The significance of monazite U–Th–Pb age data in metamorphic assemblages; a combined study of monazite and garnet chronometry

The signi¢cance of monazite U^Th^Pb age data inmetamorphic assemblages; a combined study of monazite and

garnet chronometry

Gavin Foster a;*, Pete Kinny b, Derek Vance c, Christophe Prince a,Nigel Harris a

a The Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UKb Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, Perth, W.A. 6102, Australia

c The Department of Geology, Royal Holloway, University of London, Egham Hill, Egham, Surrey TW20 OEX, UK

Received 1 November 1999; received in revised form 20 June 2000; accepted 25 June 2000

Abstract

In this study, a coupled U^Th^Pb isotopic and EMP chemical study was carried out in situ on monazite (micro-inclusions within garnet as well as matrix grains) from rocks recovered from two chronologically well-constrained areasof the Himalayan orogen. Monazite inclusions within garnets from three samples yield ages of V44^36 Ma, whereasmatrix grains within one sample are typically younger (30^26 Ma). Y depletion of these younger matrix grains indicatesthat they grew after garnet had crystallised. The recognition of episodic monazite growth during regionalmetamorphism, first at greenschist facies (before garnet growth) and secondly at upper amphibolite facies (post-dating garnet growth), allows relative growth ages of the occluding garnet to be calculated. These are in excellentagreement with Sm^Nd garnet ages from surrounding units. This approach not only provides a ready means ofobtaining porphyroblast growth ages but also allows the combination of precise U^Th^Pb data from metamorphicmonazite with thermobarometric information obtained from rock-forming minerals. ß 2000 Elsevier Science B.V. Allrights reserved.

Keywords: monazite; absolute age; isotope ratios; U/Th/Pb; Sm/Nd; secondary ion mass spectroscopy

1. Introduction

Of the U^Th-rich accessory phases commonlyused in geochronology, monazite is perhaps the

most useful for determining the timing of meta-morphism in amphibolite and higher grade gran-itic and pelitic rocks (e.g. [1,2]). This light rareearth elements (LREEs) phosphate is a relativelycommon accessory mineral in a variety of rocktypes [3,4], usually has high concentrations of Uand Th, and typically has minimal concentrationsof common Pb [4]. These characteristics coupledwith a minor risk of isotopic inheritance [4] and ahigh resistance to Pb loss [4,5] have led to thegrowing use of monazite as a chronological tool

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 1 2 - 0

* Corresponding author. Present address: The Departmentof Geology, Royal Holloway, University of London, EghamHill, Egham, Surrey TW20 OEX, UK. Fax: +44-1784-471780;E-mail: [email protected]

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in metamorphic rocks (e.g. [6]). However, unlikecommon rock-forming minerals such as garnet,the metamorphic reactions responsible for the for-mation and breakdown of such accessory phasesare poorly understood, despite numerous studiesover the last decade (e.g. [7] and references there-in). It is, therefore, di¤cult to combine the precisechronological information provided by monazitewith the pressure^temperature determinations ofrock-forming minerals. Thus, despite its chrono-logical utility, the application of metamorphicmonazite U^Th^Pb dating to geological problemsis severely limited, and will remain so until reac-tions responsible for the breakdown and forma-tion of monazite are not only recognised but alsocalibrated in pressure^temperature space.

In this contribution, we apply secondary ionmass spectrometry techniques to provide in situU^Th^Pb isotopic measurements of monazites,both included within garnet porphyroblasts andin the matrix of a rock. This approach has theenormous advantage over conventional U^Th^Pb investigations, or more traditional ion probetechniques, in that the textural relationships ofmonazite are retained. Although the techniquehas been used before [8], this contribution in-volves several novel and important advances.Firstly, we present a full U^Th^Pb dataset forthe analysed monazites so that we are not reliantsimply on 207Pb^206Pb [8,9] or 232Th^208Pb ages[10]. This approach allows us to apply the criticaltest of concordancy to our data. Secondly, wealso present estimates of the timing of local garnetgrowth, independently established using Sm^Ndchronometry (this study, [11]). We are thus ableto demonstrate, for the ¢rst time, that the ages ofmonazite inclusions in garnet are similar to, orslightly older than, the occluding porphyroblast.As a result, we can combine precise U^Th^Pbages on monazite inclusions with thermobaromet-ric data from garnet. Finally, we show that matrixmonazites commonly yield more variable andyounger ages than the included grains. By inte-grating detailed REE and trace element datawith textural observations, we are able to distin-guish several growth phases of monazite duringregional metamorphism and estimate the condi-tions under which this growth occurred.

