 1974], the
only major tectonic event affecting the entire basin was
doming and initial rifting of the Tasman Sea, which caused
rapid uplift anderosion100-70 Myr ago [Falvey, 1974].
Rockspresentlyexposedalongthe New SouthWales coast,
the rift margin,are inferredto havereachedtemperatures
of
70-200øC, implying burial to 1-3 km depths if a
paleogeothermal
gradientof 70øC/km is assumed[Middleton
and Schmidt,1982]. The temperatureestimatesare basedon
severalindependentlines of evidence:coal rank and vitrinite
reflectance[FalveyandMiddleton,1981]; regionallaumontite
to prehnitemetamorphism[Raam, 1968]; resetfissiontrack
agesin apatites[Moore et al., 1986]; and remagnetization
temperaturesof natural remanent magnetization (NRM)
[Schmidtand Embleton, 1981].
The remagnetization
temperature
T,.in natureis determined
bycorrecting
themaximumunblocking
temperature
Tu•of the
burialoverprintNRM, measured
in relativelyrapidlaboratory
heating,for the effect of much more prolongedheatingin
nature.Pullaiahet al.'s [1975] time-temperature
relationsfor
27,271
27,272
DUNLOP ET AL.' PALEOMAGNETISM OF THE MILTON MONZONITE
150030' E
ß
ß'....i.•
ß
ß
ß
ß
ß
ß
ß
'/• • Permian-Triassi
7
•-•lntrusions
I: • ZlPermian
I•• •]Sediments
Ulladulla
• bS;Veøn•;•
'Silurian
ß
- 90000 mN
35ø30'S- •
60000mE
AustralianMap Grid Zone56
70000mE
Figure 1. Sampling
sitesin theEarlyTriassicMiltonMonzonite,
nearthetownof Milton,New SouthWales,in the
southernSydneyBasin,andgeologyof the surrounding
region.
making this correction,based on Ndel's [1949] theory of
thermoviscousmagnetizationin single-domaingrains,result
in valuesof Tr that are muchhigherthan thoseindicatedby
vitrinite reflectance, metamorphic grade, or fission track
annealing. Middleton and Schmidt [1982] therefore constructedalternativetime-temperature
contoursusingWalton's
[1980] relations,which are alsobasedon N6el single-domain
theory but which use a differentcriterionfor magnetization
unblocking[seeEnkin and Dunlop, 1988]. The MiddletonSchmidt-Waltoncontoursgavemoreacceptableestimatesof
Tr of N250øC, but thesewere still significantlyhigherthan
averagetemperaturesindicatedby vitrinite reflectance.
Middleton and Schmidt [1982] suggestedtwo possible
explanationsfor the discrepancy. (1) Coalificationresults
from temperaturesmaintainedfor tens of millions of years,
whereasremagnetizationmight haveresultedfrom a shorter
pulseof highertemperature,
or (2) multidomaingrains,which
were detectedin the Milton Monzonite, might havedifferent
time-temperaturerelationsthan single-domaingrains. It is
importantto settlethis questionbecausethe remagnetization
temperatureapproachto basinanalysisis quick, inexpensive,
andmuchmorewidelyapplicablethanvitrinitereflectanceor
other measuresof organicdiagenesis. Other, independent
studieson sandstonesand limestonesin the Hercynianand
Appalachianorogensof EuropeandNorth America[Chamalaun,1964;Kent,1985]havealsoyieldedunblocking
temperaturesthatare "anomalously
high"(i.e.,implyingunrealistically
highregionalT•values).Thustheremagnetization
problemis
not limited to a particularlithology,depositional-erosional
history, or tectonicsetting,and its solutionis likely to lie in
somegeneralmagneticphenomenon.
With this backgroundin mind, we restudiedthe Milton
Monzonite with a greaterareal coverageof sites and more
comprehensivethermal treatments than in Schmidt and
Embleton's[1981] study. These treatmentsgave detailed
information about the high-temperaturethermoremanent
magnetization
(TRM) andthe low-temperature
thermoviscous
magnetizationthat overprintsit. Thermal demagnetization
DUNLOP ET AL.: PALEOMAGNETISM OF THE MILTON MONZONITE
also revealed two additional NRM componentswith still
higher unblockingtemperatures,whose directions"echo"
those of the lower unblockingtemperaturecomponents.In
section4 we will arguethat thesetwo latter componentsare
probablychemicalremanentmagnetizations
(CRMs). Chemical overprintingsuggestsa third possibleexplanationof
anomalously
highunblockingtemperatures:
the low-temperature NRM componentmay not be a purely thermoviscous
overprint. However, Dunlop et al. [this issue](hereinafter
referred to as paper 2) show that multidomaingrains can
explaintheelevatedunblocking
temperatures
thatwe observe.
Chemical overprintingis not required,in this formationat
least.
27,273
The ratio betweenthe NRM survivingafter low-temperature
demagnetization
to NRM beforelow-temperature
demagnetization is called the memoryratio R.
Low-field susceptibility)• was measuredcontinuouslyin
air, during heating and cooling, betweenliquid nitrogen
temperature(- 196øC)and •620øC for 23 samples,usingan
automaticrecordingbridge. Room temperaturesaturation
hysteresis
wasmeasured
for 35 samplesfrom sites1-10 using
a vibrating-samplemagnetometer.
3. Hysteresis and Thermomagnetic Data
At the outset, we should rememberthat hysteresisand
thermomagnetic
datacharacterizethe bulk magneticmineralogyof a rock,but theymaybe unrepresentative
of thephases
2. Sampling and Measurement Techniques
thatcarryNRM. Strong-field-induced
magnetization(hysterSchmidtand Embleton[ 1981] sampledtheMilton Monzon- esis)tendsto be dominatedby large,multidomaingrainsof
ite at six sites in two locations. Our 11 sites were distributed
magnetite or titanomagnetite. Single-domainor smaller
more broadly, with a view to examiningvariationsin the pseudo-single-domain
grains of these mineralsoften carry
compositionanddomainstateof magneticmineralswithin the most of the remanencebut contribute relatively little to
intrusion (Figure 1). Six to eight 2.5-cm-diametercores, hysteresis.Mineralslike pyrrhotiteandhematiteare difficult
orientedwith both sunandmagneticcompasses,
were drilled to resolve in hysteresis and also in weak-field-induced
at eachsite,andtwo or more2.2-cm-longspecimenswere cut magnetization(susceptibility)datafor two reasons.They are
from eachcore in the laboratory.
less magnetic than magnetite, hematite particularly so.
