WalkerJ_JGR_95(B13)21483 - KU ScholarWorks

JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 95, NO. BI3, PAGES 21,483-21,502, DECEMBER
10, 1990
Geochemistryof CrustallyDerivedLeucocraticIgneousRocksFrom the Ulugh
MuztaghArea, NorthemTibet andTheir Implications
for the Formation
of the Tibetan Plateau
L. W. MCKENNA 1
Departmentof Earth, Atmosphericand PlanetaryScience,Massachusetts
Instituteof Technology,Cambridge
J. D. WALKER
Departmentof Geology,Universityof Kansas,Lawrence
Igneous rocks collectedfrom the Ulugh Muztagh, 200 km south of the northem rim of the Tibetan
Plateau (36ø28'N, 87ø29'E), form intrusive and extrusivebodies whose magmas were producedby
partial melting of upper-crustal,primarily pelitic, source rocks. Evidence for source composition
includes
highinitial
87Sr/86Sr
ratios
(-0.711
to0.713),
206pb/204pb
ratios
of 18.72,
207pb/204pb
of 15.63
and208pb/204pb
of38.73.Thedegree
ofmelting
in thesource
region
wasincreased
by
significant heating via in situ decay of radioactive nuclides; a reasonable estimate for the heat
production
ratein thesource
is 3.9x 10'6 W/m
3. Thecrystallization
agesandcooling
ages
[Burchfiel et al., 1989] of the earliest intrusive rocks within the suite suggestcrustal thickening
began in the northernTibetan Plateaubefore 10.5 Ma, with maximum averageunroofingrates in this
part of the Tibetan Plateaufor the period between 10.5 and 4 Ma at approximately< 2 mm/yr. The
Ulugh Muztagh flows are at the northernedge of a widely distributedfield of Plio-pliestocenevolcanic
rocks in the north-central Tibetan Plateau. The crustally derived rocks described here are an endmember componentof a wide mixing zone of hybrid magmas; the other end-memberforms mantlederived, potassicbasanitesand tephrites exposedin the central section of the Plio-Pleistocenefield.
The compositional trends in these belts strike east-west, at high angle to the N30E strike of the
Plateau itself. Considerationof the chemical data and publishedgeophysicaldata argue that the subPlateau mantle is mechanically detachedfrom the overlying continental lithosphere, and that in this
section of the plateau the thermal structureof the asthenosphereis not responsiblefor the formation
or maintenanceof the plateau's topography.
INTRODUCFION
allow inferences
to be madeon boththe composition
of the
Tibetan crust and the character of Miocene to Pliocene
The collision(between40 and 50 Ma) and subsequent
magmatism.Aspectsof the structuralgeologyin the Ulugh
Muztaghareaare discussed
by Burchfielet al. [1989].
continent
createdboththe HimalayanRangeandthe Tibetan The Tibetan Plateau is typically divided into three
Plateau[Molnar, 1988]. Althoughrecentyearshaveseenan structuralblocks; from north to south these are the Kunlun,
enormous
increasein our knowledgeof the geologyof the QiangtangandLhasablocks(Figure1; Changet al. [1986]).
greater Himalayan orogen, little is still known of the
The JinshaSuture,datedas end-Triassic[Harris et al., 1988],
geologyof the TibetanPlateauand its surrounding
areas. is the surfacetrace of the north-dipping[Harris et al., 1988]
Our limitedknowledgeis derivedprimarilyfrom teleseismicor south-dipping [Pearce and Mei, 1988] subductionzone
convergence of the Indian subcontinent and the Eurasian
data,a smallnumberof seismiclines,andremotesensing which separatedthe Kunlun and Qiangtangterrainsprior to
studies
of thearea[Molnar,1988]and,withtheexception
of collision. As describedby Burchfielet al. [1989], Ulugh
the Royal Society-Academica
SinicaGeotraverse
results,few
Muztaghlies astridea seriesof ophioliticfragmentswhich
directedfield observations
are available. The samplesof may mark the western extension of the Jinsha Suture into
igneousrocks discussed
in this report were collectedfrom
this area. The mean age of crustalmaterial in the Kunlun
the Ulugh Muztagh region of the north-central Tibetan
blocksome500 km eastof UlughMuztagh,alongthe Royal
Plateau(Figure 1) and providean opportunityto constrain SocietyGeotraverseroute, is constrained
by the isotopic
both the thermalstructureof the Tibetan crust and mantle,
composition and ages of syn-collision to post-collision
and the rates of crustal unroofing within the northern granitiods exposed in the Kunlun Mountains to be mid-
TibetanPlateau. In addition,analysisof thesesamples Proterozoic
[Harriset al., 1988]. Neodymium
modelagesof
sedimentaryrockscollectedalong the Geotraverseroute also
! NowattheDepartment
ofGeology,
University
ofKansas,
Lawrence
Copyright1990 by the AmericanGeophysical
Union
Paper 90JB01427
0148-0227/90/90JB-01427
$05.00
give mid-Proterozoic
ages. Accordingto the geological
map
of the Tibetan Plateau [Ministry of Geology and Natural
Resources, 1980], country rocks of the Kunlun terrain
includeCarboniferous
to Permianrocksjuxtaposed
with units
of Triassicand Cretaceous
age. Southof the suture,upper
Triassic [Burchfiel et al., 1989] to lower to middle
21,483
21,484
MCKENNA AND WALKER: GEOC'ttEMISTRYOF LECOUCRATICIGNEOUSROCKS
50ø/•
T
T
USSR
T
T
T
T
T
500krn
o
30
ø•
ß
ß
•
.
ß
....
75øE -L
INDIA
85o
ß
ß
ß
2oo
ß
CHINA
'
•
95 ø
"•
J.
•5 ø
105 ø
Fig. 1.
Locationmap and regionalgeologicsettingof Ulugh Muztagh ("GreatSnowy Mountain"), which is
situated 200 km south of the northern edge of the Tibetan Plateau, central Asia. Areas of Cenozoic
magmatismare shaded,basinsare stippled,strikeslipfaults are shownwith half arrows indicating relative
movements,thrust faults are shownwith barbson upper plate. Dashedlines with openbarb are suturezones;
from north to south they are the Jinsha,Banggongand Indus-ZangpoSutures,respectively. International
boundariesare shownby thin lines, degreesNorth latitude and East longitudeare also shown.
Cretaceous
sediments
form the majorityof the pre-Cenozoicleucitites,phonolitesand pyroxeneandesites;thesesamples
rocks. The apparentdiscrepancyof the age of the suture are discussed in more detail below (see Regional
(end Triassic)and that of rocks which it truncates(middle Relationships). Additional major element determinations
of
Cretaceous)
is probablydue to the poorlyknownagesof the a subset of these samples, along with new trace element
sedimentary
unitsin this largelyinaccessible
region.
data, were includedby Pearce and Mei [1988], who discussed
The broadstructure
of theTibetanPlateauas illustrated
by the major and trace element chemistryof volcanicrocks
geophysical
data was recentlyreviewedby Molnar [1988]. encounteredalong the 1985 Royal Society-Academia
Sinica
From his review of seismic data, Molnar concludedthat the Geotraverse
Route,approximately
500 km east of Ulugh
depth to the Moho in the Ulugh Muztagh region of the Muztagh.
north-centralTibetan Plateau(36ø28'N,87ø29'E)is some65
Country rocks in the study area proper consist of
+ 5 kin. While sucha crustalthickness
wouldgenerallybe metamorphosed,
openlyfolded,Triassicsedimentary
rocks,
consideredabnormallyhigh, it appearsto be 5 kin, and intrudedby granitoidrocksof Mioceneage [Burchfielet al.,
perhaps10 km, thinnerthan the crustof surrounding
areas 1989]. Theseintrusiveswere thoughtby theseworkersto
withintheplateau.Molnar [1988] alsoconcluded
thatupper be responsiblefor the andalusite-grade,regional-scale
asthenospheric
temperatures
for this area are higher than contact metamorphismof the country rocks in the area.
surroundingareas, and cautiouslysuggeststemperatures
at Mineralseparates
of theseintrusive
rocksgive40Ar/39Ar
the crust-mantleboundarymay be as high as 1300 K. The coolingagesof 10.5 to 8.4 Ma ([Burchfielet al., 1989], see
neotectonics
of this area of the plateauare dominatedby of this paper for a summaryof the geochronologic
data).
north-directed thrusting of the Kunlun Shan and Tibetan The
intrusive
rocks
are
overlain,
above a local
Plateauoverthe Tarim Basinat a rate estimated
by Molnar et unconformity,
by boulderconglomerate
overlainin turnby a
al. [1987] at 6 + 4 mm/yr.
sequence
of now dissected
extrusiverocks,principallyash
The earliest(and,until recently,the only) reportsof the flow tuffs and flows. These flows are dated at 4.0 + 0.1 Ma
geology of the area are those of Littledale [1886], (40Ar/39Ar,
biotite
fusion
andK-feldspar,
[Burchfiel
etal.,
Backstrom and Johanssen[1907] and Norin [1946]. 19891).
Somewhatmore recently, Deng [1978] reportedthe Field relationsallow divisionof the magmaticrocksinto
petrologyand major elementchemistryof a numberof threegroups: (1) intrusivesamplesBKSP, UBTG, 2MGR,
samples of Plio-Pleistocene volcanic rocks from a transect and QTD whichintrudethe metamorphosed
basement;
(2)
southof UlughMuztagh.Theserocksincluded
ultrapotassicsamples(definedbelow as the PotassiumPoor samples)
MCKENNAANDW,M.,KER:
GEOCItEMISTRY
OFLECOUCRATIC
IGNEOUS
ROCKS
21,485
which crop out as small plugs (UM10, KSPO) and dikes
(QTL) that intrude the Triassic sandstoneseast of the Ulugh
Muztagh; and (3) the capping Ulugh Muztagh extrusive
series(MV1B, UM1B, MV2, UMVU, UM3V, UMQP).
in Appendix 1. The extrusive samples, with the exception
of UMQP, all contain similar phenocryst assemblages of
Sampleswere taken as 1 to 3 kg blocks from outcropsor,
in the caseof samples UM10, BKSP, and UBTG, in moraine
and stream depositsbelow outcrops,using an ice ax as the
sampling tool. Weathered faces were removed with a
diamond cutoff saw, and cut faces were polishedwith SiC to
remove sawmarks. All sampleswere cleanedby boiling and
then ultrasoundingin deionized water, then dried and hand
crushed. Whole rock powderswere made from 50 to 100 g
splits using a tungstencarbide shatterbox.