2. Geological setting

The samples for this study come from theGarhwal region of the Central Himalaya ofIndia, and from the Nanga Parbat HaramoshMassif (NPHM), in the Pakistan Himalaya.Both areas lie within the metamorphic core ofthe Himalayan orogen, and both have experiencedextensive prograde metamorphism, deformationand melting (e.g. [12,13]) as a result of the Creta-ceous/late Eocene collision between India andAsia [14,15].

Samples 97g74, 97g100U, 97g97 and 97g98were recovered from the eastern margin of theNPHM (see [12]) from garnet^kyanite^staurolitegrade metapelitic cover rocks that can be corre-lated using isotopic and ¢eld criteria with theHigh Himalayan Crystalline Series (HHCS) ofthe central orogen [16]. Samples G90 and G57were recovered from the Bhagirathi valley inGarhwal Himalaya [11,17]. Sample G57 comesfrom the base of the HHCS near the Main Cen-tral Thrust Zone and sample G90 comes fromnear the top of the series within the Harsil for-mation (see [17]).

3. Sample descriptions

Samples 97g100U, 97g98, 97g97 and 97g74 aretypical of the garnet^kyanite schists that charac-terise the cover metasediments of the NPHM[12,18]. They all contain the assemblage garnet,plagioclase feldspar, muscovite, biotite, kyaniteand quartz, with accessory phases of rutile, tour-maline, apatite and rarely allanite ; 97g74 isunique in also containing accessory monaziteand lacking apatite. Garnets in samples 97g98and 97g100U are typically around 5 mm in diam-eter, whereas those in 97g97 and 97g74 are oftenlarger (up to 20 mm). Garnets in samples97g100U and 97g98 exhibit strong major elementzonation that is indicative of that developed dur-ing garnet growth (e.g. [19]) ^ bell-shaped Xsps

from 0.15 in the core to 0.02 in the rim. Garnetsin samples 97g97 and 97g74 (see Fig. 1a) are sim-ilarly zoned but the Xsps of these garnets shows asmaller core to rim variation ^ from 0.04 and 0.1

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in the core to 0.01 and 0.02 in the rim, respec-tively. In all cases, the extreme garnet rims haveslight retrograde reversals in Sps, Alm and Pycontents. Common inclusions within the garnetare kyanite, chlorite, muscovite, quartz, rutile, il-menite, plagioclase feldspar and zircon. A detailedSEM investigation revealed that monazite, allan-ite or xenotime were not present as includedphases in samples 97g98, 97g97 and 97g100U.As such, these samples are suitable for a garnetSm^Nd dating study (see [11]). In sample 97g74,allanite and xenotime were not observed but mon-azite occurs as inclusions in kyanite and garnetporphyroblasts in addition to the matrix of therock. Matrix grains often grow in groups or trainsof several crystals along the foliation, and aresometimes associated with zircon. Monazite inclu-sions in garnet are of equal dimensions to theirmatrix counterparts : both populations are around100^50 Wm in length with short axes of around50^10 Wm. Both occurrences are equally abun-dant. BSE observations of the monazites indicatea lack of any signi¢cant zonation.

Samples G90 and G57 are both garnet^biotiteschists containing the assemblage garnet, biotite,plagioclase and quartz, with accessories of rutile,zircon, monazite and apatite. In addition, G90contains kyanite, chlorite (after biotite), orthoam-phibole (gederite) and staurolite (after garnet).Muscovite is an additional fabric-forming phasein sample G57. Garnet forms euhedral to subhe-dral porphyroblasts, up to 5 mm in diameter insample G90 and up to 1 cm in diameter in G57.In both instances, garnet is growth-zoned (Figs.2a and 3a) although zoning in Xsps is not strong.Rutile, ilmenite, quartz, zircon and monazite arecommon inclusion phases in garnets of both sam-ples notably allanite and xenotime were not ob-served. In sample G90, monazite inclusions varyfrom 25U20 Wm to 60U50 Wm, whereas in sampleG57 they are less than 50 Wm long and typicallyaround 25U15 Wm. In both samples, matrix mon-azites are less abundant than their included coun-terparts and in sample G57 matrix monazites arealso typically larger (up to 100U80 Wm) and oftenassociated with large (V250 Wm) apatites. In bothsamples, BSE observations indicate monazites arenot zoned.