Schmidt and Embleton's [1981] work had shown that the Furthermore,they have much larger critical single-domain
low-temperatureand high-temperaturecomponentsof NRM
sizesthan magnetite,and thereforetend to contributelessto
were cleanly separatedby thermaldemagnetizationbut had induced magnetizationthan to remanence,compared to
overlappingcoercivityspectrain alternatingfield demagneti- magnetiteof the samegrainsize.
zation. We thereforeconcentrated
on thermaldemagnetizaSite-meanvaluesof reducedsaturation
remanence
MrJM•,
tion, which has the addedadvantageof giving more direct coerciveforce goHc,and Curie temperatureTo are given in
informationaboutremagnetizationtemperatures.Typically, Table 1. The principalmagneticmineralin all samplesis
we demagnetizedthree specimensof each samplein 23-29 magnetite(Fe304),with Tc rangingfrom 545 to 580øC. At site
stepsto the Curie point of magnetite(580øC), or beyondif
8, samples8c and8d hadmagnetiteCuriepoints(560-580øC)
significantNRM remainedundemagnetized,
aswasthe caseat but 8a, 8f, and8g had Tc = 600øC,probablyindicatingcation
sites 7 and 8. In all, almost200 individual specimenswere deficientmagnetite,
slightlyoxidizedto maghemite
(yFe203).
stepwisedemagnetized.
TheCuriepointof puremaghemite
is •645øC[Ozdemir
and
Because measurementswith the SQUID magnetometer Banerjee,
1984;DunlopandOzdemir,
1997,pp.58-60].We
were quick,it waspossibleto operatethreelarge demagnetiz- will showlaterthathematite(aFe203)is an importantNRM
ing furnaces in tandem, each containing N20 specimens. carrier, especiallyat sites6, 7, and 8, but it is not well exWhile one batchof specimenswasbeingmeasured,a second pressedin )•-T curves.
batch was beingheatedto a programmedtemperature,and a
Thermomagnetic
curvesfrom somesites/samples(2, 3, 4,
third batchwas cooling,all within remotelycontrolledfield- 6, 8c, and 8d) are reversibleor nearlyreversible(Figures2a,
free spaces. The time required for specimensto reach a 2b, and2c), while others(1, 5, 7, 8a, 8f, 8g, 9, 10, and 11) are
uniformtemperaturethroughouttheirvolumeswasdetermined more complex and irreversible (Figures 2d, 2e, and 2f).
by comparingsurfaceandinteriorthermocouple
readingson Samples8c and8d, with Tc _<580øC,havesimple)•-T curves,
dummysamples.This equilibration
timewas5-10 min at high whereas samples8a, 8f, and 8g, with Tc >_580øC, have
temperature. Temperatureswere quite uniform within the
furnaces,but specimenswere neverthelessalwaysreturnedto
the samepositionsfor successive
heatingssothatthetempera- Table 1. HysteresisandCurie TemperatureData
ture incrementswould be the samefor all specimens.
Site
Mrs/Ms
•toHc,mT
To øC
Even with all economiesof scaleand the speedpermitted
by the SQUID magnetometer,N2 monthsof measurements 1
0.094 +_0.006
7.2 _+0.5
555
were required. The labor investedwas repaidby unblocking 2
0.069 _+0.005
5.8 _+0.4
550
3
0.097 _+0.015
7.4 _+0.8
560
temperaturedataof unprecedented
quantityandquality,which
were indispensablein testing proposedtime-temperature 4
0.104 _ 0.004
9.6 _ 0.6
550 (545-555)
5
0.106
11.5
548 (545-550)
relations(seepaper2).
0.146 _+0.010
12.4 _+ 1.2
585
Before thermaltreatment,one specimenof mostsamples 6
0.178 + 0.006
15.9 +_1.1
574 (565-580)
was cycled in zero field throughthe magnetiteisotropic 7
0.158 _+0.028
13.4 _+2.6
588 (560-600)
temperatureTI around120 K (low-temperaturedemagnetiza- 8
0.107 _+0.013
9.8 + 0.8
553 (545-560)
tion [Ozima et al., 1964]). At Ti, magnetocrystalline
aniso- 109
0.047 _+0.004
3.7 _+0.3
570
tropy dropsto low values,with the result that domainwalls 11
570
expandandescapefrompinningsites.Single-domain
remanence, pinnedmainly by shapeanisotropy,is less affected.
Mrs/M, is saturation remanence normalized to saturation
Thus low-temperaturedemagnetizationpartially erases magnetization;
}.td4c
is bulkcoerciveforce;Tc is Curietemperature
multidomain
NRM, therebyhighlighting
single-domain
NRM. (rangesof valuesin parentheses).
27,274
DUNLOP ET AL.: PALEOMAGNETISM OF THE MILTON MONZONITE
18.0
ß
ß
ß
.
(a)
o
I
-2•
-1•
I
1•
0
I
300
200
400
I
500
-200
-100
0
100
82.0
-
I
Sample:
8ci
•
'
0
.... I.,...'•
.....
-100
0
I
200
300
:
.
500
600
i
-
{...
400
ßß(.d!
.!....... !.....
•
500
-200
-100
0
100
•
200
300
400
500
600
Temperature,
T (øC)
: ,•
:
I
.
I
Sample:10a
.
ß
ß
ß
ß
0l ....•..........•......... J..........•......
-200
- 100
0
100
200
300
700
•
Temperature,T (øC)
11.2
400
Sample:8a
4
100
300
ß
•
•...
-200
200
Temperature,
T ( øC)
Temperature,T ( "0
400
Temperature,T (øC)
500
600
(o
700
-200
- 100
0
100
200
300
400
500
Temperature,
T (øC)
Figure 2. Low-fieldsusceptibility
:• asa functionof temperature
T for selectedsamplesß
Initial heatingrunsstarted
fromliquidN2temperature
(- 196øC)andcontinuedto 600-650øC;thesamplewasthencooledto - 100øC;finally,
therewasa secondheatingfrom - 196øCto roomtemperature
or 100øC. All runswerein air. (a)-(c) Relatively
simpleand roughlyreversiblebehaviorin samplesfrom sites2, 6, and 8. (d)-(f) More complexand irreversible
behaviorin samplesfrom sites7, 8, and 10.
700
DUNLOP ET AL.' PALEOMAGNETISM
OF THE MILTON MONZONITE
27,275
in site7 samples
comesfromthermaldemagnetization
of their
complex,irreversible
curves.All sampleshavea pronounced
susceptibilitypeak centeredon the magnetite isotropic
temperature(TI • -150øC), particularlyin the initial heating
curve,anda similarpeak(theHopkinsonpeak)just belowthe
magnetiteCuriepoint,with little variationof susceptibility
at
intermediate temperatures. These are hallmarks of large
pseudo-single-domain
to multidomaingrainsof magnetite,in
the 20-100 gm size range approximately[e.g., Clark and
Schmidt, 1982]. Domain wall motion is severelylimited by
self-demagnetization
in theselarge grains,so that observed
susceptibilitycan only increasesignificantlywhenwalls are
unpinnednear TI or Tc. Single-domainsusceptibility
is not
limited by self-demagnetizationand changesover broader
temperatureranges(e.g.,coolingcurveof sample2a, Figure
NRMs, describedin section4.