Major and trace elements were determined by X ray
fluorescenceon a fully automated Rigaku X ray spectrometer
at the University of Kansas. Major elements were run as
fused glass beadsfrom whole rock powders,with Li2B40 7
flux; trace elements were run as powdered disks with
cellulose binder. Uncertainties, based on repeated analyses
of standards,are 0.5-1% for major elements, 1-2% for trace
elements with concentrationsgreater than 50 ppm, and 5-
syneruptionalcrystal sorting was not significant in the ash
flow tuffs, exceptfor UMQP which has no modal plagioclase
and exceptionallyhigh K20. The intrusive samples(BKSP,
2MGR, UBTG)
are dominantly hypidiomorphic to
panidiomorphicgranulargranites,which show some signsof
post crystallization strain. The typical assemblages is
potassiumfeldspar+quartz+plagioclase +biotite :krnuscovite
:kkaolinite(?). Visible accessoryphases are rare, generally
limited to anhederal, turbid (xenocrystic ?) zircons, and
prismatic, clear allanite. Samples QTL and UM10 are
quartz+plagioclase+tourmaline
(minor)
porphyry
trondhjemites with a microlitic quartz and plagioclase
groundmass. Sample KSPO is a granodiorite containing
coarse orthoclase phenocrysts+quartz+plagioclase with
medium-grained biotite and muscovite. Allanite is a
common accessorymineral.
10%
for trace elements
with
concentrations
less than 50
quartz+sanidine+plagioclase
(An20_40)+cordierite+biotite+
tourmaline (rare) in microlitic to glassy groundmasses.The
rocks occur as ash flow tuffs (UMVU, UM3V, MV2, and
UMQP) and rhyolitic flows (UM1B, MV1B). Similar modal
abundances for five of the samples suggests that
Major Elements
ppm.
Major elementcompositionsof the samplesare shownin
Isotope ratios and concentrationswere determinedthrough
isotope dilution techniques [Hart and Brooks, 1977]. Table 1. Silica contentsrange from 71.1 to 75.5 weight
Strontium
and
rubidium
data
were
collected
at
the
MassachusettsInstitute of Technology (on a 23 cm, 60ø
spectrometer)
and Universityof Kansas(VG Sector);samples
analyzed at both institutionsgive identical results within
analyticaluncertainties.Strontiumdata are reportedrelative
percent,Fe203* (all Fe as Fe203) from 0.15 to 1.5 wt %,
MgO from 0.0 to 0.25 wt %, and total alkaliesfrom 7.2 to
9.6 wt %. Oxide variation diagramsfor K20 and Fe203*
versus SiO2 are shown in Figures2a and 2b. The majority
of the samplesdefine a trend of approximatelyconstantK20
to NBS 987 standard
87Sr/86Sr=
0.71024,andarenormalized (5 wt %), and slightly decreasingFe203* contents(1.5-1 wt
relativeto 86Sr/88Sr=0.1194. Precisions
are 0.1% for Rb, %), over a SiO2 of 71.0 to 74.5 wt %. (As noted above,
0.03% for Sr and <0.008%for 87Sr/86Sr. Strontiumand syneruptionalsorting has apparentlyimpoverishedUMQP in
rubidium blank levels are insignificantfor the analyses plagioclase, resulting in high potash.) Three samples
reported. All Pb analyseswere conductedat the University (UM10, QTL and KSPO) do not follow this trend, showing
of Kansas,where blank levels for Pb analysesare 150-300 low K20 and very low Fe203* (as low as 0.2 wt %). These
pg.
All samples are fractionation corrected; the samples are henceforth referred to as the potassium-poor
fractionation
factorof 0.1 + 0.05%/amuwas determined
by samples.
repeated analysesof N'BS 982 common Pb standard.
Instrumental neutron activation analysis (INAA) was
carriedout at MIT; proceduresand analysistechniquesare
Trace
Elements
Trace element data are given in Table 2. All samples
(except
for QTL and UM10, which consistentlydiffer from
[1982]. High U concentrations
in the samplesrequired
corrections for La, Ce, Nd, and Sm due to interferences from the other samples) show high U, Rb, and Ba concentrations
reportedby lla and Frey [1984] and Lindstrom and Korotev
U fission products[Korotev, 1985]; correctionsare listed in
Appendix2. BecauseB has an exceptionally
largethermal
neutron capture cross section, tourmaline-bearingsamples
(QTL, UM10, andQTD) mayhavereceivedanomalously
low
neutron fluxes [King et al., 1988].
However, modal
tourmaline is low in these samples, and elemental
concentrationsof Rb determinedby INAA, XRF and ID
agreewithin uncertainties,
suggesting
that this problemis
not significantfor thesesamples. Reproducibilityof the
analysesare approximately
1-5 %; the uncertainties
quoted
with the data includeall sourcesof errors,includingU
corrections.
RESULTS
Petrography
The petrography of the samples are briefly described
below, summarized in Table 1, and described in more detail
and low
Sr and transition
metal
concentrations.
Rare
earth
element (REE) distributions(see Figure 3a) are consistently
light REE (LREE) enriched, concave upward, with large,
negative Eu anomalies and constant Yb concentrations.
Trace element concentrationsof samples QTL and UM10
vary inversely to those of the other samples:Sr is high, Rb,
Ba and U are low, and Pb is very low. LREE abundancesin
the potassium-poorrocks are generally lower than, and high
REE (HREE) abundancesapproximatelyequal to, abundances
in the remainder of the samples. Despite these differences,
the REE distributions of the potassium-poor samples are
grossly similar to those of the other Ulugh Muztagh
samples.
Sr and Pb IsotopicData
Isotopic data are shownin Table 3, and isotopecorrelation
plots for both Sr and Pb are presentedin Figures 4 and 5.
MCKENNA
ANDWALKER:
GE(X2ttEMISTRY
OFLECOU••C IGNEOUS
ROCKS
a
ß
00
21,487
0
0
I
b
00•
0
vvvvv
vv
v
v
vvvvv
v
!
uncertainty
vvvvvvv
7
7
7
7
7
7
v
SiOa(wt%)
Fig.2.
Wholerockoxidevariation
diagrams
for theUlugh
Muztaghsamples.On thisandall subsequent
diagrams,
symbols
are
opencirclesfor extrusiverocks;filled squaresfor intrusiverocks;
andfilled triangles
for potassium-poor
samples.(a) K20 versus
SiO2 (wt %) showsapproximately
constant
K20 overtherangeof
SiO2 for intrusiveand extrusivesamples;the potassium-poor
1F%
ß
samples
(triangles)
showverylow K20 at relatively
highSiO2.
Sample
UMQPplotsabove
thefield. (b)Fe203*(allFeasFe203)
versus SiO2 though scattered, exhibits a small decreasewith
increasing
silica;the UlughMuztaghsamples
are characterized
by
low transition-metal
contents,
includingFe, Mg, Cr, andNi.
vv
The trondhjemitic,potassium-poor
samples(UM10 and QTD)
have87 St/86 Sr of approximately
0.7118and87Rb/86Sr
less
than 1, while the remainder of the measuredsampleshave
higher87Sr/86Sr
ratiosof 0.71555(2)
to 0.71817(3)
(95%
uncertaintiesin the last digit quotedfor all measurements
are
shown by the numbers in parentheses). Variations in Pb
isotopecompositionsof the Ulugh Muztagh samplesare the
inverse of the Sr isotope variations (Figure 5), with
potassium-poor
samples
showing
higher206pb/204pb
and
208
204
slightly higher
Pb/
Pb than the remainder of the
samples; the latter cluster within uncertainties at
206pb/204pb
= 18.73(5), 207pb/204pb= 15.63(5)and
208pb/204pb
= 38.74(10).
vvvv
vvvvv
vv
ß
21,488
MCCA
ANDWALKER:
GEOCHEMISTRY
OFLECOUCRATIC
IGNEOUS
ROCKS
lO
a
I
I
I
I
I
I
I
I
I
Element
lOOO
,-
10
a.
1
i
i
I
i
I
i
I
i
I
I
o
PAAS
o
MV1B
-
KSPO
I
I
I
i
i
i
i
i
-_
I
I
i
b
UBTG
- - •-
I
i
-
I
UM Source
I
I
I
I
I
Element
Fig.3. Chondrite
normalized
[Anders
andEbihara,
1982]REEabundances
fortheUlughMuztagh
samples.
Datain Table2, symbols
asin Figure
2. (a) All samples
show(La/Yb)cn
greater
than10;large,negative
Eu anomalies;
andsimilarYb andLu contents.Extrusive
rocksshowlittle variation,intrusive
rockshave
generally
lowerLREEabundances
thantheextrasire
rocks.Potassium-poor
samples
varyfromrelatively
LREE-enriched
(KSPO)to LREE-poor
(UM10,QTL); HREEcontents
for all threesamples
are roughly
constant.
(b) Representative
REE abundances
for thethreesamples
groups,
the post-Archean
Australian
Shalecomposite
[NanceandTaylor,1976](PAAS,opensquares
withsolidline),and0.3-PAAS
(open
squares,
dotted
line). A possible
model
fortheREEabundances
in theUlughMuztagh
source
region
is given
by the 0.3-PAASline. Seetext for discussion.
MCKENNA AND WAIXER: GEOCttEMISTRYOF LECOU•TIC
IGNEOUSROCKS
21,489
21,490
MCKENNAANDWAIXER:GEOCHEMIgrRY
OFLECOUCRATIC
IGNEOUS
ROCKS
0.720
Extrusive rocks.
