Viscosity of Himalayan leucogranites: Implications for
mechanisms of granitic magma ascent
Bruno Scaillet, Holtz François, Michel Pichavant, Michael Schmidt
To cite this version:
Bruno Scaillet, Holtz François, Michel Pichavant, Michael Schmidt. Viscosity of Himalayan
leucogranites: Implications for mechanisms of granitic magma ascent. Journal of Geophysical
Research : Solid Earth, American Geophysical Union, 1996, 101 (B12), pp.27,691-27,699. .
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101,NO. B12,PAGES 27,691-27,699,DECEMBER 10, 1996
Vscosty of Himalayan leucogranites:
Implications
for mechanisms
of graniticmagmaascent
Bruno
Scaillet,Francois
Holtz,MichelPichavant,
andMichaelSchmidt
Centre
deRecherche
surla Synth•se
et la ChimiedesMintraux
Centre
Nationalde la Recherche
Scientifique,
Orltans,France
Abstract.
Theviscosityof theGangotriHimalayan
leucogranite
hasbeenexperimentally
determined
between
80()øand1100øC,300and800MPa,formeltwater
contents
of 3.98and6.66
wt%.Themeltviscosityis independent
of pressure
andshowsanArrhenianbehaviorrelativeto
temperature
withintherange
ofconditions
investigated.
Wepresent
anempirical
relation
thatcan
beused
to determine
leucogranite
magmaviscosities
knowingtheirmeltwatercontentand
temperature.
Thisrelation
together
withphase
equilibria
experiments
constrain
theviscosity
ofthe
Itimalayan
leucogranit½s
tobearound
104.5
Pasduring
theireraplacement.
These
viscosities
and
thewidthsof dikesbelongingto thefeedersystemareconsistent
withthetheoretical
relationship
relating
thesetwoparameters
andshowthattheprecursor
magma
of theleucogranite
wasat near
liquidus
conditions
whenemplaced
within
hostrocks
withpreintmsion
temperatures
around
350øC.
Calculated
terminalasa'.ntratesfor themagmain thedikesarearound1 m/s.Magmac•mber
assembly
timeis,onth•sbasis,
estimated
tobelessthan100years
(fora volume
of 150Icm3).
In
addition,
thedynamicalregimeof themagmaflow in thedikeswasessentially
laminar,thus
allowing
preservation
of anychemical
heterogeneity
acquired
inthesource.
Theseresults
constrain
theviscosity
of meltsf{,rmedduringthefirststepsof crustalanatexis,
thoseinvolvingmuscovite
breakdown,
tobealso•round104.5Pas.Thuscompaction
maynotbetheonlymechanism
of
meltsegregation
in partiallymeltedcrustal
rocksin viewof theveryshorttimescale
inferredfor
magma
ascent
andemplacement.
Introduction
the recent mathematical analysis of Rubin [1995] suggests
that the thermal viability of granitic dikes should prevent
The diking mechanismis the widely acceptedmode of
magma
transportfor bas',dticmagmas,as testifiedby abundant
fieldevidenceof basalticdikes [e.g.,Wada, 1994]. In contrast,
thecommoncircularplan view of many graniticintrusionsis
perhaps
the argument that has convinced many authorsthat
diapiric
upriseis a viable mechanismof transferfor granitic
magnaas
[e.g.,Ponsetal., 1995] (seealsoreviewsby Paterson
and Fowler [1993], Paterson etal. [1991], Brown etal.
them from propagatingfar from the sourceregion, thus
showing that the diking model has its shortcomingtoo.
Clearly, the questionof whether granite ascentthroughthe
crust occurs only via dikes or only via diapixic rise is not
likely to be resolved through theoretical analysis alone,
inasmuchas both the diapir and dike mechanismsneednot to
be exclusive to each other [Rubin, 1993].
Althoughthe casefor graniticdikes is clear by the analyses
of Clemens and Mawer [1992] and Petford et al. [1993],
[1995]).This modeof ascentwent virtually uncontested
for
decades
untilClemens
andMawer [1992]proposed,
onthermal assessment of this mechanism of ascent for natural felsic
andmechanicalgrounds, that diking was a more viable magmashas been, as ye.; seriouslyhamperedby two factors.
alternative
thandiapirismfor felsicmelt migration[seealso The first is that in marred contrast to basaltic dikes, wellPenford
etaI., 1993].A majorcriticismof thediapirmodelis identified examplesof leeder dikes of plutonicsystemsare
that
rates
of magmaascent,asobtained
fromearlythermal
and exceedingly
rare [e.g., /•e Fort, 1981;John, 1988;Scailletet
mechanical
modeling[e.g., Mahoneta/., 1988],areextremely a!., 1995a], a direct consequenceof the fact that the bottom
low,whichshouldallow for extensive
heatlosstowardcountry contactof most plutonic intrusionsis not visible. The second
rocks
andthuscrystallizationat depth,while a numberof is that the determinationof the viscositiesof magmafrozenin
geologic
arguments
showthat graniteintrusions
are clearly dikes is, for obvious reasons, not straigthforward.Current
disconnected
from thei• sourcezone[Miller etal., 1988]. attempts[e.g., Petfordet aL, 1993;Wada, 1994]rely on the
However,
Weinberg
andPodlachikov
[1994]haveshown
that empiricalmodelof Shay,.[1972] whichrequiresknowledge
of
rates
of diapiricrisecansignificantly
increase
(e.g.,100m/yr) the melt composition,including its water content,melt
if thecrustbehaves
in a non-Newtonian
fashion.