Mrs/Msvaluesfor mostsitesarein therange0.1-0.15and
goHcvaluesrangefrom7 to 13 mT (Table1), consistent
with
moderate
to largepseudo-single-domain
grains.Valuesat site
10 arecharacteristicof ideal multidomainbehavior(Mrs/Ms <
0.05,goHc< 5 mT formagnetite
[seeDunlop,1969;Dayetal.,
1977;DunlopandOzdemir,1997,Chapter11]. Unusually
highvalues(M,dMs= 0.18,go• = 16mT) atsite7 mayreflect
thecontribution
of single-domain
pyrrhotite.A plotof Mrs/Ms
versusHc suggests
a classification
of sitesinto five groups
(Figure3). Site 10 hasthe lowest(mostmultidomain-like)
hysteresis
parameters,
withsite2 somewhat
higher.Sites1, 3,
4, and 9 form a largepseudo-single-domain
size grouping,
2a).
withhighervaluesanda poorerlinearcorrelation
betweenthe
Site5 valuesfall attheupperendof thisgrouping
Many of the initial heatingcurveshavean increasein slope parameters.
highin goHc
. Finally,sites6, 7, and8, on
around100øC, which is absentfrom the coolingcurve. This butareanomalously
"shoulder"isjust resolvablein 8c, morenoticeablein 6d, and the basisof bulk hysteresisalone, appearsimilar in their
size
stronglyexpressedin 8a and 10a. It might be due to the linearly correlated,moderatepseudo-single-domain
character
productionof magnetiteor maghemitefrom somelessmag- parameters.Of thesetrends,onlythemultidomain
of
netic or nonmagneticprecursorduringheating. Sample7d of site 10 samplesalsotumsout to be fully characteristic
also has a shoulderin its z-T curve around100øC, but it is the NRM carriers.
differentfrommostothersamplesin thatZ decreases
markedly
between300 and400øC (Figure2e). This broadpeakcentered
4. Thermal DemagnetizationData
on the pyrrhotiteCurie point, Tc= 320øC, may be a singleStepwise
thermaldemagnetization
reveals
thattheNRM of
domain Hopkinson peak. However, the thermomagnetic
evidenceis notcompelling.The main evidencefor pyrrhotite most Milton Monzonite samplescomprisesfour distinct
0.20 ß
ß
0.18
0.16
0.14
0.12
ßß
:/
ß Site5
'/
_
//Sites!3,4,9
0.08
0.06
0.04
0.02
0
2
4
6
8
10
12
14
16
l•oHc (mT)
Figure 3. A classification
of sitesby domainstateof thebulk magneticminerals,basedon measurements
of the
hysteresis
parameters
MrJMsandPorte.
Theseparameters
areapproximately
linearly
correlated,
andincreasing
values
of eitherparametercorrespond
to decreasing
grainsize. Only site 10 sampleshaveresultscharacteristic
of true
multidomaingrainsof magnetite(> 100 gm approximately).
27,276
DUNLOP ET AL.' PALEOMAGNETISM
components(Figures4 and 5). The first to be demagnetized
we will refer to asthe low-temperature(LT) component,and
the next highestin unblockingtemperatureTuB,we will call
thehigh-temperature
(HT) component.SchmidtandEmbleton
[1981] reported only these two NRM componentsfrom
thermal demagnetizationof their samples,which camefrom
the general locality of our sites 3 and 4 (Figure 1). They
interpretedHT to be the primarythermoremanent
magnetization (TRM) of the Milton Monzonite and LT to be a viscous
partial TRM acquiredduringuplift and cooling 100-70 Myr
ago, i.e., a Late Cretaceousthermoviscous
overprintof HT.
The LT andHT components
areusuallycarriedby magnetite
(Figure 4 andTable 2), or occasionallyby pyrrhotite(Figure
OF THE MILTON MONZONITE
nents. Their highunblockingtemperaturesare mostnaturally
explainedif they are chemicalremanentmagnetizations
(CRMs) or thermochemical
remanentmagnetizations.
We will
therefore refer to them as CRM1
and CRM2.
CRM1 is an upward(normalpolarity)vector,parallelto LT
(Figures4 and5) andpresumably
of similarLateCretaceous
age. CRM1 is usuallya smallcomponent
compared
to LT, but
occasionally,
it is of comparable
or evenhigherintensity(e.g.,
Figure 5). CRM1 is carriedmainly(sites7, 8) or entirelyby
magnetite(see maximumTuBvaluesin Table 2), but its
unblockingtemperaturesare higher than thoseof the HT
component,which it overprints. It thereforecannotbe a
thermoviscous
overprintandis probablya CRM.
5; see discussionbelow).
CRM2 is a smallcomponent,carriedmainly by hematite.
The two components
with the highestunblockingtempera- Its TuBvaluesaremostly>_
600øC,andat sites7 and8, where
tureshavedirectionssimilarto thoseof the lower-TuBcompo- CRM2 is mostprominent,theunblocking
temperatures
extend
W, Up
a
Sample:6fl
b
324
7OO
(mNm)
307
LT
265
224
551
184
CRM1
CRM2
•
LT
HT
CRM1
CRM2
•o•
CRM1
I
324
20 - 410 øC
410-512øC
512- 561øC
> 561 øC
N
CRM2
384
125
NRM
Figure 4. Thermal demagnetization
behaviorof a typicalsampleshowingfour components
of NRM: LT, HT,
CRM1, andCRM2. The components
arerevealed(a) by linearsegments
on orthogonal
vectorprojections
(triangles,
vertical plane;circles,horizontalplane)and (b) by endpointor subtracted
vectordirectionsin stereographic
projections(open,solidcircles,upper,lower hemisphere).The LT and HT directions(starsin Figure4b) are
subtracted
vectorswhosedirections
correspond
to thevectorsshownasshadedarrowsin Figure4a. The CRM 1 and
CRM2 directionsalsocorrespond
to shadedvectorsin Figure4a and are closeto the 512øC and 561øC points,
respectively,in Figure 4b. The four NRM components
have unblockingtemperature
(TUB)rangesthat are
nonoverlapping
or nearlyso,resultingin relativelysharpjunctionsbetweenvectorprojections
of thecomponents.
DUNLOP ET AL.: PALEOMAGNETISM OF THE MILTON MONZONITE
N,Up
323
--
a
27,277
Sample: 7d2
600
(mA/m)
300
334
ß
ß600
408
242
ß
NRM
ß
ß
440
168
CRM1
265
242
NRM
•o
CRM1
290
LT
3O0
440
CRM2
LT
2O - 222 øC
HT
222-
CRM1
323 - 620 oc
323 øC
CRM2
> 620 øC
HT
168
193
222
Figure5. Thermal
demagnetization
behavior
typical
offoursamples
fromsite7. TheLT andHT components
appear
tobecarried
bypyrrhotite
(Tc=320øC)rather
thanmagnetite
inthese
samples.