Field relationships of these units
indicate that they were extruded approximately
synchronously,an observation supportedby the similar
ß typical
uncertainty
0.718
40Ar/39Arages(biotiteand K-feldspar)
for two of the
ß
samples(MV2 and UM1B) of 4.1 + 0.1 and 4.0 + 0.1 Ma
[Burchfiel et al., 1989]. The major elementvariationsof
these rocks are similar to those producedexperimentallyby
meltingof pelitic schist. The K20 versusSiO2 andA1203
0.716
0.714
'
I
'
I
"
I
'
I
"
I
38.9
0.712
0.710
0
10
20
30
87
4O
50
38.7
86
Rb/
Sr
Fig. 4. Sr isotopecorrelationdiagramfor the Ulugh Muztagh
samples.Symbolsas in Figure2; 2o uncertainties
are smallerthan
the symbols. Althoughthe samplesdo not define an isochron,a
referenceline with a slopeequivalentto an age of 11 Ma is shown.
38.5
typical
uncertainty
Thepotassium-poor
samples
have
verylow87Rb/86Sr
ratios
and
indicate their initial ratios of-0.7117.
The intrusive and extrusive
samples
have
similar
contemporary
87Sr/86Sr
ratios,
despite
the
,
38.3
differencein crystallization
timesof at least4-6.5 Ma [Burchfietet
18.0
al.,1989].Average
initial87Sr/86Sr
ratios
fortheintrusive
and
I
I
i
I
•
20.0
206
extrusive rocks (corrected for an age of 10.5 and 4.0 Ma,
respectively)
are 0.7123(7) and 0.7154(5).
I
19.0
I
21.0
204
Pb/
Pb
20.5
I
DISCUSSION
Petrogenesisof the Ulugh Muztagh Samples
20.0
typical
uncertainty
We hypothesize below that the composition and
mineralogyof the extrusive and intrusive samplegroupsare
consistentwith derivation by partial melting of chemically
19.5
similar pelitic or psammiticsourcerocks. Ideally, evidence
for such an argument would include observations and
analysesof the restitic material. The Tibetan Plateauis,
however, hardly an ideal place in which to conductfield
19.0
research, and such observations and samples are not
available.
Therefore we support our hypothesis by
comparing the compositionof the Ulugh Muztagh samples
to the expected composition of liquids producedby partial
18.5
,
I
,
I
,
I
,
melting of crustal material. The characteristicsof partial
0.7100
0.7125
0.7150
0.7175
0.720
melting of pelitic rocks precludes detailed petrogenetic
modeling of the processbecause(1) a wide variety of phases
are potentially stable in the restitc; (2) the apparent
andPb-Srisotope
correlation
diagrams
for the
partition coefficientsfor many of thesephasesare poorly, if Fig. 5. Lead-lead
at all, constrained, and; (3) the apparent partition Ulugh Muztaghsamples,symbolsas in Figure 2, typical
(2o) areshown.(a) 208pb/204pbversus
coefficients of these phases can vary strongly with liquid uncertainties
plotshows
theextrusive
samples
andallbutoneof
composition in highly silicic liquids. Instead, we make 206pb/204pb
quantitative arguments on the composition of the source the intrusive samples (QTD) plotting within uncertainty at
= 18.72
and208pb/204pb
= 38.73.
Potassium-poor
area, the degree of partial melting in the sourcearea, and the 206pb/204pb
samples QTD and UM10 (triangles) have considerably higher
temperatureof the system during melting by comparingthe
206pb/204pb
ratios,
duetotheirverylowPbcontents
of<2ppm.
composition of the melts to those expected through partial With sampleKSPO, the potassium-poorsamplesdefine a trend that
melting of crustal materials. We then present evidence that terminatesat the primary extrusive-intrusive
group. (b) The plot of
the potassium-poorsampleswere producedby a combination 206pb/204pb
versus
87Sr/86Sr
shows
variable
87Sr/86Srwith
of partial melting of amphibolitic or tonalitic source rocks, constant
206pb/204pb
for boththeintrusive
andextrusive
rocks.
along with variable addition of a potassium feldspar Thepotassium-poor
samples
scatter
athigh206pb/204pb
andlow
componentto the liquid. Each sample group is discussedin 87Sr/86Sr.
QTDappears
anomalous,
withhigh206pb/204pb
and
turn, beginning with the extrusive rocks.
lower87Sr/86Sr.
87Sr/86Sr
MCKENNA
ANDWAIXER:G•MISTRY
OFLECOUCRATIC
IGNEOUS
ROCKS
versus SiO2 variations for the extrusive samplesare shown
superimposedupon the 10-kbar, "pelitic, fluid absent "melt
compositionsof Vielzeuf and Holloway [1988] in Figure 6.
These compositions represent partial melts of a quartzplagioelase(An30)-kyanite-museovite-biotitegarne•staurolite+chlorite source with a pelitic bulk
composition,melted over a temperaturerange of 1150 K to
1520 K. In these experimentsthe SiO2 contentof the melt
decreasesmonotonicallywith increasingtemperature. Also
shown in these figures are the water-saturatedhaplogranite
21,491
6
ß
5
a
oco•OO
•
'
4
minima of Nekvasil [1988], calculated at 5 and 10 kbar.
As shown in Figure 6, the composition of the Ulugh
Muztagh extrusive rocks is distinctly different than
compositions of haplogranite melts, as modeled by
Nekvasil's calculated compositions [Nekvasil, 1988]. As
noted by Nekvasil [1988, p. 979], "partial melts derived
from source regions with relatively high H20 contents
shouldcluster aroundthe haplograniteminimum composition
unless the An[orthite] content of the source area is high."
Because the Ulugh Muztagh extrusive rocks do not cluster
about any composition,we infer that the rocks were derived
either from an H20-undersaturatedor anorthite-rich source
2
1
0
area.
17
Further insight into the chemical composition of the
sourcearea is gained by comparingthe experimentalpelitic
meltsof Vielzeuf and Holloway [1988] to the Ulugh Muztagh
data. The Ulugh Muztagh extrusive rocks show a smooth
variation in K20-A1203-SiO 2 composition space that
parallels, although is not coincident with, the
0
15
ß
o
experimentallyderived melt composition. Higher potash
ß
o
o ß
lO kb
contentsin the Ulugh Muztagh samplescould be due to
small differences
14
in anorthite contents of the source area.
5 kb
The calculations
of Nekvasil [1988] indicatethat increasing
the An contentof plagioclasein the sourcefrom 30 to 50
mole percentwould causean increasein both quartz and
orthoclase
components
in the melt. The effect of adding5
wt % quartz and 5 wt % orthoclase to one of the
13
,
65
I
70
,
I
75
,
80
SiO
2(wt%)
experimental
melts is illustratedby the arrow in Figure 6.
The resultantchangein melt compositionis sufficient to
explain the differences in potash contents between the
Fig. 6.
Oxide variation diagrams for laboratory melting
experimental
liquidsof Vielzeufand Holloway[1988] andthe experimentsof Vielzeuf and Holloway [1988] (shadedfield) and the
Ulugh Muztagh extrusive rocks. The lower alumina contents calculated dry haplograniteminima of Nekvasil [1988] at 5 and 10
in the Ulugh Muztagh samplesrelative to the Vielzeuf and
Holloway [1988] liquids is also partly explainedby
increasesin the quartz and orthoclasecomponents
in the
melt. The sensitivity of the alumina compositionto
pressurechanges(as shownby Nekvasil'scalculatedminima)
suggests
that the pressurein the sourcearea may also have
influencedthe aluminacontentof the UlughMuztaghmelts.
kbar. The data from Vielzeuf and Holloway [1988] representthe
measuredcompositionof a melt producedby fluid-absentpartial
1200 K and 60%, respectively. This value for F v is an
approximation
only and probablyrepresentsan upperlimit
estimatefor the actual degreeof partial melting in the
source. An estimateof the relativedifferences
in F v for the
extrusiverocks can be made by assumingthat La acted
arrowleadingfromthe shaded
field. (b) TheA1203versusSiO2 for
datafrom Vielzeufand Holloway [1988] demonstrates
the relatively
melting
(at10x 108Pa)ofa pelitic-composition
source
rock;
SiO
2
decreases
with increasingdegreeof melting. Superimposed
upon the
melt data are the data for the Ulugh Muztagh samples(symbolsas in
Figure2). (a) K20 versusSiO2 illustratesthe similaritiesin bulk
compositionof the experimentallydeterminedmelts and the Ulugh
Muztagh intrusive and extrusive samples; the potassium-poor
This discussion
indicatesthat the Ulugh Muztaghmagmas samples (triangles) show no relationship to the partial melt
have compositionsconsistentwith derivationby partial composition. The potash composition of the Ulugh Muztagh
samplesis parallel to, but greaterthan, the trend of the Vielzeuf and
melting of a source of pelitic composition:the SiO2 Holloway data. This differenceis probablydue to higheranorthite
contents
of the magmaswouldthencorrespond,
according
to contentsin the Ulugh Muztagh source area, which increasesmelts
the dataof Vielzeuf and Holloway[1988], to a temperaturequartzand K-feldsparcomponents.The changein melt composition
and volumefractionof melt (Fv) of approximately1100- due to additionof 5 wt % quartzand K-feldsparis shownby the
low alumina contentsof the Ulugh Muztagh samplescomparedto
the10x 108Pamelts.Notetheslight
increase
in A120
3 with
decreasingSiO2 in the Ulugh Muztagh extrusivesamples,a trend
which parallels that in the experimentalmelts. The differencesin
completelyincompatiblyduring melting, in which case the alumina may be due to lower pressureof fusion in the Ulugh
relative ratio in Fv is equal to the ratio of La concentrations Muztagh sourceregion.
21,492
MCKENNA
ANDWALKER:
GEOCHEMISTRY
OFLECOUCRATIC
IGNEOUS
ROCKS
in the samples.The extremeLa ratiofor the extrusive
rocks intrusiverocks range from <1040 K to 1150 K. Becausethe
degree of partial melting within a source of pelitic
is 1.2, suggesting
that the variationin Fv was small.
Additional evidencethat the extrusiverocks were produced composition changes dramatically within this temperature
by meltingof a peliticsource
is provided
by theSr andPb range (10-60%, [Vielzeuf and Holloway, 1988]), it is
isotoperatios of the samples. The Pb data are shown possible only to suggest a broad range of Fv, from 15 to
superimposedupon Zartman and Doe's [1981] Pb 60%. Again assuming that the LREE acted completely
composition
diagrams
in Figures7a and7b. All extrusiveincompatiblyduring melting, the relative differencein F v is
samplesplot tightly within or adjacentto the "Pelagic small, <1.1.