Similarly, temperature,
andamountof crystalspresentat thetimeof dike
,
intrusion. These parameters (in particular, the water content
which most of the time •.sfixed arbitrarily) are poorly known
formostplutonic
rocks.Thelackof precise
determination
of
Copyright
1996
bytheAmerican
Geophysical
Union.
magmaviscosity,
a mast{,rvariablethatmayinfluence
whether
graniticmagmaswill a•cendas a diapiror as a dike [e.g.,
htl•rnumber
96J'B01631,
0148-0227/96/96JB.0163
! $09.00
Emermanand Marret, 1990; Rubin, 1993], is a weaknessin the
presentstatusof the diking model that has obscuredthe
27,691
SCAILLETET AL.:HIMALAYANLEUCOGRANITEVISCOSITIES
27,692
contacts
relationship
between
dikewidthandviscosity,
particularly
for eta/., 1987].Owingto thegenerallackof intrusive
withinthe laccolith(i.e., betweenthe different
magma
felsiemagmas[Petfordeta/., 1993].
thegrowingtimeof eachindividual
laccolith
must
In thisstudy,we usean exceptional
geological
areawhere batches),
shortin orderto prevent
anysignificant
meltmigration
undoubtextly
occurred
viadiking,asseenin the havebeenextremely
field,andultimately
fil!el batho!ithic
sizedbodies,
toexamine cooling of each individualunit during the growthof the
that
theoretical
modelsfor magmaascentvia dikingas appliedto laccolith.Preliminarynumericalthermalmodelingshows
laccolithshouldcoo! belowits soliduswith:m
graniticmagmas.
The regionof interest
is theHighHimalaya a 2-kaxt-thick
on the thermal
regime
of
rangewherehighly dissected
relief providesnaturalcross 30,000to 100,000years,depending
sectionsof great vertical extentenablingaccuratethree the crustand on the thermaldiffusivityvaluechosenforthe
dimensional
representation
of the plutonitbodies[e.g., magma(B. Scaillet,mm•uscriptin preparation,1996).These
providean upperboundfor the assembly
timeof
Scaillet et al., 1995a]. These are the High Himalayan calculations
Petrographic
and
Leucogranites
(HHL) whichhavebeenextensively
studied
in the laccolithsat the level of eraplacement.
that
therecentyearsandwhosepetrogenesis
is wellknown[e.g., experimentalstudies[S•aillet et at., 1995b]haveshown
Le Fort et al., 1987; Casrelliand Lombardo,1988;lnger and thesemagmas
wereeraplaced
ascrystal-poor
melts(< 5 vol%•
rangingfrom 800ø to 750øC(biotite-muscovite
Harris, 1993].In particular,
important
parameters
thatcontrol at temperatures
facies),
with
magmatheologysucha• temperature,
weightpercent
H20 in facies)to 750ø to 700ø(: (tourmaline-muscovite
melt,crystallinity,
andnteltcomposition
of themagmaat the melt water contentsvarying between5.5-7 (Bt-Ms)and>7
is
time of its eraplacement
in the higherlevelsof the crustare (Tur-Ms) wt % H20. The pressure of eraplacement
constrained
to
have
been
between
300
and
400
MPa,
on
the
now well constrained
by experimental
data [Scailletet al.,
1995b]:Ratherthanusingtheempirical
modelof Shaw[1972]
basis of thermobarometric results obtained on associated
we have experimentally
determinedthe melt viscosityof a
thermal aureoles[Guillot eta/., 1993], while the pressure
of
magmagenerationwas between700 and 1000 MPa[e.g.,
dropof
conditionsrelevant to its entire petrogenetichistory (as P•chcr, 1989]. Thusthe magmaunderwenta pressure
inferredfrom phaseequilibriumand petrographic
studies). around300-400 MPa during eraplacement.