Symbols
andprojections
asinFigure
4.
upto =670øC(Table3 andFigure6). CRM2 is a downward that both CRM1 and CRM2 occur at sites 3 and 4, where
was
(reversepolarity)vector,approximately
parallelto HT. It Schmidtand Embleton[ 1981] sampled,buttheirpresence
by theverydetailedthermaldemagnetization
couldbepartof theprimaryTRM, thehematitecounterpart
to onlyrevealed
in the presentstudy. A similarsituationin the
HT, butwe considerit morelikelythatCRM2 is a Triassicage undertaken
CRM.
Mauch ChunkFormationof Pennsylvania
wasdescribedby
Four samplesfrom site7 behavein themannershownin Kentand Opdyke[1985].
It is alsointerestingthat chemicaloverprinting,
hitherto
Figure5. Four NRM components
are presentbut LT has
issowidespread
in theMiltonMonzonite.
There
maximumunblockingtemperatures
of ~220øC(Table2) and unsuspected,
withNorthAmerican
rocksof theAppalaHT hasa maximumTtmof ~310øC(Table3). Our interpreta- areagainparallels
[McCabeand Elmore,
tion is that HT is a Triassicage NRM carriedby pyrrhotite chiansand adjacentmidcontinent
(Tc= 320øC),and LT is a Late Cretaceous
thermoviscous1989]. Abouthalf the Milton sitesbeara Triassichematite
overprint,
alsoin pyrrhotite.Theusualmagnetite
HT compo- NRM, probablya CRM, whilealmostall siteshavea magnethesameageasLT, theupliftviscous
nentis completely
absentin thesesamples.However,magne- titeCRM of essentially
largerthanLT
tite with the unblockingtemperature
rangetypicalof the HT partialTRM. At site7, CRM1is sometimes
arises:
Is LT reallya thermoviscous
component
in othersamples
carriesCRM1, echoingtheLT (Figure5). Thequestion
direction.
overprint
of HT or is it tooa CRM, in wholeorin part?
Figure6 showstotalintensity
decaycurvesfor theseparate
CRM1 is foundat all sitesexceptsites10 and11, although
constructed
fromthethermaldemagnetization
onlyat sites6, 7, and8 wasit largeenoughto permitreliable components,
haveremarkably
estimatesof its mean direction(Table 2). CRM2 is found at dataof Figures4 and5. All components
unliketheusualthermally
discrete
curves
sites3, 4, 6, 7, and8 andis largeenoughat the lattertwo sites lineardecaycurves,
justbelowtheCuriepointof
to givea reliablemeandirection(Table3). It is interesting in whichdecayis concentrated
27,278
DUNLOP ET AL.: PALEOMAGNETISM
Table 2. LT and CRM1 PaleomagneticData After Thermal
Demagnetization
Site Location N
N'
D,
I,
deg deg
k
{x95, (TUB)
....
deg
øC
n
LT ComponentData a
1
2
3
4
5
6
7
8
9
(10
11
603/914
596/915
663/846
656/866
634/854
646/853
651/851
652/851
644/875
6
6
7
8
6
6
7
7
6
6
2
7
8
5
6
7
5
4
620/883
6
6
617/884
6
4
LT mean
10
1
238
14
12
324
9
3
10
346
22
241
-58
-78
-78
-79
-79
-75
-70
-64
-78
89
684
258
91
104
203
86
49
75
-15
122
-75
22
20
35
8
350.5 -77.5
7
4
6
8
5
7
11
11
401_35
333_ 15
395_+18
428+22
440+15
394+27
220_+12
267+61
427+
6
6 516_+14
538_+
8
11
12
14
15
16
12
15
14
6
12)
8
CRM1Component
Datab
1
2
3
4
5
6
6
7
7
8
7
9
CRM1 mean
Combined mean c
6
7
5
3
13
25
304
298
-79
-81
-83
334.6 -83.3
348.3 -79.0
97
101
143
121
42
551 - 570
555- 577
568 _+ 6
571_+ 5
-575
7 570_+ 5
6 604_+16
6 600_+ 6
570
11
6.5
9
7
14
16
2
18
16
17
3
Sitelocationsaregridreferences
for AustralianMap Grid Zone
56 (seeFigure1). For example,603/914 (site1) is an abbrevia-
OF THE MILTON MONZONITE
two, occupiedby HT unblockingtemperatures.If LT were
even in part a CRM similar in origin to CRM1, it would
inevitablyhaveattackedandoverprinted
HT in thisintermediateTtminterval. The sharpseparationof LT and HT unblockingtemperatures
(discussed
in detailin paper2) is compelling
evidence
thatLT is a thermoviscous,
nota chemical,
overprint.
5. PaleomagneticData
Site-mean cleaned NRM directions, Fisher statisticsk and
•95, and maximumunblockingtemperatures
for eachof the
four componentsare summarizedin Tables2 and 3. The site
10 LT directionsare anomalouslyshallow and have been
omittedfrom averagingto obtaintheLT mean. Fourof thesix
samplesfrom site2 alsohad shallowinclinations,in this case
streakedalonga greatcircle betweenLT and HT directions,
and were omittedin averaging. Site-meanand grand-mean
directionsfor LT, CRM1, andHT areplottedin Figure7.
LT andCRM 1 are of normalpolarity(as expectedfor the
Cretaceous
normalsuperchron
(118-83 Ma): seesection6) and
have mean directionsthat are indistinguishable
at the 95%
confidencelevel. The combinedLT-CRM1 grand mean
directionis D=348.3ø, I= -79.0 ø (k= 42, •95=6.5ø),in exact
agreementwith Schmidt and Embleton's[1981] result, D=
348ø,I= - 79ø. The grandmeanis distinctlydifferentfromthe
presentEarth'sfield direction but is similar to directionsfor
dated Late Cretaceousunits. The corresponding
(south)
paleopolepositionis 56øS, 158øE (dp= din= 12ø), falling
around100Ma on the Australianapparentpolarwanderpath
(Figure 8).
tion for 260300 m E, 6091400 m N, i.e., the references omit the
first one or two digits(which are obviousfrom the maplocation
Table 3. HT andCRM2 Paleomagnetic
DataAfter Thermal
of Milton) and are accurate to the nearest 100 m. N, N' are
Demagnetization
numberof samplestakenandnumberusedin averagingat each
site,respectively.D andI aredeclinationandinclinationof NRM
after cleaning;k is Fisherprecision;•95 is theradiusof the 95%
cone of confidenceabout the mean direction. (TuB)max
is the
maximumunblockingtemperature
of eachcomponent,
determined
from vector diagrams(e.g., Figures4 and 5) and averagingn
specimenresultsper site. The semiaxesof the oval of 95%
confidenceaboutthe paleopoleare dp anddm.
apaleopolefor LT component is 157.8øE, 58.7øS, with
dp=14.0ø and dm=15.0ø.
bPaleopole
for CRM1 component
is 158.7øE,47.0øS,with
dp=21.1ø and dm=21.6ø.