4. Analytical techniques

U^Th^Pb isotopic measurements were carriedout in situ on polished probe sections of samples97g74, G57 and G90 using the Sensitive HighResolution Ion MicroProbe (SHRIMP-2) atCurtin University, Western Australia. BSE obser-vations, EDS and re£ected-light microscopy car-ried out on the same probe sections allowed therecognition and mapping of monazite micro-in-clusions both within garnet porphyroblasts andin the matrix of each sample. Monazite grainsas small as 10 Wm diameter were targeted, usinga 2.5 nA O3

2 primary beam, and with the second-ary ion analyser set to a mass resolution of5000. During the analytical sessions, sensitivityfor the Pb isotopes was 26 counts per s/nA/ppm.A single analysis constituted seven countingcycles through the isotopic species of interest :LaPO2, CePO2, 204Pb, background near 204Pb,206Pb, 207Pb, 208Pb, 232Th, 238UO, 232ThO2 and238UO2. The total analytical time was 20 minper spot.

Prior to analysis, chips of standard monaziteMAD-1 were cast into each section, providing abasis for normalisation of Pb/U and Pb/Th iso-topic ratios measured by SHRIMP. The MAD-1standard is judged to be 514 Ma, based on themean 206Pb*/238U ratio determined by TIMSanalyses. Importantly for this study, this age iswithin error (0.5%, 2c) of its Th^Pb age alsodetermined by TIMS (L. Heaman, written com-munication). EMP analysis, following the proce-dure outline below, of the MAD-1 standard in-dicates that in terms of trace element and REEcomposition, MAD-1 is similar to the unknownsof this study, albeit with slightly lower U andhigher Si concentrations. Normalisation of mon-azite Pb/U ratios was based on a plot ofln(206Pb*/238UO) versus UO2/UO for analyses ofthe standard, which routinely produces a corre-lation line of slope V0.7. Using this relationship,measured 206Pb/238UO ratios of unknowns weredivided by the equivalent value for the standardat the same UO2/UO, and multiplied by thestandard's known 206Pb/238U ratio (0.0830). Nor-malisation of Pb/Th ratios was based on an anal-ogous relationship between 208Pb*/232Th versus

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ThO2/Th established for the standard (cf. [10]).Common Pb correction was based on back-ground-corrected 204Pb counts, with the isotopiccomposition of the common Pb component mod-

elled on that of 40 Ma average crustal Pb [20].2c errors on Pb/U and Pb/Th ratios followingthis approach are typically between 3 and 6%.In addition to counting statistics, these ¢guresinclude errors arising from uncertainties in the¢t of the correlation line, the reproducibility ofthe standard and errors resulting from the com-mon Pb correction.

Subsequent to isotopic analysis, monazites fromeach population were chemically characterised us-ing the Cameca SX100 electron microprobe. Dueto interference problems, only the LREEs alongwith structurally important trace elements wereanalysed. In particular, Y has been measured asa proxy for the heavy REE. All analyses wherecarried out using an accelerating voltage of 20 kV,a beam current of 20 nA and counting times ofaround 100 s per element. High-spatial resolutionwas achieved with a 1 Wm electron beam. Carefulcalibration and selection of background positionsensured that interference among the LREEs wasminimised. Only analyses with totals s 97% wereused in later chemical characterisation and all el-ements except Eu and Pb were routinely abovethe detection limit of the EMP. X-ray maps ofTh, La and Y were carried out on selected mon-azites from each sample following the methodol-ogy of Williams et al. [21].

A garnet Sm^Nd isotopic investigation, follow-ing the methodology and rationale of Cohen et al.[22], was carried out upon samples 97g100U,97g97 and 97g98. In all cases, around 10^20 mgof optically clean garnet separate was obtainedfrom either whole rock (97g100U and 97g98) orentire crushed garnet (97g97). Furthermore, sam-ple 97g97 contains garnets large enough (V1.6cm) to allow the mechanical separation of acore and rim fraction. Sm^Nd isotopic determina-tions were carried out using the Finnigan MAT262 mass spectrometer at ETH, Zu«rich, Switzer-land.