Theseviscositymeasurements
combinedwith the field data
HighHimalayan
leucogr'•itewithinP, T, weightpercent
H20
providethe first comparison
of the theoretical
relationshipExperimental Determination
relatingdike width to the magmaviscosity[Petfordetak, of Leuc0granite Viscosities
1993]. We show below that such a relation is entirely
consistent
with the eraplacement
of the HHL as nearliquidus
magmasandthat theem•lacement
timerequiredfor completeExperimental Method
assemblyof the laccolifl:was only a few yearsin uppercrust
The experimentalmethodused to determinethe viscosity
of
whosetemperature
was mound350øC.
hydrousmelts is the falling spheretechnique.In this studythe
The Himalayan Leucogranite Laccoliths
The HHL were producedduring the collision betweenthe
Indian
subcontinent
and Eurasia
which
started circa
65 Ma
[Beck et al., 1995]. Geological mapping carried out over the
past 30 years has shown that theseleucogranitesform discrete
bodiescroppingout regularly all along the 2000 km of extent
of the Himalayan rang•:. Each body has invariably a lens
shape, being eraplaced most of the time within
metasedimentary rocks with clear intrusive contacts [e.g.,
CasteIll and Lombardo, 1988; France-Lanord and Le Fort,
!988; Searle et al., 1993; Inger and Harris, 1993; Scaillet et
al., 1995a]. In this teloft we will focus on the Garhwal
Himalaya, where both the granite and its feeder systemare
exposedin the Gangotriregion along verticalcliffs of more
than 2500 m of relief.
Detailed
structural studies of this area
have shown that magma eraplacement took place during
extensional tectonism in the Himalayan orogen
contemporaneously with, or subsequent to, the crustal
thicknening process [Searle eta!., 1993; Scaillet et al.,
1995a]. The feeder systemis made up of hundredsof dikes
whose vertical extension is at least 1000 m, with thicknesses
same procedure as that used by Schulze et al. [1996] was
followed, and the reader is referred to that work for full
technicaldetails. Only the salient featureswill be repeged
here. A hydrousglasscylinderwas preparedby melting
the
rock powder (natural tourmaline-muscovite bearing
leucogranite
GB4of Scailletet aL [1995b])loaded
within
aPt
capsuletogetherwith an appropriate
amountof distilled
and
deionizedwater. The meltingwas doneat 300-400MPa,at
1100ø-1200øC for 3-4 days to ensure a homogeneous
distributionof water. A glass cylinderfree of bubbles
approximately
4 cmlongand0.5cmin diameter
wasobtained.
Thecylinder
wassawed
.•cross
soasto obtain
a long(2-3cm)
and a short (0.5 cm) cylinder. The two cylinders
subsequently
stucktogether
withina newPtcapsule
(diameter
0.4 cm),withPt powder
in between.
ThisPtsurface
(thickn•s
1-2I.tm)wasused
asarelerence
leveltomeasure
theposition
of
thePt spheres
before
andaftertheexperiment.
Ontheother
endof thecylinder,
a smalllayerof glass
powder,
obtain•
fromcrushed
fragments
ofthesame
glass
cylinder,
was
added.
TwoPtspheres
were
placed
within
thepowdered
glass.
The
Pt
capsule
wasfinally
welded
shut
andbrought
tothedesired
P-T
conditions.
Afteranexperiment
under
a given
setofP-T
varying between 10 and 50 m, most of them intersectingthe conditions,
the nextonewasperformed
by usingthesam•
bottom contact of the laccolith (Figure 1). This general glass
cylinder
simply
turned
upside
down
toallow
thespheres
disposition led Scaillet et al. [1995a] to proposethat the tosettle
back
intheopposite
direction.
Thisprocedure
intrusive bodies representgenuine laccoliths,having grown ustouse
thesame
glass
,:ylinder
(with
thesame
water
coateat)
Pand
T conditions
(upto10experiments
were
by lifting up the overlyingmetasediment.
The heterogeneity forvarious
in Rb/Sr isotopessuggeststhat the whole bodycorresponds
to withthesame
cylinder).
Thewatercontent
of theglass
the assembly of diffenmt magmas batches which did not
undergoany subsequentvigorousmixing process[e.g,, Deniel
cylinder
wasmeasured
byKarlFischer
titration
(KFT),
bofore
and/or
after
(onboth
ends
oftheglass
cylinder)
agiven
set
SCAILLETET AL.:HIMALAYANLEUCOGRANITE
VISCOSITIES
27,693
b
•
I
i
i
i
i
I
Figure 1. (a) Photograpbof the southfaceof the Shivlingpeakshowingthe baseof a leucogranitelaccolith
with the array of feeder dikes that still connectto the bottomof the intrusion.The height of the cliff is about
1300 m. The leucograniteis intrudedinto black schistthat overliesa thick layer of orthogneisses.
In the
latter,the dikes are nearly vertical,while they are tilted in the schistas a consequence
of the late orogenic
extensionalcollapse [see Scaillet et al., 1995a]. (b) Schematicline drawing of the Figure la enhancingthe
geologicalcontoursof the main units.Only the upperreachesof the thickerdikesare drawn,whenthey cross
the black schistlevel: crosses,leucogranite;horizontalcrosses,orthogneisses.