Lead and strontium isotope ratios of the intrusive rocks
also
argue for a source region of pelitic composition.
externallyto fields containing80% of measuredupper
crustal,lowercrustalandmanfiederivedrocks. The measured Strontium isotope ratios for some of the intrusive samples
87Sr/86Srvaluesfor the extrusive
samples
rangefrom must be recalculatedto accountfor postcrystallizationdecay.
Crystallization ages for these rocks are not available:
Sediments" field in both variation diagrams, and plot
0.71636(2) to 0.71735(8); recalculatingthese ratios to
however,
threesamples
havebeendatedby the40Ar/39Ar
correctfor postcrystallization
decay(basedon a 4.0 + 0.1
and give ages of 10.5 + 0.1 Ma (UBTG, Muscovite
Ma agefrom40Ar/39Ar
geochronology
[Burchfiel
et al., method
plateau)to 8.4 + 0.1 Ma (QTD, K-feldspartotal gas). Thus a
1989]givesa source
87Sr/86Sr
of 0.7154(5).Thishigh minimum crystallization age for the intrusive suite is 10.5
value falls well within the "crustal"field of Faure's [1986]
Rb/Srevolutiondiagram(his Figure10.4), andrequiresa
sourceareawith a hightime-integrated
Rb/Srratio.
Miller
Ma, and recalculationof initial isotope ratios basedon this
age will representmaximum estimatesfor the initial ratios.
[1985, p. 674] described12 "Criteria for Age-corrected Pb isotope ratios are not considered
significant enough (<0.05) to require their use in the
Identifying
PeliticParentage
of Igneous
Rocks,"
reproduced
discussion. Lead isotope data are presentedin Figure 7,
hereasTable4, whichincludesfive criteriainvolvingmajor
superimposed upon Zartman and Doe's [1981] Pb
compositiondiagram. With the exceptionof sampleQTD,
involvingisotoperatios. With the datadescribed
above, the intrusiverocks plot, like the extrusiverocks, within or
only eight of thesetests are possible. Comparisonof the
elementchemistry,
four involvingtraceelements,
andthree
criteria in Table 4 with the data in Tables 1 through4 shows
adjacent
to thepelagic
sediments
field. Initial87Sr/86Sr
that all pelitic parentagecriteria are met or surpassed
by the ratios range from 0.7123 to 0.7133, and are higherthan the
extrusiverocks (with the exceptionof UMQP) collectedfrom 87Sr/86Sr mantleevolutioncurveof Faure [1986],
Ulugh Muztagh. We concludethat althoughour knowledge suggestingthe intrusivemagmasformed by melting of a
of source area chemistry and the chemographyof melting source with "crustal" composition. Comparison of the
relationships is imprecise, the compositionsof the Ulugh Miller's [1985] criteria in for pelitic parentagein Table 4
Muztagh extrusiverocks are consistentwith derivation as a with the data in Tables 1 through 4 show that all pelitic
partial melt of a pelitic sourcearea. The degreeof partial parentage criteria except for normative corundum and
melting is broadly constrainedat 50-60%, at a temperature slightly high Na20 in QTD are met or surpassedby the
of 1100-1200
K.
intrusiverocks collectedfrom Ulugh Muztagh.
cooling
agesfortheintrusive
andextrusive
Intrusive rocks. The geochemical,geochronologicaland The40Ar/39Ar
rocks
exposed
at
Ulugh
Muztagh
imply
minimum
petrologicaldata presentedabove suggestthat the extrusive
rocks analyzed in this study form a genetically related, emplacement ages for these units of 10.5 and 4.0 Ma
comagmaticseries. However, the relationshipbetweenthese respectively. Despite these age differences, the extrusive
extrusive rocks, the intrusive rocks (UBTG, QTD, 2MGR, and intrusive samples plot together on the Sr isotope
and BKSP) and the potassium-poorsamples(KSPO, UM10, correlation diagram in Figure 4. Correcting for their
and QTL) exposed in Ulugh Muztagh is less certain. respective ages, the intrusive and extrusive rocks have
average
initial87Sr/86Sr
ratiosof 0.7123(7)
and
Evidencefor a pelitic sourcearea for the intrusivesamplesis distinct
suggestedby evidencesimilar to that used for the extrusive 0.7154(5), respectively. The scatter in the initial ratios
samples. The K20 and A120 3 variations for the intrusive may reflect $r isotopeheterogeneityof the sourcearea; such
rocks are illustrated in Figure 6 along with the heterogeneityin leucocraticrocks formed by partial melting
experimentally determinedpelitic partial melts of Vielzeuf of sedimentsis common, as discussedby, amongstothers,
and Holloway [1988]. The intrusive rocks have similar to Le Fort [ 1981].
higher SiO2 and lower A1203 and K20 than the extrusive Potassium poor samples. The three potassium-poor
rocks. By comparison to the experimentally produced samples(KSPO, UM10, and QTL) differ substantiallyfrom
liquids of Vielzeuf and Holloway [1988], temperaturesof both the intrusive and extrusive rocks discussed above. The
formation for the magmasrepresentedby the Ulugh Muztagh most striking differences, seen in Figures 2a and 2b and
TABLE4. Criteria
forIdentiftinl•
PelticSource
Areas
Major'Element
Concentration
Trace
Element
Concentrtion
Paragenesis
(wt%)
quartz
Na20
-3.5 - 4
Rb
> 100
A12SiO5
CaO
< 2
Sr
< 300 - 400
+ cordierite
SiO2
> 65
Ba
< 600 - 1000
+ garnet
Norm C*
>5
Rb/Ba
> 0.25
+ muscovite
Modified from Miller [1985, Table 3]. Sr-Nd isotoperelationsare not shown.
* Nonnative
Conmdum
Isotopic Composition
(ppm)
87Sr/86Sr> 0.701
180/16
O > 11-12%o
MCKENNAAND WALKER:GEOCHEMISTRY
OFLECOUCRATIC
IGNEOUSROCKS
21,493
15.9
a
Pelagic
Seds. M
Upper
Crust
15.7
15.5
Lower
Crust
15.3
15.1
40
I
I
!
I
I
b
UpperCrust
M
Pelagic
Seds.
Arc
Oceanic Volc Rocks
Lower
Crust
37
16.5
I
17.5
206
19.5
20.5
204
Pb/
Fig. 7.
I
18.5
Pb
Leadisotopedatafor the UlughMuztaghsamples(typical20 uncertainties
are shownin Figure5)
superimposedupon the reservoir summary diagram of Zartman and Doe [1981]. Solid lines enclose
approximately80% of all data points derived from each reservoir,including "probableaveragevalues"for
pelagic sedimentsof Mesozoic and Cenozoic age. Range of whole rock Pb ratios for the Manaslu
leucogranite
(Nepalese
Himalaya
[Vidal
etal.,1982]
areshown
byboxlabeled
"M."(a) The207pb/204p
versus
206pb/204pb
data.Allsamples
except
forQTL,UM10(triangles)
andQTD(solid
square)
plotwithin
or adjacentto the Pelagic Sedimentsfield. As noted in text, correctionfor postcrystallizationdecay would
probably
relocate
QTLandUM10to withinthePelagic
Sediment
field. (b)The208pb/204pb
versus
206pb/204pb
dataalso
plotwithin
thePelagic
Sediments
field,except
forsamples
QTL,UM10(triangles)
and QTD (solid square). As notedin the text, correctionof QTL and UM10 ratiosfor postcrystallization
decaywouldrelocatethe samples
to withinthe PelagicSediment
field.
Tables2 and 3, are the low to very low K20, Fe203, MgO, examination and repeated staining of slabs failed to show
Pb and Rb contentsof the potassium-poor
samplesat SiO2 any modal K-feldspar in these two samples, while the
contentsequivalentto the remainderof the Ulugh Muztagh relatively K20-rich, granodioritic KSPO has 13.5 wt %
samples. As noted in Appendix 1, samplesUM10 and QTL normative K-feldspar. The chemical relationship between
are trondhjemites composed entirely of porphyritic these samples and the remainder of the Ulugh Muztagh
plagioclase,quartz, and rare tourmalinein a microcrystalline samples is investigated in Figure 8, a Pearce-typevariation
groundmass
of the sameminerals,with accessoryapatiteand diagram [Nichols, 1988] displaying the variation of 3K/Yb
very rare zircon. Although these sampleshave 0.8 and 2.6 versus Si/Yb. In this diagram, addition of componentswith
wt % normarive K-feldspar, respectively, thin section a cation ratio K:Si of a:b has slope 3 a/b; thus the trend
21,494
MCKENNAANDWALKER:GEOCttEMISTRY
OFLF•OUCRATIC
IGNEOUS
ROCKS
1.0
.
[
'
I
'
I
'
largestresiduals
arein A1203,CaOandNa20, dueperhaps
to
I
removalof a plagioclasecomponentfrom the QTL or UM10
liquid. Thishypothesis
wasnot testedbecause
theresulting
0.8
model would have been overdetermined.
0.6
Thesemassbalancecalculations
requirethe subtraction
of
12 and 13 wt % K-feldsparfrom KSPO to producethe
compositionof QTL and UM10, respectively. Such a
dramatic
changein bulkcomposition
shouldbe accompanied
by a parallelchangein the concentration
of traceelements;
Ksp, Mica
0.4
thosecompatible
with K-feldspar
shouldbe greatlydepleted
in QTL andUM10 in comparison
to KSPO. Thishypothesis
could be testeddirectly by analysisof K-feldsparand
plagioclaseseparatesfor thesesamples,but becausethese
dataarecurrently
lackingwe usea morequalitative
approach
of comparing
wholerockRb andSr contents
to changes
in
total normativeweightpercentK-feldsparandplagioclase
•V&
0.2
Qtz, Pig
0.0
I
2.4
I
2.6
I
I
2.8
3.0
3.2
3.4
Si/Yb
Fig. 8. A Pearce-typeelementratio diagram[Nichols, 1988] for K
and Si. On this diagram, trends controlledby minerals with K:Si
ratios of 1:3 (K-feldspar, micas) will have a slope of unity, as
shown by line labeled "Ksp". Addition or subtractionof phases
with K:Si of 0 will have a slope of zero, as indicatedby the line
labeled "Qtz, Plg." Symbolsas in Figure 2. Data for intrusiveand
extrusive groups fall approximately on a common line with slope
0.2 + 0.09 (2o); this line has a nonzero intercept at a 90%
confidence
level [seeNichols, 1988]an r2 of 0.81,andis
significant(nonzero)abovethe 99% confidencelevel. Data for the
threepotassium-poor
samplesdefine a line of slope0.9 + 0.7 (2o);
betweenUM10 and QTL, as normalizedrelative to KSPO.