5. Results

5.1. EMP study

A complete set of results is given in the EPSL

Fig. 1. Chemical and isotopic data for sample 97g74. a:Composition of garnet 97g74 (1), showing mol fraction of Fe(diamonds), Mn (squares), Mg (triangles) and Ca (crosses)and the Fe/(Fe+Mg) ratio (circles) in a traverse from rim torim. Arrows show the location of selected monazite inclu-sions. Although more are present, these show the extent ofmonazite localisation within garnet. b: Plot of ThO2/UO2 vs.(La/Y)N for all monazite EMP analyses from sample 97g74.Inset shows a detail of the principal data ¢eld. Matrix mona-zite analyses are shown as crosses and included monazitesare shown as circles. c: 206Pb*/238U vs. 208Pb*/232Th concor-dia plot of monazites from sample 97g74. Matrix monazitesare shown as heavy grey ellipses and included monazites areshown as ¢ne black ellipses.

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Online Background Dataset1 associated with thiscontribution. The important results for this studyare shown in Figs. 1b, 2b and 3b. In each sample,matrix and included monazites share many chem-ical characteristics. However, in all three samples,

Y concentrations, and hence HREE concentra-tions, are typically lower in the matrix grains. Inaddition, in sample G90, matrix monazites alsohave typically lower Th/U than included grainsdue to elevated U contents (0.4^0.6 wt% com-pared to 1^1.2 wt%). The REE pattern variationwithin and between monazite grains is approxi-mated by the chondrite-normalised La/Y ratio((La/Y)N). This ratio is plotted against ThO2/

Fig. 3. Chemical and isotopic data for sample G57. a: Com-position of garnet G57 (1), symbols as for Fig. 1a. b: Plotof ThO2/UO2 vs. (La/Y)N for all monazite EMP analysesfrom sample G57. Details and symbols as for Fig. 1b. c:206Pb*/238U vs. 208Pb*/232Th concordia showing monazite U^Th^Pb data from sample G57. Details as for Fig. 1c. Mona-zite analyses mentioned in text are labelled.

Fig. 2. Chemical and isotopic data for sample G90. a: Com-position of garnet G90 (1), symbols as for Fig. 1a. b: Plotof ThO2/UO2 vs. (La/Y)N for all monazite EMP analysesfrom sample G90. Details and symbols as for Fig. 1b. c:206Pb*/238U vs. 208Pb*/232Th concordia showing monazite U^Th^Pb data from sample G90. Details and symbols as forFig. 1c. Monazite analyses mentioned in text are labelled.

1 http://www.elsevier.nl/locate/epsl; mirror site: http://www.elsevier.com/locate/epsl

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UO2, as measured by EMP, in Figs. 1b, 2b and3b. These plots show that matrix grains are typi-cally more HREE-depleted relative to chondritethan the included grains. Notably, included mon-azites rarely exhibit the elevated (La/Y)N thatcharacterise the matrix grains within a particularsample, whereas a small percentage of matrixanalyses do show included characteristics (seeFigs. 1b, 2b and 3b). Trace element and REEzonation within individual monazites, obtainedby EMP spot analyses, do not show any consis-tent zonation patterns. X-ray maps of selectgrains con¢rm the lack of systematic Th and Lazonation. However, maps of Y content indicatethat in samples G90 and G57, matrix monazitesare typically characterised by cores of equal Ycontents to the included monazite populationand outer zones where Y is severely depleted(Fig. 4). In addition, matrix monazites from sam-ple G57 often exhibit an increase in Y content (tos 2 wt%) at the extreme rim (Fig. 4a). Notably,the boundaries between these zones are sharp andoften arcuate.