The intermediatelevel of
schist is black.
experiments
and was &termined to be constant,within the
H20 . For a fixed meltwater content and temperaturethis
analytical
uncertainty
of theKFT technique
(0.3wt % H20).
difference
in meltdensity
produces
a variation
of 2.5x10
-4 log
The
falling
distances
of thePtspheres
weremeasured
withan
unit on the calculated viscosity. Therefore for convenience,a
optical
microscope
equipped
with an automated
X-Y-stage constant
meltdensity
of 2300kg/m3 wasusedin all viscosity
enabling
measurements
to be performed
witha precision
of determinations. All experiments were performed within an
0.001
cm.Given
thefactthattwoPtspheres
wereused,
each internally heated pressurevessel, pressurizedwith
argon and
experiment
gavetwo independent
measurements
of the working vertically. Total pressure is known within 50 bars.
•iscosity.
The viscosi
B wascalculated
fromStokes'
law, The temperature was read by at least three PtRh30
corrected
for bordereffects(Faxencorrection),
usingtherun thermocouplesand is kr.own to within +10øC (including the
duration,
theradius
ofthePtsphere
(determined
to+ 5 gmby
gradientacrossthe capsule[seeRouxeta/., 1994]).The length
optical
andweighting
methods)
andthedensities
of thePt of the glass cylinders allowed us to perform runs of long
duration, between about 3 and 78 hours, with settlingdistances
•phere
(21,450kg/m
3) andmelts.In our experiments,
calculated
meltdensities
varybetween
2290kg/m
3 at800øCrangingfrom 0.2 to 1 cm, which virtuallyeliminatesanyerror
priorto attainment
of
•nd
6.66
wt%H20and2310kg/m3
at1100øC
and3.98wt% arisingfrom the settlingof Pt spheres
27,694
SCAILLET ET AL.: HIMALAYAN LEUCOGRANITE VISCOSITIES
experimentalconditionsandduringthe quench.Uncertainties within
0.01logunit.
It predicts
aviscosity
of104-6_105
Pas
for settlingtimes are + 5 min.
fora magma
with5.5wt% H20 at800ø-750oC
andof104.1.
Results
thattheArrhenian
approximation
isstrictly
valid
only
f0•
small
temperature
ranges.
Useofequation
(1)well
outside
•
104.5
Pasforamagma
with
7wt%H20at750ø-700oc.
(Note
The resultsof 11 experimentsare listed in Table 1 in
chronologicalorder. Melt water contentsof 6.66 and 3.98 wt
% havebeeninvestigated.
Temperatures
rangedfrom800øCup
to 1100øC and pressure•between300 and 800 MPa. In all
experiments,viscositiesobtainedfrom the two Pt spheresare
within 0.02 log unit of eachother.Replicateexperiments
at
around 860øC give viscositiesthat agree within 0.05 log
units. Overall, the uncectainty
in viscositymeasurements
is
experimental
range
ofcalibration
may
give
erroneous
results.)
These
conditions
correspond
tothose
during
theemplacetaeat
of biotite-muscovite
andtourmaline-muscovite
bearia•g
leucogranites,
respectively,
asinferred
from
phase
equilibri•
experiments
[Scaillet
eta!., !995b].However,
given
•
narrow
rangeof theseviscosity
determinations
andfor•
sakeof simplicity,
in thefollowing
section
wewillconsi•
average
values
of
750øC
for
the
temperature
ofemplacem•
estimatedto be better than 0.05 log units. Experiments and104.5
Pasforthemagma
viscosity.
performed at 860øC and 500 and 800 MPa with a meltwater
contentof 6.66 wt % H20 yield viscositiesthat are identical
within error (Table 1), indicatingthat pressurehas no
detectable
effecton hydrousmeltviscosities
withinthisrange
of pressurevariation,as shownalsoby Burnham[1964]and
Implications for the Diking Mechanism
Theresultsobtained
in thisstudyprovide
directconstrains
existing
between
dikewidth
andmagma
$chulzeet aI. [1996].The two setsof experiments
display ontherelationships
Dike widthis an easilymeasured
parameter
inthe
linear behavior in an Arrheniusplot (Figure 2), with viscosity.