CCombined
paleopoleis 158.0øE,55.9øS,with dp=l 1.7ø and
dm= 12.3 o.
eachmagneticphase.A verybroadsizespectrum
of magnetic
carriers (mainly magnetite)is one possibleexplanation.
However,thebulk hysteresis
andthermomagnetic
properties
favoredmoderateto large pseudo-single-domain
grains,in
somecasesapproachingidealmultidomainsize. Multidomain
andlargepseudo-single-domain
grainshavemuchmorelinear
thermaldecaysof TRM andpartialTRM thanmostsingledomain grain assemblages[Bol'shakovand Shcherbakova,
1979; Xu and Dunlop,1994;Dunlopand Ozdemir,1997,
Figures 10.11, 16.11] and can also explain the observed
decays.
More germaneto the questionposedearlier is the wide
separation
of theTtmrangesof theLT andCRM1 components.
In samples7d-f, there is an -100øC window, and in other
samplesthere is often an even wider window, betweenthe
Site
N
N'
D,
deg
1
2
3
4
5
6
7
6
6
7
8
6
6
7
6
6
7
8
5
6
6
22
88
43
50
59
29
60
+62
+77
+70
+72
+80
+68
+70
79
186
42
40
242
244
81
8
5
10
9
5
4
8
8
9
7
6
5
6
45
34
+74
+73
45
101
12
7
11
6
2
133
+80
> 103
HT mean
10
47.4
+74.4
71
I,
deg
k
{X95
,
deg
(TuB)max
,
øC
HT ComponentData a
538
547
534
544
541
503
309
508
475
542
-575
_+ 18
_+ 19
_+33
_+ 15
_+ 10
_+ 15
_+ 10
_+17
_+20
_+ 8
15
18
18
18
16
18
12
6
11
5
3
>577
>577
>580
665_+ 9
672 _+ 5
7
2
18
13
6
CRM2Component
Data•
3
4
6
7
7
6
7
6
mean
2
Combined mean •
12
8
CRM2
64
79
+79
+82
55
158
70.2
+80.6
890
49.9
+75.5
77
9
5
15
5
Symbolsare the sameas in Table 2.
apaleopole
for HT component
is 172.1øE, 13.6øS,withdp=9.9ø
and dm= 10.9 ø.
•Paleopole
forCRM2component
is 169.9øE,
27.5øS.
•Combinedpaleopoleis 171.8øE,15.8øS,with dp=8.4ø and
dm=9 .2 ø.
DUNLOP ET AL.: PALEOMAGNETISM
OF THE MILTON MONZONITE
27,279
1000 -
HT component,
7d2
800 -
aFe203
100
LT
component,
CRM2,
7d2
600
•
4OO
CRM1,
LT
component,
sample7d2
HT,
6fl
o
200
½RM1,
6fl
o
0
100
200
300
400
,
500
600
Temperature,
T (øC)
Figure 6. Thermaldemagnetization
intensitydecaycurvesplottedseparately
for eachof thefour components
in
samples
6fl and7d2 (determined
fromtheorthogonal
vectorprojections
of Figures4 and5). The decaycurvesare
almostlinear("thermallydistributed")
ratherthan"thermally
discrete."Notetheenlargedscalefor CRM2.
Milton
monzonite
N
N
N
o
o /
Av½iIncli•ati.øn:177'5."
/ Av½ilncli.nati.øn:183
ø / Ave.Inclination:74.4
o
EqualAreaNet
O - Upperhemisphere
ß - Lowerhemisphere
A or ß - Average
direction
Figure 7. Site-meancleanedpaleomagnetic
directionsfor theLT, CRM1, andHT NRM components
of the Milton
Monzonite.Trianglesandenclosing
circlesdenotegrandmeansandcirclesof 95% confidence
for eachcomponent.
27,280
DUNLOP ET AL.' PALEOMAGNETISM OF THE MILTON MONZONITE
180E
-/.
Milton
20S
HT + CRM2
ß
-247
ß
251-260
40S
-280
4-
4-
ß
Milton
LT+CRM1
60S A"
120E
168-174
90-60
150E
Figure8. Permianto mid-Cretaceous
apparent
polarwanderpathforAustralia.Numbers
onpaleopoles
areages
in Ma; individualpaleopoles
arelistedin Table4. Paleopoles
andapproximate
ovalsof confidence
for LT+CRM1,
CRM1,andHT+CRM2areindicated.NotethatHT+CRM2andLT+CRM1havenonintersecting
ovalsandcannot
be of similarages.
HT and CRM2 are of reversepolarity and have similar calloopis of aboutthesamelengthasthesubsequent
"Coral
directions.The CRM2 meanfallsjust outsidethe95% circle Sealoop"linkingthe -•170Ma MiddleJurassic
polesto the
of confidencefor HT, but sinceCRM2 has low intensity -•100Ma Cretaceous
poles.Theoverallpicturewouldthenbe
comparedto LT, HT, or evenCRM1 (compareFigure6) and of quasi-static
apparent
polarwanderduringthePermianand
is measurable
withconfidence
at onlytwosites,it is probably EarlyTriassicandagainduringtheEarly andMiddleJurassic
not greatlydifferentin directionor time of acquisition
from with 40-60 Myr long periodsof morerapid platemotion
HT. The combinedHT-CRM2 mean,usingHT datafrom all following eachstandstill.
11 sitesand CRM2 from only 2 sites,is D= 49.9ø, I= 75.5ø
(k= 77, •95=5 ø),whichis indistinguishable
from Schmidtand
6. Discussion
Embleton's
[1981]result,D=-60ø,I= 78ø. Thecorresponding
(south)paleopole
position,16øS,172øE(dp=8ø,din=9ø),falls
We have confirmed earlier determinations of the HT and
around150Ma ontheAustralianapparent
polarwanderpath LT mean directions for the Milton Monzonite and in addition
(Figure8) andis ratherfar removedfromPermianandEarly havediscovered
thesesamedirections
echoed
in highunblockTriassicpaleopoles
for eastern
Australia(polesrangingfrom ingtemperature
magnetite
andhematiteoverprints,
CRM1 and
280-290Ma to235-245Ma in agein Figure8). Thediscrep- CRM2. In eachcase,theCRM hassteeper
inclination
thanthe
ancy cannotbe explainedby tilting becausethe Milton• TRM orpTRM it resembles.
Thedifferences
in directions
are
Monzoniteintrudesflat-lying strata,as notedin the Introduc- notstatistically
significant,
buttheymayreflectslightdiffertion.
encesin the time of remanenceacquisitionbetweenHT and
One possibilityis thatthewhole rock K/Ar dateof 245 Ma
CRM2 and between LT and CRM1.