Because
Rb and Sr are, relativeto one another,strongly
partitionedinto K-feldsparand plagioclase,
respectively
[Nashand Craecraft,1985],andotherphases
in the rocks
(quartz and minor tourmaline)have small massesof these
elements, a large fraction of the whole rock Rb and Sr
shouldresidein the K-feldsparand plagioclase
phases,
respectively. This comparisonis shownin Table 5 and the
results,thoughvery qualitative,supportthe hypothesis
that
the chemicalevolutionof the potassium-poor
rockswas
controlledby removal of K-feldspar. Changesin Rb
contentsin QTL and UM10 measuredrelative to KSPO are
very similar to the relative K-feldsparcontents,while the
ther2 of 0.98is significant
at the95%confidence
level,althoughchange
in relativeSr contents,
thougha factorof 2 greater
the regressionitself is significantonly at the 90% confidencelevel.
The 3K/Si ratio of the experimentsof Vielzeuf and Holloway [1988]
(and hence the slope on the Pearcetype diagram used here) for the
SiO2 range sampledby the Ulugh Muztagh rocksaverages0.203 +
of change. We concludethat removalof K-feldsparand
0.003, and falls to a value of 0.134 at an SiO2 of 66 vet%. Thus
formation
for thepotassium-poor
samples,
eventhough
we
than the relative plagioclaseincrease,show the samesense
quartz from an initial melt was the dominant mechanismof
the slope of the intrusive and extrasire line in this projection is
are not able to offer a testablehypothesis
to explainthe
consistentwith derivationby melting of a pelitic source. The 3K/Si mechanismby which this removal occurred.
ratio for a granite minimum melt is 0.24, which is greaterthan that
Otherscenarios
for theproduction
of thepotassium-poor
derived from the Ulugh Muztagh at greaterthan a 95% confidence
interval. The slope of the trend line for the potassium-poor samplescould includelate weathering
of an initially
samples, although subject to great uncertainty, is consistentwith
potassium-rich
rockor alteration
of an initiallypotassiumthe unit slope of the trend line expectedif removal of K-feldspar
richrockby metasomatic
fluids. The trendsin Figure8
from the system was the dominant mechanism for chemical
differentiation.
argues against the metasomatism model: such alteration
wouldhavehad to removenot only potassium,
but alsoA1
and Si in the ratio expected
for K-feldspar,
an unlikely
Furthermore,
thereis little petrographic
defined by addition of K-feldspar (as well as biotite or happenstance.
evidence
of
plagioclase
replacement
of K-feldspar
in samples
muscovite)has a slopeof unity, while the slopefor quartz
UM10
and
QTL.
Although
late
stage
weathering
of the
and plagioclaseis zero. Figure 8 clearly illustratesthat the
potassium-poor
sampletrend KSPO-QTL-UM10 could be
producedsimplyby removalof K-feldspar(or mica) from a
liquid. A simpleleastsquaresregression
of this trendgives
a slopeof 0.9 ñ 0.7 (2s) and while the uncertaintiesare
clearly enormous,additional information is available to
potassium-poor
samplescouldexplainthe odd Pb/U ratio in
thesesamples
(seebelow),thesesamples
wereamongthe
freshestwe analyzed,and no petrographic
evidencefor
alterationis presentin the samples.
Samples
QTL andUM10 haveveryhigh206pb/204pb
ratios,but 207pb/204pb
and208pb/204pb
thatareonly
support this conclusion.
A more detailed examination can be carried out through slightly higher than those of sample KSPO. The Pb ratios
mass balance calculations. Whole rock contents of SiO2, of the latter sample are approximatelyequal to thoseof the
A1203,CaO,Na2¸ andK20 in QTL andUM10 werealtered intrusive and extrusive samples collected from Ulugh
in 206pb/204pb
ratios
by additionof orthoclase
(Or) and quartz(Qz) to give the Muztagh.Partof thediscrepancy
bestfit to the KSPO compositions.Resultsand assumptionswithin the potassium-poor samples is due to
decayof 238Uin samples
UM10and
are given in Table 5. KSPO can be manufactured
from postcrystallization
mixtures
of
0.8 1QTL+0.12Or+0.07Qtz
and QTL, which have extremely low Pb contents (less than 2
0.8UM10+0.13Or+0.07Qtz(weight fraction). These models ppm for both). Unfortunately, the Pb analysesfor UM10
havehigh,but not terriblyhighZ2 of 36 and16.3, and QTL are not sufficiently accurateto allow correctionfor
respectively,with two degreesof freedom. In both casesthe this decay, and thus their initial lead isotope ratios are not
MCKENNA AND WALKER: GEOCHE••Y
OF LF•OUCRATIC IGNEOUSROCKS
21,495
TABLE 5. Mass Balance Constraints on the Formation of the
Potassium
PoorSamples
Major Element Constraints
ModeledCompositions
Final(KSPO)
QTL
KSPOmodeledas0.80 QTL+
SiO2
74.84
74.09
74.69
A1203
14.13
14.69
14.13
CaD
0.42
1.20
0.96
4.80
7.48
5.98
2.55
75.50
76.09
15.10
14.27
0.68
0.53
7.16
5.57
0.23
2.51
0.54
2.63
0.13Or+0.07
Qz (•2=35.7)
UM10
KSPO modeled as 0.778 UM10+
0.138
Or+0.084
Qz (Z2=9.8)
Trace Element
Sample
Constraints
Plag,wt
Ksp,wt
%
%
KSPO
41
QTL
Fractionalchange
67
1.7
UM10
62
Fractional
chanse
15
1.5
St, ppm Rb, ppm
54
388
3.2
0.2
171
3.2
49.7
0.13
1.4
135
21.7
0.09
2.5
0.06
Z2 values
inthetable
arecalculated
assuming
fractional
(notwt%)
uncertainties
of 1% for SiO2 andA1203and10%for otheroxides.Thefractional
changerowsin the bottomtableare calculated
by dividingthe quantityfor QTL or
UM10 by the corresponding
quantitiesin KSPO. In this sectionof the table,wt
% are nonnative, not modal, percentages.
in the hanging wall of thrust faults by release of fluids from
footwall rocks ("flux melting") [LeFort, 1981]; and in situ
radioactive decay and mantle heating in thickened
continental crust [Molnar et al., 1983]. The amount of melt
producedby "flux melting" is probably minor without the
input of additional heat and is not consideredfurther here.
intrusive
andextrusive
samples.
The87Sr/86Sri
forsamplesShear heating, while capableof producinglarge quantitiesof
QTL and UM10 is approximately equal to their measured heat obviouslyrequiresthe existenceof a fault in the source
ratios due to the low Rb/Sr ratio of the rocks, the samples area, and while some Chinese workers have suggestedthrust
may underlie
average 0.7118(1).
This ratio plots well within the faults of Cenozoicage with large displacements
"crustally derived" field of Faure [1986], suggestingthat the Ulugh Muztagh region (B.C.
Burchfiel, personal
these magmas were derived from a source with crustal communication, 1988), little detailed information is
composition.
available to constrainheat productionby this mechanism.
Heat productionin the Ulugh Muztagh sourceregion from
In Situ Melting of Source Rocks
in situ radioactive decay can be estimated from the
A necessary (although not sufficient) requirement for a radionuclide concentration in the extrusive rocks, and their
crustalsourcefor the Ulugh Muztagh magmasis that the P-T- estimated degrees of partial melting, F v. Assuming bulk
X conditions within the crust were sufficient to produce a distributioncoefficientsfor Th and U of 0 during melting, an
partial melt. A number of mechanismshave been proposed F v of 0.2 to 0.6, K20 concentrationin the sourceof 4-5 wt
density
of 2.8 x 103kg/m
3, the calculated
for in situ melting of crustal rocks, including frictional %, anda source
heating along thrust faults [LeFort, 1975]; melting of rocks heat productionin the source would range from 1.7 to 4.6
well constrained. If, as suggestedabove, KSPO, QTL and
UM10 are comagmatic,than the Pb isotoperatios for KSPO,
which has not changed significantly since crystallization,
should give the initial ratio for all three potassium-poor
samples:this sampleplots within the PelagicSedimentfield
in Figure 7, althoughit does appear to be distinct from the
TABLE
'
6.
' Minimum
U
7h
4
5
K20
2.6
Source-Rock
Concentrations
BestEstimate
Concentration*
10
12
" Maximun•
12
14
3.6
4
HeatProduction
t
•tW/m
3
1.7
3.9
HGU
4.0
9.4
4.6
11
* 'Uand"•Th
concentrations
inpans
Permillion,
K20inwt%,andare'
basedon U and Th concentrations
of the Ulugh Muztaghvolcanicrocks,
dividedby degreesof partial meltingof 0.2 (minimum),0.5 (bestestimate)
and 0.6 (maximum), and assumeddistribution coefficients of 0 for both Th
and U.
, Heatproduction
assuming
a density
of2.8x103kg/m
3. Heat
productionratesfrom Turcotteand Schubert[1982]
•
21,496
MCKENNAANDWALKER:GEOCI•MISTRYOFLECOU•TIC
!.tW/m
3. Detailsof thecalculations
aregivenin Table6.