field[e.g.Wada,1994],andit haslongbeen
recognized,
at
least
qualitatively,
that
the
less
viscous
the
magma
the
and 0.993 for 6.66 wt % H20). The activationenergyof
correlationcoefficientsclose to 1 (0.999 for 3.98 wt % H20
the
viscousflow decreases
slightywith increasing
watercontent, narrowerthe dike. Petford et al. [1993, 1994]used
analysis
of BruceandHuppert
[1989,1990]
to
as observedin the haplogranite
system[Schulze
eta!., 1996]. mathematical
between
dikewidthandmagma
viscosity:
All the experimental
datahavebeenfit with thefollowing derivea relation
equation:
Wc
= 1.5[c(Tsol-Tw)2/L(Tliq-Tsol)13/4(r!kHIgA@
(2)
log •1 = 16280/T - 7.5461 + [0.59784
wherewc is the criticaldike widthnecessary
to prevent
its
- •235.4/T1wt.%H20
(•)
thermal
lock-up
during
meltflow,Tliqisthemagma
initial
temperature
(750øC),and Tso1 is the temperature
at which
the
magma near the dike wall becomesimmobile, heretakentobe
where'1 is theviscosity
in pascal
seconds,
T is thetemperature
thatof the solidus(645øC).Strictly,thetemperature
atwhich
in kelvinsand weightpercentH20 is thewatercontentof the magma stop flowing differs from that of the solidusbat
melt. This equationreproduces
the experimental
viscosities leucogranitemagmashave a strongeutectic-like
behavior
[Scai!letet al., 1996], being 80% liquid 15øCabove
solidus, and this difference should not exceed 10øC.The
Table1. Experimental
Viscosities
of Leucogranite
Melts specificheat,c (1600J/kgøC),wascalculated
using
theb•
Run P,
MPa
T,
øC
duration,log'11, log•12,
s
Pa s
Pa s
6.66wt%H20 inMelt*
4
5
6
7
8
9
10
11
500
500
500
500
500
800
500
650
860
910
855
800
865
860
955
813
63420
18660
46860
72000
28680
45900
9000
75300
3.58
3.25
3.55
3.94
3.54
3.5 2
3.00
3.86
kg/m
3 forthecountry
rocks[e.g.,Corry,!988]and
amelt
3.57
3.22
3.53
3.93
3.54
3.51
2.99
3.86
3.98wt%H20inMeltõ
12
13
14
300
300
300
1100
1001
907
14880
35770
280300
3.09
3.79
4.45
composition
andthe modelof LangeandNavrotsky
[1992]
for
all oxidesexceptfor water whosepartialmolarspecific
heat
was takenfrom Clemensand Navrotsky[1986].Thedensity
differencebetweenthe magmaand the hostrock,Ap,was
calculatedwith an averagedweightedmeandensity
of 2700
densitycalculated
usingthe bulk composition
andpartial
molar volumes of Knoche et al. [1995] for all oxide
componentsexcept for FeO and H20 whosepartialm,lar
volumeswere takenfrom Langeand Carmichael[1990]
and
Holtzet al. [1995],respectively.
The thermaldiffusivity
k was
setto8 x 10'7 m2/s,andg, thegravitational
constant,
was
set
to 10m/s2. All theprevious
inputparameters
of(2)can
be
considered
as well constrained.
In contrast,
Tw, thefar-tick!
wallrocktemperature,
L. thelatentheatof crystallization,
H, the dike length,are .moredifficultto assess.
A miniram
3.10
3.77
4.45
valuefor H is thepresent
verticalextent
of dikes
(1000
whilea maximum
valueis givenbythedifference
between
'the
depthof magmageneration,
whichis estimated
to bebe•eea
7 and10 kbar,andits levelof eraplacement.
Varying
//
P, pressure;
T, temperature.
*Ptspheres
ofradius
0.0110
cmand0.0127
cm.
{}Ptspheres
ofradius
0.0132
cmand0.0136
cm.
between
1000and20,000m, keeping
allother
values
co
(withL = 3x105J/kgandTw = 300øC,
seenext
increases
the criticaldike widthfromabout1 to2 m,
shows
thatthisparmeterhaslittleinfluence
onthe
ofwc.Although
reasonable,
these
thicknesses
are~ !
27,695
SCAILLET ET AL.: HIMALAYAN LEUCOGRANITEVISCOSITIES
I 100
•
1000
900
800
700
øC
6
6.66 wt.% H20
8
9
11
10
10000 / T(K)
Figure
2. Arrhenius
plotofexperimental
viscosities
determined
fora High
Himalayan
leucogranite
attwo
meltwater
contents.
Theboxindicates
therange
ofmagma
viscosities
during
theeraplacement
of Himalayan
leucogranites,
asinferred
from
phase
equilibria
and
petrographical
studies
[Scaillet
eta/.,1990,
1995b]
and
fromthe presentwork.
nonlinear,beingminimal
at near-liquidus
conditions,
magnitude
lower
thanthose
observed
(10-20
m,oreven
more).highly
theamount
of
However,
thisfirstcalculation
is based
onthepremise
thatthe anda valueof 3x105J/kggreatlyoverestimates
heat
released
by
crystallization
during
dike
intrusion.
The
latentheat is being released uniformly acrossthe
effectof latentheaton thecriticaldikewidthis illustrated
on
crystallization
interval.
In fact,crystallization
paths
obtained
Figure
3
for
a
magma
having
a
viscosity
of
104-5
Pa
s
at
from
phase
equilibrium
diagrams
show
thatsuch
leucogranites
eraplaced
in a hostrockat300øC.
Alsoshown
is the
arestill70-80wt % liquid15øCabove
theirsolidus
[Scail!et
et 750øC
amount
of accumulated
heatreleased
by the
a/.,1996].In otherwords,thecontribution
of latentheatto approximate
of a leucogranite
magma
between
750ø and
theheatbudget
of themagmaduringits crystallization
is crystallization
ß
1000
I
'
I
'
I
Latent heat released by cooling
from 750øC to 660øC
!