For this mason,we
for theMiltonMonzoniteis slightlyolderthanits trueage. proposethatCRM2 is likely a CRM acquiredby authigenic
Most of the paleopolesin Figure8 andTable4 arefor extru-
hematiteandthereforeslightlyyoungerthanHT, ratherthana
sive or shallow intrusiverocks,but the Milton Monzonite is a
TRM in primaryhematite.
hypabyssal
intrusionthat may containsomeexcessargon.
CRM1 is carriedby authigenicmagnetite,probablya
productof hydrothermal
alteration.Therearetwopossibilities. CRM1 couldbetheresultof deeppenetration
by hydrothermalsolutions
duringheating,domingandinitialrifting
over a risingplumepriorto full rifting andspreading
of the
Judgingby theHT+CRM2 paleopole
position,a LateTriassic
ageof -•220-230Ma for theMilton HT magnetization
would
be plausible.A loopjoiningthePermianandEarlyTriassic
paleopolegroup(nearsoutheastern
Australiain Figure8) to
the Early to Middle Jurassicgroup(-•165-200Ma, southof
New Zealand)couldthenincorporate
theMilton HT+CRM2
paleopole
in theapparent
polarwanderpath.Thishypotheti-
Tasman Sea (-• 80 Ma). In this case,CRM1 would be some-
whatolderthanLT, whichwasproduced
by slowcoolingin a
laterstageof uplift.Alternatively,
CRM1 couldbetheproduct
DUNLOP ET AL.: PALEOMAGNETISM
OF THE MILTON MONZONITE
27,281
Table 4. AustralianPaleopolesof Permianto CretaceousAge
Age,Ma
Formation
Reference
~290
FeatherbedVolcanicsD
Klootwijk et al. [1993]
280-290
280-290'
MountLeyshon
TuckersRange
Clark [ 1996a]
Clark [1996b]
-280
FeatherbedVolcanicsA
Klootwijk et al. [1993]
270-300
Bulgonunna
Volcanics
Lackieet al. [1992]
Database
251-260
NewcastleCoal Measures
H. Th6veniaut(personalcommunication,
1996)
249-257
~247
-247'
235-245
Gerringong
Volcanics
DundeeIgnimbrite
DundeeRhyodacite
PatongaClaystone
IrvingandParry [1963]
Lackie[ 1989a]
Lackie[1989b]
EmbletonandMcDonnell[1980]
220-230?
Milton MonzoniteHT+CRM2
thispaper
189-199
187-207
172-185
Western Victoria Basalt
Garawilla Volcanics
JurassicIntrusives
Schmidt [ 1976a]
Schmidt [ 1976b]
Schmidt [1976b]
168-174
167-193
Barrenjoey
Dyke
Jurassic
compilation
Robertson
[1979]
Embleton[ 1981]
1239
166-182
TasmanianDolerites
Schmidtand McDougall [ 1977]
1113
166-178
130-160
~ 150
~ 145
Kangaroo
IslandBasalt
HornsbyBreccia
BendigoDykes
LateJ-EarlyK compilation
Schmidt[1976a]
Schmidt
andEmbleton[1981]
Schmidt[1976a]
Embleton[ 1981]
781
1238
100-160
Erskine Park Sill
Robertson [1979]
1239
110-120
100-110
OtwayFormation
CygnetAlkalineComplex
Idnurm[1985]
Robertson
andHastie[ 1962]
1201
974
~ 100
= 100
90-105
~90
90-60
MountDromedary
Milton MonzoniteLT+CRM1
BunburyBasalt
PatongaClaystone
overprint
HornsbyBrecciaoverprint
Schmidt[ 1976c]
thispaper
Schmidt[ 1976a]
Embleton
andMcDonnell[1980]
Schmidt
andEmbleton[1981]
781
1238
1238
995
2566
1610
781
780
780
Databasenumberis theGlobalPaleomagnetic
Databasereferencenumber[McElhinnyandLock,1996].
of low-temperature
hydrothermal
alterationlatein uplift,after
the LT magnetizationhad been blockedby cooling. The
paleopolepositionfor CRM1 alone,47øS, 159øE,falls in an
older position along the polar wander path than LT or
LT+CRM1 (Figure8), favoringtheformerscenario.
The HT-CRM2 paleopoleis puzzling. It doesnot match
Late PermianandEarly Triassicpolesfrom Australiabut lies
dencearedistinctby a largeangle. Equivalently,theovalsof
close to 150 Ma on the Coral Sea loop of the Australian
Monzoniteis of LateTriassicage(~ 230-220 Ma), ratherthan
apparentpolar wanderpath. Around 150 Ma, the Sydney
Basin was tectonicallyquiescent. Basin subsidenceand
depositionhad ceasedby the Early Jurassic(-•200 Ma) and
uplift did not beginuntil 100 Ma at the earliest.The reverse
polarity of HT and CRM2 is also somewhatunexpected
because150 Ma wasa time of predominantly
normalpolarity,
althoughnot exclusivelyso. Late Jurassicremagnetization
thereforeseemsunlikely, althoughit cannotbe ruledout.
A magnetizationage for HT and CRM2 significantly
youngerthan150 Ma canbe ruledout on bothrockmagnetic
and paleomagneticgrounds. If HT and CRM2 are younger
CRMs, theirprobableageis no olderthan ~ 100 Ma, the time
of domingand initiationof marginalrifting seawardof the
SydneyBasin. They would thereforebe expectedto have
EarlyTriassicasindicatedby its 245 Ma K/Ar date,andthat
HT is the primaryTRM datingfrom initial coolingof the
intrusion.CRM2 is thoughtto be a slightlyyoungerhematite
CRM. The HT-CRM2 paleopoledefinesa new loop(-•240200 Ma) in theAustralianapparent
polarwanderpath,linking
Permian/Early
Triassicpolesto Early/MiddleJurassic
poles.
Thisloopcrosses
theyoungerCoralSealoop(•-165-100Ma)
around150Ma, producing
theambiguityin magnetization
age
confidenceaboutthe LT-CRM1 andHT-CRM2 paleopolesin
Figure8 arenonintersecting
by a largeangle. HT-CRM2 is
thereforenot similar in ageto LT-CRM1.
Since it is unlikely that HT and CRM2 are either Late
Jurassicor Cretaceousremagnetizations,we prefer the
hypothesis
advancedearlier. We assumethat the Milton
of HT-CRM2.
7. Conclusions
1. The predominantmagnetic mineral in the Milton
Monzoniteis magnetitewith Tc = 545-580øC. Hematiteand
pyrrhotite also carry NRM but are not well expressedin
similar directions to LT and CRM1 and normal, not reverse,
susceptibilityand hysteresis.