While these rates of heat production are higher than those
generallyquotedfor leucogranitesourceareas[e.g.,Pinet and
Jaupart, 1987], these estimatesrepresentconservativelimits
for the Ulugh Muztagh sourceregion: while decreasingFv
would cause a linear decreasein the heat production rate,
increasing the density of the source or the distribution
coefficients for Th and U would increase heat production
within the source. The presenceof stable zircon in the
restitc would tend to increase the apparent partition
coefficient of U in the system, and the model described
above would then underestimate the source U concentration,
IGNEOUS
ROCKS
for the Ulugh Muztagh intrusive and extrusiverocks, leading
to temperature increasesof 70 to perhaps 200 K. These
thermal perturbations can be maintained for long time
scales, due to the long half-lives of the radionuclides.
Withdrawal of the radioactive nuclides by partial melting,
such as invoked for the intrusive and extrusive rocks, would
reduce (to zero, with the assumptions above) the
concentration of the radionuclides, effectively halting the
temperature increase by this mechanism. For nonzero
apparent partition coefficients, residual U and Th would
remain
in the source and could continue
to heat the restitc.
The pressure-temperature-composition
conditionsnecessary
to producepartial melts in pelitic rocks to form magmasof
granitic compositionhave been discussedby a number of
authors [Hyndman, 1981; Thompson, 1982; Vielzeuf and
Holloway, 1988]. Figure 9 illustratesthe approximateP-T
loci of water present and water absent liquidii for pelitic
compositions,along with granite solidii and approximate
steady state geotherms for the Ulugh Muztagh region.
M'mimumtemperatures
necessaryto producepartial melts are
and hence the sourceheat production. The inferred Th and
K20 contentsof the source region are similar to that of
average shales, while our "best estimate" for the U
concentrationis approximately 3 times that in the average
post-Archeanshale [Taylor and McClennan,1985].
The effects of these heat production rates on crustal
temperaturescan be inferred from the study of Molnar et al.
[1983]. With slight modification of their results,this study
can be used to determine the thermal perturbationsof thin ~920 K if the systemis saturatedwith externallyderived
horizons of uraniferous rocks.
Thermal effects of discrete
water and 1120 K for the more likely scenarioof "fluidhorizons of radioactive material can be calculated from the
absentmelting"of a systemwherein all fluid is suppliedby
shear heating calculationsof Molnar et al., exchangingthe dehydrationof hydrousphasessuchas muscoviteand biotite
productAoD (heat productivityfrom radioactivedecaytimes [Thompson, 1982; Vielzeuf and Holloway, 1988]. The
layer thickness) for c•v (resolved shear stress times steadystategeotherms
shownin the diagramare constrained
of 65 :l: 5 km [Molnar, 1988],
velocity). While these substitutionsappear ad hoc, both are by (1) the crustalthickness
suggestedby Molnar et al. [1983].
(2) TMoho<1300 K [Molnar, 1988], and (3) nominalcrustal
Temperatureperturbationsdue to in situ radioactivedecay heatproduction
ratesof 0-1 x 10-6 W/m3. Thisillustration
(ATr) in isolatedhorizons, using the parametersof Table 7, demonstratesthat temperaturesnecessaryto partially melt
are functions
of both the thickness
and volumetric
heat
pelitic rocks can be attained within the thickenedcrust of
production within the layer. For heat productionrates in the Tibetan Plateau, with a wide variety of plausible heat
Table 6, and a radiogeniclayer 10 km thick, ATr within this sources.
horizon range from 100 to 250 K some 20 Ma after the layer
For the minimumgeothermmodeled,wherethe assumption
was created. These A Tr estimates are of course very of no heat productionwithin the crusthas beenmade,the
intersects
the dry liquidusat -1100 K and70 km
sensitive to the assumed parameters; the uncertainty in k geotherm
(thermal conductivity,here assumedto be 2.1 W/m-K [Pinet depth;additionof heatby in situdecayof radioactive
nuclei
and Jaupart, 1987] is particularly large. Increasingk will (100-200 K) produceslocal intersectionof the liquidusat
anddepthsof lessthan1050K and40 km. A
lead to lower ATr becausethe heat will be transportedfrom temperatures
the source at a greater rate. Nonetheless, this procedure geothermdeterminedwith an average volumetricheat
demonstrates that in situ radioactive decay may have production
rateof 1 x 10-6W/m3 in thetop30kmof crust
contributedsignificantquantitiesof heat to the sourceregion intersectsthe dry liquidusat 1000 K and ~30 km depth;
TABLE7. Temperature
Perturbations
Time, Ma*
AoD,
W/m
2
10
20
30
40
z•Tr (•0
0.017
0.039
0.046
73
170
205
95
220
265
0.017
0.039
0.046
78
183
220
105
245
295
105
240
290
110
255
305
Z•Tr (30 •n)?
120
285
•35
305
340
380
D=10km;Ao-l.7x 10-6,3.9x 10-6and4.6x 10-6W/m3;
•- 2.1W/m-K;
•-1x 10-6mY/s.Heatproductions
arecalculated
fromFigure
5 ofMolnar
etal.
[1983].
*Elapsedtime of decay.
tTemperature
increase
(K) in the centerof a 10-km-thick
layerduesolelyto
heatingby in situradioactive
decay,giventhe Ao of Table6 anddepthbelow
surfaceof 20 (upperset)and30 km (lowerset). Otherparameters
are givenabove.
MCKENNA AND WALKER: G•MISTRY
OF LECOUCRATIC IGNEOUS ROCKS
21,497
Temperature (K)
300
500
700
900
1100
0
-.
' --.
A=0".,
15
-'q::i
'•::i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:?,•'
A=I .....
0
•
"-
10
"
0 ATr(K)
200
B•e ofContinental
Cr•t
"'•
Fig. 9. Summa• diagramof the •e•d
stmctu• of •e TiPton Plateau•d dep•-temperatu•relationsof
•litic •d graniticliquidii, adapted• paa from Hyndman [1981], Thompson[1982] and Vielzeuf and
Holloway
[1988].Tem•rature
isgiven
• Kelv•andp•ssu•• 108Pa(=kba0.•i•er l•es show
l•ati•
of •uminosflicatestab•ity fields (A=andalusite,K=kyanite,S=s•anite).
we• constructed
us•g cu•nt crustal•ic•esses (65 2 5 •) •d Moho t•ratures
1988],
•d n••
Steadystategeothe•s
(g13• K) [Molnar,
h•t production
of0 and1x 10'6 W• 3 (straight
andcu•edl•es labeled
A=0andA=I,
respectively). •rge, shadedtriangleillustratesl•ely temperature•creases • sourceregi• of Ulugh
Mu•agh magmasdueto • sireradioactive
decay(seeTables7 •d 8). •e tem•ramre increaseis a •nction
of •e •ickness of •e sourcelayer; •is •c•ase is given, at a s•cdic •ickness indicatedby the veaical
l•b of the triangleby •e dist•ce from the verticall•b to the hypot•euse of the •gle.
D• •d wet
melt•g •nes •dicate the P-T l•ii of water saturatedand water absentmelting of pelitic sourceareas
[Vielzeufand Holloway,1988]. L•ely P-T field for eraplacement
of •e UlughMu•agh •tmsive r•ks is
stippled.
addition of heat from in situ decay allows temperaturesin
excessof 1200 K at depths as shallow as -20 km. These
calculations show that superposition of reasonable
geotherms for the Tibetan Plateau and temperature
perturbations from radioactive decay can produce the
temperaturesnecessary to partially melt even dry pelitic
source rocks and to produce large volumes of melt. The
upper temperaturelimits derived in Figure 9 are compatible
with the temperaturerange of the experimentsof Vielzeuf
and Holloway [1988], demonstrating that the pressuretemperature-composition
relationshipswithin the sourcearea
for the Ulugh Muztagh intrusive and extrusive rocks are
sufficientto producean in situ partial melt.
REGIONAL RELATIONSHIPS AND TECTONIC IMPLICATIONS
The recent magmatismin the Ulugh Muztagh area is a part
of a regionally extensive province of post-Middle Miocene
vulcanism,which covers an elliptical area at least 300 km in
extent
north-south
and 600
km
east-west
within
the north-
central Tibetan Plateau. The approximate age and type of
volcanism is shown in detail in Figure 10, adapted from the
GeologicalMap of the Tibetan Plateau [Ministry of Geology
and Natural Resources, 1980]. Two features of interest in
this map are the latitudinal zonation of the chemistryof the
volcanics and the sharp, linear northern and easternedges of
the province. This northernfront strikes obliquely relative
to the edge of the plateau (as representedby the trace of the
Altyn Tagh Fault), but is subparallel to, and 100-50 km
north of, the trace of the Jinsha Suture. Ulugh Muztagh is
found along the westernsectionof the northernedge of the
province.
Deng [1978] reportedthe resultsof a reconnaissance
field
trip, detailed optical petrography, and major element
chemistryof Quaternaryvolcanic rocks exposedsouth of
Ulugh Muztagh. He separatedtheseflows into threegroups:
the southernBamogiongzong,central Yongbohu and the
northernQiangbaqiansequences. As shown in Figure 10,
these units lie 200, 100, and 50 km, respectively, south of
21,498
MCKENNAANDW•:
+
82ø
+•
•
G•STRY
+
86ø
OFLECOUCRATIC
IGNEOUSROCKS
+
zsz
-•
9•
+
oLhasa
+30øN
94øE
Fig.10. Simplified
geologic
mapof post-Early
Miocene
volcanic
rocks
ontheTibetan
Plateau,
modified
andadapted
fromTheGeologic
MapoftheTibetan
Plateau
[Ministry
of Geology
andNatural
Resources,
1980]
and[Coulon
etal.,1986].Rock
types
andapproximate
ages
areshown
byshading:
1,Neogene
andesitic
volcanic
rocks;2, Pliocene
to Recentandesitic
volcanic
rocks;3, undffferentiated
andesitic
volcanic
rocks;
4, potassic
Pliocene
to Recent
andesitic
volcanic
rocks;
5, Pliocene
to Recent
basicvolcanic
rocks;6, potassic
Pliocene
to Recentbasicvolcanic
rocks;7, undffferentiated
basicvolcanic
rocks.
Unshaded
areas
areolder,undifferentiated
volcanic
rocks.Thecentral
Tibetan
Plateau
volcanic
fieldis shown
inthecenter
ofthefigure,
dotted
lines
separate
areas
ofdistinct
rock
type.TheAltynTagh
Fault
forms
the
northern
structural
andtopographic
frontof thePlateau;
notetheangular
discordance
between
thisstructure
andtheE-Wtrending
northern
boundary
of thevolcanic
province
andcompositional
subprovinces.