!
i
!
!
100
ii
ii
ngotri dikes
,.•.,
width of G
!
!
10
,,
i
i
!
!
f . I ,...I , I. ..•I • I •
ii
i
1
o
50
lOO
!5o
200
250
300
Latent heat (j/kg, /1000)
Figure
•. Effect
oflatent
heat
ofcrystallization
onthe
critical
dike
width
foraleucogranite
magma
at750øC
with
aviscosity
of104'5
Pas.Also
shown
istherange
ofwidths
forthe
Gangotri
dikes
that
areclearly
connected
tothebase
ofthelaccolith
(Figure
!).Theapproximate
amount
ofcumulated
latent
heat
released
during
thecrystallization
ofaleucogranite
magma
betweem
750
øand
660øC
isreported.
SCAILLET ET AL.: HIMALAYAN LEUCOGRANITEVISCOSITIES
27,696
morethana hundred
dikes,thetimeof
660øC, as estimatedfrom phaseequilibriumexperiments As thereareprobably
as instantaneous
if all
[ScaitIet et aL, 1995b]. Clearly, the preserveddike chamberbuildingcan be considered
thicknesses indicate that the amount of latent heat released
dikeswereactive
together.
Thusthe2-1ma
laccolith
assembly
during
dikeintrusion
wassmall,
probably
below
lx105J/kg. time shouldlie between1 and 100,000years,themaximum
The dike thicknesses are thus consistent with a magma
coolingtime to attain solidusconditionsas obtained
from
emplacement
at near-liquidusconditions,
as shownby
petrographic
and experimental
studies
[ScailIet
et al., 1990,
1995b;!ngerand Harris, 1993].Theyalsofit withtheoverall
trendestablished
by Wada [1994]between
dikethickness
and
numerical
simulations
(B. Scaillet,
manuscript
in preparation,
1996).However,a time at the lowerendof thisrange
is
presently
favoredbecause
of thenearconstancy
of dikewidths,
aspreserved
now in the field. In fact,equation
(2) predicts
that
the criticaldike width will decreasewith increasing
hostrock
temperature.This is what is shownon Figure4, wherethe
viscous
felsic(100m, 106-107Pas) magmas.
Thecombinedrelationbetweencriticaldike widthandhostrocktemperature
magma
viscosity,
fallingmidway
(10-50m, 104-5Pas)
between
those
reported
formafic(1 m, 101-10
2 Pas)and
useof experimental
an(l field datasupports
the theoreticalis shown for a magma at 750øC and for different valuesof
at
approach
followedby Pe½ord
et al. [1993,1994]andshows latentheat.The widthsof the Gangotridikesarereproduced
below 400øC,with the mostreasonable
that,in thisparticular
case,dikewidthis a precise
indicator
of hostrock temperatures
magmaviscosity,
providedthatothercontrolling
factors
such preintrusiontemperaturesbeinglocatedat around350øC.Had
asthe latentheatbudgetduringcrystallization
or thehostrock the processof magma incomingbeen protracted(e.g.,
hundredsof thousands{,f years), then the host rockswo•d
temperature
(seebelow)arealsowellconstrained.
have had time to warm up, whichshouldhavesignificantly
decreasedthe width of the latestdikes intruded.For instance,
with a valueof Tw = 600øC the critical dike widthdrops
to
around 0.5 m (i.e., almost 2 orders of magnitudebelow
Implications for Magma
Emplacement Time
The minimumtimerequiredto assemble
themagmachamber
canbe obtainedby usingtheequation
of fluidflow in a tabular
conduitthatapproximates
thedikegeometry.
The generalform
of thisequationis [e.g.,Petfordet al., 1994]
Va½
e =gApw2/12•l
(3)
whereVave is the averagevelocityof magmaflow andw is the
dike width. Taking a minimum value of 10 m for the dike
width, the flow velocity varies between2.6 m/s and 0.3 m/s
for a magma
witha vi,,cosity
B of 104-1and105-01Pas,
observed thicknesses). Significantly, a temperaturearouM
350øC is that expected at a depth of 15 km in a continental
crusthavinga normal geothermalgradient.If the preintmsioa
temperaturewas 500øC at 15 km depth,thentemperatures
of
around 1000øCwould prevail at 30 k.m of burial,conflicting
with existing thermal models of thickenedcontinentalcrust
[e.g., England et aL, 1992], which show that evenwhenthe
thermal relaxationprocessis completed(i.e., ~ 60 Myr after
the stackingof the crust},temperaturesat suchdepths(in the
hangingwall of the thru;t sequence)
hardlyexceed700øC.
In
addition, numerical simulations of magma cooling (B.