2. The magnetitedetectedby bulk hysteresis
measurements
polarity. The Milton samplescontaina mixtureof magnetic
size (>_10 gm),
phases,
principallymagnetite,
hematite,andpyrrhotite(Figure is of moderateto largepseudo-single-domain
true multidomainsize (> 100 gm) at site 10.
2). Self-reversalin one of thesephasesis conceivable,but in approaching
3. The NRM of the Milton Monzonite consists of four
both magnetite (HT) and hematite (CRM2) it is highly
low temperature(LT), high temperaunlikely.Furthermore,
theHT-CRM2 directionis not antipo- distinctmagnetizations,
dal to theLT-CRM 1 direction(e.g.,Figures4 and7). If either ture (HT), and magnetiteand hematite CRMs (CRM1 and
the LT or HT groupof directionsin Figure7 is reversedand CRM2), with nonoverlappingor only slightly overlapping
comparedto the other,the grandmeansandcirclesof confi- rangesof unblockingtemperatures.
27,282
DUNLOP ET AL.: PALEOMAGNETISM OF THE MILTON MONZONITE
and Frontiers,573 pp., CambridgeUniv. Press,New York,
4. The HT and LT componentsare believedto be of
1997.
thermal origin. HT is the primary(Triassic)TRM. LT is a
D. J.,O. Ozdemir,
andP.W. Schmidt,
Paleomagnetism
Middle to Late Cretaceousthermoviscous
overprintacquired Dunlop,
and paleothermometry
of the SydneyBasin, 2, Origin of
in slow cooling during uplift. HT and LT are carriedby
anomalously
highunblockingtemperatures,
J. Geophys.
Res.,
magnetitein mostsamplesandby pyrrhotitein four samples
this issue.
from site 7.
of Australia
5. CRM1 is carriedby authigenicmagnetiteproduced Embleton,B. J. J., A reviewof thepalaeomagnetism
and Antarctica, Paleoreconstructionof the Continents,
duringuplift around100 Ma. It is similarin directionto LT
Geodyn.Ser., vol. 2, editedby M. W. McElhinnyand D. A.
but may be slightlyolder.
Valencio,pp. 77-92, AGU, Washington,D.C., 1981.
6. CRM2 is similar in direction to HT. It could be a
Embleton,B. J. J., andK. L. McDonnell,Magnetostratigraphy
in
primaryhematiteTRM but is more likely a CRM slightly
the Sydney Basin, southeasternAustralia, J. Geomagn.
Geoelectr.,32, suppl.III, 1-10, 1980.
youngerthan HT.
7. The HT-CRM2 mean direction is D= 50 ø, I= 75.5 ø
Enkin,R. J., andD. J. Dunlop,The demagnetization
temperature
necessary
to removeviscousremanent
magnetization,
Geophys.
similar to that foundby Schmidtand Embleton[1981]. The
Res.
Lett.,
15,
514-517,
1988.
paleopolefalls at 16øS,172øE,near150Ma on theAustralian
apparentpolar wanderpathbut a considerable
distancefrom Facer,R. A., andP. F. Carr, K-Ar datingof PermianandTertiary
igneousactivityin the southeastern
SydneyBasin,New South
PermianandEarlyTriassicpolesfrom Australia. There is no
Wales, J. Geol. Soc. Aust., 26, 73-79, 1979.
known tectonicor other magnetizingevent in the Sydney Falvey,D. A., The development
of continental
marginsin plate
Basinaround150Ma, andsotheHT-CRM2 poleprobablylies
tectonictheory,APEA J., 14, 95-106, 1974.
on a new Middle Triassicto Early Jurassicsegmentof the Falvey,D. A., andM. F. Middleton,Passivecontinental
margins:
polarwanderpath.
Evidencefor a prebreakupdeepcrustalmetamorphicsubsi8. The LT-CRM1 mean direction is D= 348 ø, I= -79 ø,
identicalto thatfoundby Schmidtand Embleton[ 1981]. The
paleopolefalls at 56øS,158øE,near100Ma on theAustralian
apparent
polarwanderpath. Thisageis consistent
with uplift
andcoolingrelatedto initial rifting of the TasmanSea.
dence mechanism, Oceanol. Acta, 4, 103-114, 1981.
Herbert,C., Depositionaldevelopment
of theSydneyBasin,Geol.
Surv. N.S.W. Bull., 26, 10-52, 1980.
Idnurm, M., Late Mesozoicand Cenozoicpalaeomagnetism
of
Australia,I, A redeterminedapparentpolar wander path,
Geophys.
J. R. Astron.Soc.,83, 399-418,1985.
of somePermianrocks
Acknowledgments.
Most of thepaleomagnetic
researchwas Irving,E., andL. G. Parry,The magnetism
from New SouthWales,Geophys.J. R. Astron.Soc.,7, 395donewhileD.J.D.andO.O.wereVisitingScientists
atCSIRO.
411, 1963.
We thankMark Huddlestonfor thermomagnetic
measurements
Joplin,
G. A., The shoshonite
association:
A review,J. Geol.Soc.
andJeffWood for hysteresis
determinations.
Helpful reviews
Aust., 15, 275-294, 1968.
andsuggestions
by GillianTurner,Bob Butler,andan anonyKent,
D. V., Thermoviscous
remagnetization
in someAppalachian
mousrefereegreatlyimprovedthe paper. The analysisand
limestones,
Geophys.
Res.
Lett.,
12,
805-808,
1985.
interpretation
phasesof the work weresupported
by NSERC
Kent,D. V., andN. D. Opdyke,Multicomponent
magnetizations
researchgrantA7709 to D.J.D.
from theMississippian
Mauch Chunkformationof the central
Appalachians
andtheirtectonicimplications,
J. Geophys.Res.,
References
90, 5371-5383, 1985.
Klootwijk, C., J. Giddings, and P. Percival, Palaeomagnetic
of Upper Palaeozoicvolcanics,northeastern
Bol'shakov,A. S., andV. V. Shcherbakova,
A thermomagnetic reconnaissance
Queensland,Aust. Geol. Surv. Org. Rec., 1993/36, 1993.
criterionfor determining
thedomainstructure
of ferrimagnetics,Izv. Phys.SolidEarth, Engl. Transl.,15, 111-117, 1979. Lackie, M. A., The rock magnetismof graniticand ignimbritic
rocks in the Lachlan and New Englandfold belts,N.S.W.,
Branagan,D. F., C. Herbert,andT. Langford-Smith,
An Outline
Ph.D. thesis,MacquarieUniv., Sydney,Australia,1989a.
of the Geologyand Geomorphology
of the SydneyBasin,60
Lackie,M. A., The palaeomagnetism
andmagneticfabricof the
pp.,Universityof Sydney,Sydney,Australia,1976.
Late
Permian
Dundee
Rhyodacite,
New
England,NewEngland
Chamalaun,F. H., Originof thesecondary
magnetization
of the
Orogen: Tectonics and Metallogenesis,edited by J. D.