Following
Molnar,
[1988],
observe
thatthevolcanic
province
islocated
overanarea
ofanomalously
hot
upper
mantle;
it appears
fromthedataofDeng,[1978];
andPearce
andMei,[1988]thatunits5, 6, and7 are
formed
bypartial
melting
of enriched
mantle.
Theband
ofunit2 (which
includes
sample
TQ3) between
Ulugh
Muztagh
andTQ2 represent
hybrid
magmas
formed
through
mixing
of crustally
derived
mantle
melts
similar
to those
exposed
at Ulugh
Muztagh
andmantle
melts
similar
to those
exposed
at TQ2. Fault
ornamentation
as in Figure 1.
thevolcanicunitsexposed
at UlughMuztaghandlie on the trachydacites
to subalkalic
rhyolites[Deng,1978;Pearceand
northernsectionof the Qiangtangterrain. The southern Mei, 1988]. Theageof all of these
unitswassuggested
by
sequence
includesultrapotassic
rocksvaryingfrom tephrite Deng [1978]to be Quaternary,
basedon relationships
with
basanitesto phonolites. Typical phenocrystsinclude underlying
rocksof assumed
Plio-Pleistocene
age. The silica
leucite, analcite,nephiline,nosean,olivine (Fo75) and andpotashconcentrations
of thesesamples
agreereasonably
aegerine-augite. The central sequenceare transitionalto well with the compositions expected from their unit
calc-alkaline tephrite basanitesto trachyandesites
and identificationon the GeologicalMap of the TibetanPlateau.
dacites,while the northerngroupare typicallycalc-alkalineIn that map's terminology,and in the terminologyfollowed
MCKENNAANDWALKER:GEOCHEMISTRY
OFLECOU••C
in Figure 10, samplesTQ1, 2 and 3 are ultrapotassic
basic
rocks (unit 6 in Figure 10), ultrapotassicandesiticrocks
(unit 4), and intermediaterocks (unit 2), respectively. The
variation in rock type recorded by Deng [1978] in the
Yongbohuvolcanicsmay indicate that the central unit 4
provincein Figure 10 includesa rangeof lithologiesfrom
IGNEOUSROCKS
21,499
While we recognizethe potentialdangerof drawing
conclusions
from sucha small data set, the GeologicalMap
of theTibetanPlateausuggests
thatthepatterns
seenin the
datadiscussed
heremaywellbe representative
of patterns
in
the centralvolcanicprovinceas a whole,andarecertainly
testablewhen additionaldata become available. These
basanite to andesRe.
resultsarecompatible
with a modelfor Pleistocene-Recent
One samplefrom eachof the sequences
identifiedby Deng
[1978] was re-analyzedby Pearce and Mei [1988]. As noted
by Pearce and Mei [1988], the TQ samples all show
extraordinaryenrichmentof the REEs and the LIL elements
(Rb, St, Th) indicativeof derivationfrom a highly enriched
source area, due perhaps to assimilationof a subduction
component [Pearce and Mei, 1988]. Despite this
enrichment,all three samplesdemonstratea consistenthigh
field strength elements depletion [Salters and Shimizu,
1988], a characteristic not atypical of subcontinental
lithosphere. The high MgO contentsand Mg numbersfor
the basanite indicate that these magmas are direct mantle
melts, apparentlyextrudedthroughthe 65 km thicknessof
magmatism
alongthenorthern
Qiangtang
terrainwhichis
dominated
by mixingof twoend-member
compositions.
The
silicarich end-member
(represented
by the UlughMuztagh
extrusives)
formsthrough
in situpartialmeltingof pelitic
rockswithin the thickenedTibetancrust. The mafic endmember(represented
by the TQ 2, the Yongbohusequence)
forms through partial melting of enriched, perhaps
metasomatized subcontinental mantle, and occurs, as shown
in Figure 10, in a band50 km thick betweenthe southern
ultrapotassic
and the northernhybridprovinces.Theseendmembers occur approximately 100 km apart, while the
hybridmagmalies 50 km southof Ulugh Muztagh. Most
likely, neither of the chemical end-membersare point
sources;we do not envisionveins of magma stretching50
km to mix together in the central belt. Rather, we
hypothesizethat partial melting of the crust, and hence
productionof the silicic end-member,occurs laterally
throughoutthe crust. To the north, unadulterated
crustal
meltsreachthe surfaceowingto a lack of mantlemelt, while
in the Qiangbaqianarea, subequalvolumes of the end
membersare present. In the Yongbohu area,an entiresuite
of basaniteto rhyoliteindicatesthe presenceof both hybrid
liquidsandunadulterated
mantlemelts. Finally,southof the
Yongbohubelt, a wide areaof ultra-potassic
rocksrepresents
Bamogiongzong
sequence,
as represented
by sampleTQ I of an area of hybrid liquids stronglymodifiedby low-pressure
by sampleTQ 1.
Pearce and Mei [1988], apparenfiy formed by fractional fractionation,as suggested
A numberof studies(see Molnar [1988] for a review) have
crystallizationof hybrid melts.
the plateau.
Basedon the availablemajor and traceelementdata for the
TQ rocks,we suggestthat the volcanicrocksexposedin and
southof Ulugh Muztagh are related by magmamixing: the
end-membersare representedby the high silica crustalmelts
exposedat Ulugh Muztagh and the basanitesamplesof the
centralYongbohuprovincesouthof Ulugh Muztagh (Figure
11). Mass balance calculationsindicate that the analyzed
samplefrom the northernQiangbaqian
sequence(sampleTQ
3) formedfrom approximatelyequalfractionsof the two endmembers.
The major element chemistry of the
Tarim
Tibetan Plateau
Basin
TQ2 UM
TQ1 TQ3
Fig. 11. A sketch of the sourcesand mixing areas of magmas in the central Tibetan province, modified
from Molnar, [1988]. The areasof mantle upwelling are shown by arrows; approximatelocations of Ulugh
Muztagh, TQ 3 and TQ 2 are also shown. Silicic magmasare producedthroughcrustalmelting of pelitic or
perhapsamphiboliticmaterial over a wide region of the plateau (N-dipping hatching), while manfie-derived
melts occur only southof the area of active upwelling (S-dippinghatching). The resultinghybrid magmas
can cover wide areas of the central plateau, but the northernterminusof the marie magmasshould indicate
the northernterminusof the upwelling. If so, data for the Ulugh Muztagh extmsiverocksindicatethe length
of the boundarybetweenerodedand unerodedlithosphereis of the order of 100 km.
21,500
MCKENNA AND WALKER: GE•MISTRY
OF LECOUCRATICIGNEOUSR•
demonstratedthe existenceof a high impedancezone in the the time-integrated unroofing rate for the Ulugh Muztagh
upper mantle below the north-central Tibetan Plateau, the region is less than 1.25-2 mm/yr. Interestingly,Molnar et
al. [1987] estimated that thrust faults along the northern
same area occupied by the volcanic province. Molnar
[1988] hypothesized that this anomaly was due to the edge of the Tibetan Plateau have vertical slip rates of 1-2
presenceof relatively hot, perhapspartially melted, mantle mm/yr, in good agreementwith the crustal thickeningrates
material driven convectively against the base of the Tibetan estimatedby examination of the Ulugh Muztagh igneous
continental lithosphere. This thinning of the mantle rocks.
lithosphere beneath the central Plateau has produced the
CONCLUSIONS
lateral thermal gradients resolved by teleseismic studies.
Molnar also suggestedthat the approximate correspondence
The granites, granodiorites,rhyolites, and trondhjemites
of the volcanic province with the thermal anomaly was
collected
from the Ulugh Muztagh region of the northern
evidenceof a causal relationship.
The large-scale mixing observed in the volcanic rocks Tibetan Plateau are leucocratic, potassic, primarily
south of Ulugh Muztagh confirms this model and allows peraluminousrocks which formed from crustally derived
some detail to be added to it. The geochemicalgradient partialmelts. Thesemelts were producedat depthsof 20-40
between the central Yongbohu sequence and the crustally km by heating of the source region through in situ
derived melts is causedby the thermal gradient imposedby radiogenicdecay and mantle heat flux into thick, and hence
mantle convection.
The mantle-derived
melts form south of
the line at which hot, superadiabaticmaterial is juxtaposed
with continental lithosphere. North of this line, mantle
temperaturesare too low, or lithosphericthicknessestoo
high, to allow formation of melts or their transportto the
surface.
If true, the distribution of volcanic rocks on the
insulative, continental crust. The igneousrocks currently
exposedat Ulugh Muztagh form both extrusive and intrusive
bodies;regional contact metamorphicrelationshipssuggest
that the intrusiverocks were emplacedat pressuresless than
4 x 108 Pa.
Burchfiel
et al. [1989]havereported
40Ar/39Ar
datathat
plateau suggest that the width of the thinned zone of documentsa minimum age difference between the intrusive
asthenosphere
is approximatelyequal to the width of the and extrusive samplesof 4-6.5 Ma. The youngerrhyolitic
element abundances, trace
mixing area defined by the Ulugh Muztagh and Yongbohu extrusive rocks have major
87
86
element
abundances,
and
St/
Sr and206pb/204pb
ratios
sequences,or approximately100 km. This would require
similar
to
those
of
the
intrusive
rocks.
Although
the
significantlateral thermal and mechanicalgradientsin the
extrusives appear to be significantly younger than the
mantle below the plateau.
intrusives,the chemical data suggestthat both the extrusive
If the northward limit of Cenozoic volcanism does follow
and the intrusive
rocks considered
here were derived from the
subcrustal thermal anomalies, the angular discordance
between this anomaly and the topographicand structural same, or very similar, source rocks of pelitic composition.
front of the Tibetan Plateauhas importantimplications. The This history of crustally derived magmatism requires
topographicrelief (from the westernmost
exposureto the maintenanceor episodic attainment of super-soliduscrustal
easternmostexposureof mafic melts, see Figure 10) along temperaturefor time scalesof 5 to 10 million years. The
the northern limit of volcanism is over 2 km.