Scaillet, manuscriptin preparation, 1996) reproduce
the
respectively. The horizontal length of the dikes is not
metamorphic
peaktemperatures
of around550øC
in
preciselyknown but shouldbe no more than a few hundred observed
temperatures
below
meters. A single dike ,vith 300 m of horizontal extension the contactaureoleonly for preintrusion
in e.xcessof thisvalueyieldmuchhigher
couldtherefore
fill a 150km3 magma
chamber
(themaximum 400øC.Temperatures
gradesin the aureole,in somecasesreaching
volume of a laccolith in lhe Gangotriarea) in less than2 years. metamorphic
1000
100
width of Gangotri dikes
latentheat
(j/kg, /1 000)
.Ol
300
ß ,.........
I
400
• .......
I
......
500
•
ß
:
,,
600
Host rock temperature (øC)
Figure4. Effectof hostrocktemperature
onthecritical
dikewidthfora leucogranite
magma
at750øC,witha
viscosity
of 104.5Pas andforthreevalues
of latent
heat:0.3x105,
lx105,and3x105J/kg.Alsoshown
is
therangeof widthsfor theGangotri
dikesthatareclearlyconnected
to thebaseof thelaccolith(Figure1).
SCAH•ET ET AL.: HIMALAYAN LEUCOGRANITEVISCOSITIES
27,697
conditions
of incipientpartialmelting(> 650øC).Such indicatesthat virtually any chemical heterogeneityinherited
features
arelacking
in theGangotri
region,
andwethereforefrom the sourcewill survive the ascentperiod.Thereforethe
conclude
thatmagmaintrusion
in thisareatookplacein an chemical heterogeneityfound at the outcropscale does not
akeady
coldupper
crust[seealsoCopeland
eta/., 1990],
the necessarilyimply the coexistenceof different magma pulses
eraplacement
process
beingachieved
in anextremely
short havingtravelledin sepm'atedikes,but it may alsoreflect the
period,
afactendemic
tomost
laccolithic
intrusions
[Corry, existenceof
a former single magmabatch that was already
in the sourceregion.Besidesthe implications
1988].
Thiscold environment
is in agreement
with the heterogeneous
thatsuchfindingmay haveon the mechanisms
andkineticsof
hypothesis
of Scaillet
etal. [1995a]
of themagma
fractures
being
arrested
bycollapse
foldsin theupper
crustata level partial melting processesoccurring in the crust (i.e.,
½tose
tothebrittleto ductiletransition
[seealsoHoganand equilibriumor disequilibriummelting,rate of melt extraction,
Gilbert,1995].
Implications
of High Rates
of Magma Flow
see [Brown et al., 1995]) and which are beyondthe scopeof
this paper,a first consequence
is that it explainsthe existence
of strong variations in somes isotopes(e.g., St) while
physicalboundariestestifyingof the coexistenceof several
magmabatchesare lackh•g[Denielet al., 1987].
Theratesof magmaflow suggested
by thisstudyare among
thehighest
evercalculated
for silicicmagmas.
Thesehigh Concluding Remarks
roeshavetwo majorimplications.First,suchratesof magma
The High Himalayanleucogranites
are a mostspectacular
flow should minimize, or even impede, any chemical
'.metaction
with the countryrocks encountered
duringuprise exampleof the assemblyof large granitic batholithsvia
['Clemens
andMawer,1992].Thusthemagma
composition
at dikingon very shorttiatescales(i.e., of the orderof years).
itsarrivallevel is probably close to that producedby the This is not to say that every singlegraniticplutonor large
melting
reaction,
just belbreit left thesource
zone.It follows batholithbehavesin the way exemplifiedabove.The HHL are
thattheseleucogranites
can be possiblyusedas directprobes derivedfrom a crustalmelting processalone with no mantle
oftherheological
propertiesof the melt in the meltingsource, input [Le Fort eta/., 1987]. In this respectthey differ from
owing
to thelackof pressure
dependence
on meltviscosity. mostof the granitesthat belongto coastalbatholithssuchas
Thus
it canbeconcluded
thatmeltviscosity
wasaround
104-5
the Sierra Nevada Batholith in California. In the latter, the
Pas in the source area. The HHL are inferred to have been
mantlecomponentrepresents
a significantfraction,if not a
produced
by a meltingreactioninvolvingthe breakdown
of
dominantone, of the intrusive rocks. A consequence
of this
mantle involvementis that Cordillerangranitesspana wider
muscovite
with melt fractions in the source of about !0-15 %
range than do the HHL. This compositional
[e.g.,
LeForteta/., 198'/;Harris andlnger,1992].Therateof compositional
petrogenetic
stories
which
meltsegregationof a partially molten rock througha diversityimpliesmorecomplicated
compaction
mechanism
can be evaluatedwith the physical in turn open the pos:dbilityfor having more complex
behaviors
(i.e., differentcrystal/melt
ratiosduring
model
of McKenzie [1984]. Calculationsdone with a melt rheological
viscosity
of 104.5 Pa s and a porosityof 10% give extraction,ascentand eraplacement).Finally, it needsbe
compaction
timesof theorderof 105years
[Wickham,
1987; stressedthat the volume of magmaticmaterial involvedin
Laporte,1994]. Therefore,in view of the extremelyshort
'timesca!es
duringwhichmagmaascentandemplacement
occur,
thisresultindicatesthat compaction
aloneis probablynot an
efficient
processfor melt segregation
in the continentalcrust
[seeWickham,1987].