Old Red Sandstone
of theAnglo-Welshcuvette,J. Geophys.
Kleeman,Univ. of New England,Armidale,New SouthWales,
Res., 69, 4327-4337, 1964.
Australia, 1989b.
Clark,D. A., New Permian,DevonianandSilurianpolesfromthe
ChartersTowersProvince- Implicationsfor the Palaeozoic Lackie,M. A., D. H. French,andD. A. Clark, Magneticmineralogyof felsicvolcanicsof theConway-Bimurra
area,northeast
APWP of Gondwana(abstract),Eos Trans. AGU, 77(22),
Queensland:Relationshipsto aeromagneticanomaliesand
West.Pac.Geophys.Meet. Suppl.,W19, 1996a.
hydrothermal
alteration,CSIRODiv. Explor. Geosci.Restrict.
Clark, D. A., Palaeomagnetism
of the MountLeyshonIntrusive
Rep., 274R, 1992.
Complex,theTuckersIgneousComplexandtheRavenswood
Batholith,CSIRO Explor.Min. Rep.318R, 1996b.
Mayne,S. J., E. Nicholas,A. L. Bigg-Wither,J. S. Rasidi,andM.
Clark, D. A., and P. W. Schmidt,Theoreticalanalysisof
J. Raine,Geologyof the SydneyBasin-A review,Bull. Bur.
thermomagnetic
properties,low-temperature
hysteresis
and
Mineral. Res.Geol. Geophys.Aust.,149, 229 pp., 1974.
domain structureof titanomagnetites,
Phys. Earth Planet. McCabe,C., andR. D. Elmore,The occurrence
andoriginof Late
Inter., 30, 300-316, 1982.
Paleozoicremagnetization
in the sedimentary
rocksof North
Conolly, J. R., and J. C. Ferm, Permo-Triassicsedimentation
America,Rev. Geophys.,27, 471-494, 1989.
patterns,SydneyBasin,Australia,Am.Assoc.Petr. Geol.Bull., McElhinny,M. W., andJ. Lock, IAGA paleomagnetic
databases
55, 2018-2032, 1971.
with Access,Surv.Geophys.,17, 575-591, 1996.
Day,R., M.D. Fuller,andV. A. Schmidt,
Hysteresis
properties
of Middleton,M. F., andP. W. Schmidt,Paleothermometry
of the
titanomagnetites:
Grain sizeand compositional
dependence,
SydneyBasin,J. Geophys.Res.,87, 5351-5359, 1982.
Phys. Earth Planet. Inter., 13, 260-267, 1977.
Moore, M. E., A. J. W. Gleadow,and J. F. Lovering,Thermal
Dunlop, D. J., Hystereticpropertiesof syntheticand natural
evolutionof rifted continentalmargins:New evidencefrom
fissiontracksin basement
apatitesfromsoutheastern
Australia,
monodomain
gr.a.
ins..,
Philos.
Mag.,19,329-338,1969.
Earth Planet. Sci. Lett., 78, 255-270, 1986.
Dunlop,D. J., and O. Ozdemir,RockMagnetism:Fundamentals
DUNLOP ET AL.: PALEOMAGNET!SM
OF THE MILTON MONZONITE
27,283
N6el,L., Th6oriedutrainage
magn6tique
desferromagn6tiques
en Schmidt,P. W., and B. J. J. Embleton,Magneticoverprintingin
grainsfinsavecapplications
auxterrescuites,Ann.Gdophys.,
southeasternAustralia and the thermalhistoryof its rifted
5, 99-136, 1949.
margin,J. Geophys.Res.,86, 3998-4008, 1981.
Ozdemir,
O., andS. K. Banerjee,
Hightemperature
stability
of
Schmidt, P. W., and I. McDougall, Palaeomagneticand
maghemite,Geophys.Res.Lett., 11, 161-164, 1984.
potassium-argon
datingstudiesof theTasmaniandolerites,J.
Geol. Soc. Aust., 25, 321-328, 1977.
Ozima,M., M. Ozima,andS. Akimoto,Low temperature
characteristicsof remanentmagnetization
of magnetite:Self-reversal Walton, D., Time-temperature
relationsin the magnetization
of
and recovery phenomenaof remanentmagnetization,J.
assembliesof singledomaingrains,Nature, 286, 245-247,
1980.
Geomagn.Geoelectr.,16, 165-177, 1964.
Packham,
G. H. (Ed.),The geologyof New SouthWales,J. Geol. Wellman, P., and I. McDougall, Potassium-argon
ageson the
Soc. Aust., 16, 654, 1969.
Cainozoic volcanic rocks of New South Wales, J. Geol. Soc.
Aust., 21,247-272, 1974.
Pullaiah,G., E. Irving,K. L. Buchan,andD. J. Dunlop,Magnetizationchangescausedby burialanduplift, Earth Planet.Sci. Xu, S., and D. J. Dunlop,The theoryof partialthermoremanent
Lett., 28, 133-143, 1975.
magnetization in multidomain grains, 2, Effect of
Raam, A., Petrologyand diagenesisof BroughtonSandstone
microcoercivity
distributionandcomparison
with experiment,
(Permian), Kiama District, New South Wales, J. Sediment.
J. Geophys.Res., 99, 9025-9033, 1994.
Petrol., 38, 319-331, 1968.
Robertson,W. A., Palaeomagnetic
resultsfrom someSydney
Basinigneousrockdeposits,
J. Proc.R. Soc.N.S.W., 112, 3135, 1979.
Robertson,W. A., andL. Hastie,A palaeomagnetic
studyof the
CygnetAlkalineComplexof Tasmania,J. Geol.Soc.Aust.,8,
259-268, 1962.
Schmidt,P. W., A newpalaeomagnetic
investigation
of Mesozoic
igneousrocksin Australia,Tectonophysics,
33, 1-13, 1976a.
Schmidt,P. W., The non-uniqueness
of theAustralianMesozoic
palaeomagnetic
poleposition,Geophys.J. R. Astron.Soc.,47,
D. A. Clark and P. W. Schmidt,Divisionof Explorationand
Mining,CSIRO, North Ryde,New SouthWales2113, Australia.
(e-mail:[email protected].
eg.:csiro.au)
D. J. DunlopandO. Ozdemir,Department
of Physics,Erindale
College,Universityof Toronto,3359 Mississauga
RoadNorth,
Mississauga,
Ontario,CanadaL5L 1C6.(e-mail:dunlop@physics.
utoronto.ca)
285-300, 1976b.
Schmidt,P. W., TheLatePalaeozoic
andMesozoicpalaeomagnetism of Australia, Ph.D. thesis,Aust. Natl. Univ., Canberra,
1976c.
(ReceivedJanuary16, 1997; revisedJuly 7, 1997;
acceptedAugust27, 1997.)