If the depthof emplacementof the intrusiverocks and the inferred
northern terminus of the mafic rocks maps the thermal depth of the source rocks suggests that rates of crustal
anomaly within the mantle, the subcrustalthermal structure thickening and unroofing in the north-central Tibetan
is clearly not affecting the formationof the plateau. In Plateau may be approximately equal, at 1-2 mm/yr. These
otherwords,the mantle-crust
systemin centralAsia appears rates are similar to those estimatedby Moltmr et al. [1987]
to be decoupled,with the mechanisms
of crustalthickening from ground level reconnaissanceobservationsand suggest
operating independently, but synchronously,with the that the north central Tibetan Plateau may be in a steady
mechanismsaffecting the mantle. If a link between the state condition, with the rates of crustal thickening
mantle thermal structure and the creation and maintenance of approximatelyequal to the rate of unroofing.
Considerationof the extrusivesamplesat Ulugh Muztagh
the plateau's elevation does exists, it appears to be
operatingon time scaleslonger than that recordedin the and other Pliocene to recent volcanic rocks south of Ulugh
upperCenozoicvolcanicrockspresentin the centralplateau Muztagh suggeststhat large-scale mixing of crustal melts
and mantle derived melts is occurring in the north central
that are examined here.
Tibetan
Plateau. The east-westtrendingcompositionalzones
The time scaleand rate of thickeningin this part of the
are
due
to similarly trending thermal gradientsin the upper
plateau
canbeestimated
fromthe40Ar/39Ar
geochronology
resultsof Burchfielet al. [1989], alongwith the estimatefor mantle below the plateau. The oblique angle between the
maximumdepth of emplacementfor the intrusiverocks (4 x subplateauthermal structure and the Tibetan Plateau itself
108Pa,or approximately
10-12km). Theminimum
ageof indicates that the mechanisms of crustal thickening may
crystallizationfor the intrusiverocks is providedby the operate independently of mechanisms controlling the
10.5 Ma muscoviteplateauage for UBTG; the Sr isotopic
datapresentedabovesuggestpossiblecrystallization
agesof
11-12Ma. The intrusivesuitewasexposed
at the surfaceby
4.0 Ma, as the well-dated extrusive rocks overlie a boulder
conglomerate which contains abundantclasts of the intrusive
thermal
and mechanical
structure
of the mantle.
APPENDIX 1
MV2, UMVU and UM3V are rhyolitic tuffs characterized
by
glomerophyricplagioclase(An20_25) and sanidine,phenocrystic
quartz (up to 7 mm dianemr)and biotite, and subhederalto anhederal
rocks. Thus the maximum averageunroofingrate between cordieritein a microliticgroundmass.
CordierRe
is surrounded
by a
10-12 Ma and 4 Ma is 10-12 km/6-8 Ma=1.25-2 mm/a. The
felty, colorlessrim 10-30 •.m thick, and locally has altered to
actualunroofingrate may well have exceededthis average pinite. Zircon includedin cordieritecrystalsare anhederal,broken
rate for shorttime intervals;our estimatemerelystatesthat grains,with no mandes. Sanidineand biotite separatesfrom MV2
MCKENNAANDWAIXER: GEOCHEMISTRY
OFLECOUCRATIC
IGNEOUSROCKS
APPENDIX
2: REE Corrections From U Fission
Extrusives
Element MV1B
ls
1.03
UM1B
1.03
MV2
21,501
Intrusives
UM3V
UMVU
UMQP
0.72
0.69
0.63
0.47
QID
BKSP
UBTG
0.33
0.57
0.46
Potasslum-poor
2MGR
KSPO
QIL
UM10
0.3
0.54
0.25
0.27
oLa*
0.24
0.2
0.1
0.15
0.1
0.06
0.04
0.12
0.1
0.05
0.07
0.03
0.04
Ce
7.02
7.02
4.91
4.7
4.27
3.19
2.24
3.89
3.13
2.08
3.67
1.73
1.86
oCe
Ni
1.62
1.35
0.68
1.03
0.7
0.43
0.3
0.84
0.65
0.32
0.49
0.22
0.24
5.46
5.46
3.82
3.65
3.32
2.48
1.74
3.02
2.44
1.62
2.86
1.34
1.45
oNd
1.4
1.18
0.6
0.89
0.62
0.39
0.27
0.73
0.56
0.29
0.44
0.19
0.22
Sm
0.1
0.1
0.07
0.07
0.06
0.05
0.03
0.06
0.05
0.03
0.05
0.03
0.03
0.04
0.03
0.02
0.02
0.02
0.01
0.01
0.02
0.02
0.01
0.02
0.01
0.01
•Sm
Valuesin the tableare the concentrations
(in partsper million) of elements
produced
in the samplesby inducedU fission[Korotevand
Kindstrom,
1985]. Thesevaluesweresubtracted
fromtheINAA results
to givethetrueconcentrations
of theelements
reported
in Table2.
* Fully propagated
2s uncertainties
in the corrections,
in partsper million.
consists
of quartzand plagioclase
with
were
analysed
for39Ar/40Ar.
Thesanidine
hadaplateau
ageof4.0 The isotropicgroundmass
_+0.1 Ma (72% of gas in threesteps,total gasage was4.1 Ma), and diameters less than 30-50 mm. In UM10, faces of tourmaline and
quartzareroughon the scaleof 30-50ram,aparently
dueto reaction
biotite gave a total gasage of 4.2 _+0.1 Ma.
UMQP is a porpyritic rhyolite with quartz, sanidine, cordieritc
and biotite phenoctystsin a microlitic groundmass. Tourmaline is
present in trace quantities,and althoughtypically of small size is
clearly a phenocrysticphase. The isotropicgroundmassconsistsof
10-50 •tm diameter quartz+plagioclase+sanidine
(70%), opaques
(20%) and biotite (10%)
MV1B and UM1B are glassy rholitic flows with porphyritic to
locally glomerophyricplagioclaseand porphyrititc sanidine,quartz,
cordieritc and biotite in a glassy matrix. Plagioclase is locally
fluid-inclusion rich, individual laths are up to 4-5 mm in length.
Cordieritc grains are typically smaller in size than the plagioclases,
and some display overgrowths of cordieritc around central, zoned
grains. A sanidine separate from MV1B had a 4.0 _+ 0.1 Ma
39Ar/40Ar
plateau
age(fivesteps,
100%
ofthegas).
BKSP and UBTG are coarsegrained,panidiomorphicgraniteswith
phenocrystsof perthitic, poikolitic alkali feldspar and quartz with
smaller phenocrysts of plagioclase (An25.45) and biotite in a
medium grained, equigranulargroundmassof quartz, alkali feldspar
and biotite. Zircon inclusionsin biotite included clear to very dark
coloredgrains all less than 50 mm long. Clear grainsare euhederal
and show no radiation haloes, dark grains are anhederal and are
surroundedby radiation-damage
haloes. Three mineral separateswere
of the grainswith the groundmass,
while the facesof plagioclase
grainsare quitesharp.In QTL, the plagiodase
grainsdisplaythis
roughtexture,whilethequartzandtourmaline
facesaresharp.Trace
quantitiesof zircon (cloudy,anhederal)are presentwithin the
tourmaline.
KSPO is a rathercoarsegrainedgranodiorite
with phenocrysts
of
alkali feldspar(2-15mmin length),quartzand plagioclase
with
interstitial,fine-grainedbiotiteand muscovite. Lathsof allanitc
occurboth as inclusions in otherphasesand as interstitialgrains.
In the formerenvironment,
the grainsare stubyandanhederal,
while
the interstitialgrains are long, thin, optically clear laths upto
5001•min length.
Acknowledgments. We would like to thank Drew Coleman for
XRF preparation;S. R. Hart for the use of the Geoalchemylab at
MIT; W. R. Van Schmus and M. E. Bickford for consultationat the
XRF and mass-specfacilities at the University of Kansas; and F.
Frey for use of, and P. Ila for her help in all aspectsof, the INAA
facility at MIT. Ye Hongzhuankindly translatedmap legends and
tables for us: his assistancewas crucial to our understandingof ideas
presentedhere. Kevin Furlong wrote the program"Geotherm"used
in producing Figure 9. B. Nelson and an anonymousreviewer
providedexcellentreviewsof this manuscript;B.C. Burchfiel,P. Le
analyzed
by39Ar/40Ar.
Muscovites
gave
a 10.5+ 0.1Maplateau Fort, K. Burke and V. Salters read early versions of the manuscript
(78% of the gas in two steps),biotite a total gas age of 10.1 + 0.1
and suggested improvements to it.
Although they may not
Ma and sanidine displayed a minimum age of 9.1 Ma with a total necessairily agree with any of the ideas presented herein, we
gas age of 10.2 + 0.1 Ma.
appreciate their efforts. The paper was typeset by Elizabeth
2MGR is a medium grained hypidiomorphicgranular,two-mica- Spizman. Financial supportprovided by the StudentResearchFund
granite. Quartz grains typically show undulatory extinction, Committee of the Department of Earth Atmospheric, and Planetary
muscovite locally shows very minor kinking. Mucovite has very Sciencesat MIT (L.W.M.), the National Science Foundation (EAR
few inclusions of any sort except for rare zircon, while biotites 8805125 to S. R. Hart, EAR 8517889 to W. R. Van Schmus) and
containabundant(a few volume percent)opaqueinclusions. Zircons the Shell Oil Company (J.D.W.).
occursas both inclusionsin other phasesand as interstitial grains;
in both environments the zircons are irregular, anhederal and
optically cloudy. A biotite separate from this sample gave an
Anders, E., and M. Ebihara, Solar-system abundances of the
39Ar/40Ar
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elements, Geochim. Cosmochim. Acta, 46, 2363-2380, 1982.
feldspar separatedisplayeda 9.8 + 0.1 Ma total gas age with a
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and J. Sutter, Geology of the Ulugh Muztagh Area, Northern
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Tibet, Earth Planet. Sci. Lett., 94, 57-70, 1989.
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of the Royal Societyand
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Academia Sinica 1985 Geotraverse of Tibet, Nature, 323, 501507, 1986.
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(Received
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revised3anuary17, 1990;
accepted
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23, 1990)