The second implication concerns the chemical
l•terogeneity
(e.g.,Sr isotopes)
of leucogranitic
magmas.
The
fluiddynamic
regimeof a magmaflowingin a dike canbe
assessed
throughthe Reynoldsnumberwhich is definedas
theseorogenicbelts(from 20% up to 90%, [Patersonand
Fowler, !993]) is much more importantthan that in the
Himalayanrange, where exposedplutonsrepresentno more
than 2% of this intracontinenta!belt [Le Fort eta/., 1987]. A
majorconsequence
of this1 orderof magnitude
of difference
in
the volume of intrusive rocks is the heat budget of the
orogenicbelt. Repeatedinjectionsof magmaticbodiesover
protracted
periods
associ•tted
withlargeandsustained
heatflux
at the baseof the crustmay affectthetheologicalbehaviorof
the mediumthroughwhich graniticmagmasascend.In this
[e.g.,
Jaupart
andAtl•gn',1991]
context, first eraplacedmagmasmay have emplacement
Re = pVaveW/11
(4) mechanism(s)
different(s)fromlatterones[seePatersonand
Fowler,
1993].
For instance,it can be envisaged
that fu-st
Taking
average
valuesof 104-5Pas and20 m formagma
magmas
willtraverse
a coldcrustthrough
dikes,
while
viscosity
anddikewidth,respectively,
thecomputed
Reynolds injected
number
is 5, which is well below the critical value of 2000
latter oneswill ericoutera hottermedium,be it a heatedcrustor
intrusion.
In thiscasethe
beyond
whichthe onsetof turbulentflow is predicted. a fully or partiallycrystallized
Increasing
thedikewidthupto themaximun
observed
in the viscositycontrastbevaeenthe intrudingmagmaand its
(i,e., lower)to a
f•eld
(50m) andtakingthelowest
magma
viscosity
found
in hostingrocksmay be morefavourable
the Gangotrileucogranites
thisstudy
(104.1Pa sitgivesa Reynolds
number
of 603. diapiricrise. In comparison,
a short-lived
magmatic
eventwithlimitedthermal
Therefore
thedynamical
regime
ofthemagma
flowing
throughrepresent
• feeding
system
of theGangotri
!accolith
canbepredictedeffectson the traversedterranes.At a largerscalethe spacing
tobedominantly
laminar.
Thelackof turbulence
means
that betweenthe differentmajorplutons(100-200km) belonging
thermal
interferences
between
them.
•e homogeneization
of anheterogeneous
batch
of magmatoHHL beltalsoprecludes
arcs,
.•rAng
itsuprise
proceeds
mainly
through
chemical
diffusion.Thus,in contrastwith whatprobablyoccursin magmatic
properties
of thecrustwerenotsignificantly
Th•veryslowrateof cationicdiffusion
in silicatemelts therheo!ogical
duringtheinjection
of theGangotri
magma
(notethat
compared
to the rateof magmaascentfoundin thisstudy affected
27,698
SCAILLETET AL.:HIMALAYANLEUCOGRANITE
VISCOSITIES
thismay not be entirelytruefor thelargestHHL plutonssuch
as the Everest-Makalu or Manaslu granites). In summary,
althoughtheGangotrileucogranites
validatedikingas a viable
mode of ascentfor silicic magmas,they do not excludeother
mechanismsof granitea,•ent anderaplacement.
Hogan,J.P.,andM.C. Gilbert,The A-typeMount
Scott
GraniteSheet:Importance
of crustal
magma
traps,
I.
Geophys.Res., 100, 15,745-15,765, 1995.
Holtz,
F.,H.Behrens,
D.B.Dingwell,
andW.Johannes,
H20
solubility
in haplogranitic
melts:Compositional,
premium
and
temperature
dependence,
Am.Mineral.,
80,94-108,
1995.
Acknowledgments. '['his studywas financiatedby grants
Inger,S., and N.B.W. Harris,Geochemical
constraints
from the IDYL researchprogramof the CNRS.The first glass
leucogranite
magmalism
in
the
Langtang
valley,
Nepal
cylinder usedto measurethe viscositywaskindly preparedby
Himalaya, J. Petrol., 34, 345-368, 1993.
Frank Schulze. An informal review done by Bernard Evans Jaupart,
C., andC.J. AlJ.•gre,
Gascontent,
eruption
rateand
greatly improved the quality of the manuscript.The official
instabilities
of eruptionregimein silicicvolcanoes,
Earth
Planet. Sci. Lett., 202, 413-429, 1991.
reviewsdoneby J.P. Hogan,D. Dingwell,andG. Bergantzare
gratefully acknowledged.
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