5.2. U^Th^Pb isotopic study

Unlike previous studies of this kind, we presenthere a full U^Th^Pb isotopic dataset for the mon-azites analysed with SHRIMP. These data can befound in the EPSL Online Background Dataset1

associated with this contribution. The amount of

radiogenic Pb in these samples is relatively lowdue to their young age. Therefore, although com-mon 206Pb and 208Pb are only a small fraction(typically 6 1%) of the measured Pb, commonPb can constitute a signi¢cant portion of the mea-sured 207Pb (up to 50% but typically V10%). Thecalculated 207Pb*/235U ratio is therefore very sen-sitive to incorrect assumptions concerning the iso-topic composition of the common Pb and to er-rors in the measurement of 204Pb arising frombackground interference. Indeed, an overestima-tion of the background under the 204Pb peak byas little as 1^3 counts per s would be su¤cient tochange the 207Pb*/235U age by up to 10% and tocreate an apparent minor discordance. Notably,such an overestimate does not change the 206Pb/238U ratio by more than 0.5%, but is su¤cient todecrease the 207Pb*/235U ratio to disconcordantvalues. Therefore, a more reliable test of the con-cordancy of these analyses is provided by a208Pb*/232Th vs. 206Pb*/238U concordia (Figs. 1c,2c and 3c). In the ensuing section, these plots areutilised to apply the critical test of concordancy tothese data.

Thirteen monazite micro-inclusions within gar-net and three monazites in the matrix of 97g74were analysed by SHRIMP. The radial locationsof a number of the analysed monazite inclusionsare shown in Fig. 1a. Included and matrix mon-azites both appear to be concordant in Th^Pb vs.U^Pb space and also appear to be of similar age,

Fig. 4. X-ray maps of Y content of matrix monazites (a) G57mat1, (b) G90mat1 and (c) G57mat4. The lighter the grey scale,the higher the Y content. Note, however, that images have been manipulated and grey scale between images is no longer equal.Ellipses mark the position of ion probe analyses labelled with age. Circles mark the location of EMP analyses labelled with wt%Y2O3 and (La/Y)N in brackets. Scale bar is 50 Wm in each image.

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with most analyses clustering around 39^45 Maon concordia. Multiple analyses on single grainsshow that, despite the overall variation in the ageof the population, individual monazites did notgrow over such a protracted period. There is noapparent relationship between the ages of mona-zites and their radial distribution within the gar-net (Fig. 5).

Ten monazite micro-inclusions within a singlegarnet and two matrix monazites from sampleG90 were analysed by SHRIMP. The radial loca-tion of a number of the analysed monazite inclu-

sions is shown in Fig. 2a. Fig. 2c shows that insample G90, the analyses are all concordant withthe exception of G90mat1, and that includedmonazites grew over a short period from V44Ma to V36 Ma. Multiple analyses of singlegrains indicate that individual monazites did notgrow over such a protracted period. The age ofG90mat1 is problematic but the discordance ofthis analysis may be an artefact of its high com-mon Pb content (V30% of measured 206Pb andV90% 207Pb). There is no apparent relationshipbetween the ages of monazites and their radialdistribution within the garnet (Fig. 5).

Fifteen monazite micro-inclusions within twogarnets and three matrix monazites were analysedfrom G57 using SHRIMP. The location of anumber of the analysed monazite is shown inFig. 3a. Fig. 3c shows that all monazites are con-cordant and that included monazites grew over ashort period from V44 Ma to V36 Ma. Matrixmonazites, on the other hand, range in age fromV34 Ma to V25 Ma. Once again, multiple anal-yses on the same included grain indicate individ-ual monazites did not grow over such a pro-tracted period. In contrast, Fig. 4a shows thatmatrix grains appear to be composed of domainswith distinct ages. Fig. 5 shows that there is,again, no relationship between Th^Pb age ofmonazite inclusions and their position within gar-net.

Table 1Sm^Nd isotopic data of garnets and whole rocks for samples 97g100U, 97g97 and 97g98

Sample Analysis [Sm] [Nd] 147Sm/144Nda 143Nd/144Ndb Agec Error MSWD(ppm) (ppm)

97g97 whole rock 6.512 35.01 0.1124 0.511950 (04)bulk garnet 2.268 0.646 2.124 0.512468 (34) 39.6 2.9garnet core 2.130 1.038 1.241 0.512232 (08) 38.2 1.5garnet rim 2.327 1.483 0.9485 0.512150 (07) 36.6 1.8all gnt-Wr 38.0 4.9 2.4

97g100U whole rock 6.654 35.92 0.1120 0.511970 (05)bulk garnet 1.281 0.201 3.857 0.512992 (36) 41.7 1.6

97g98 whole rock 7.198 35.01 0.1243 0.512138 (04)bulk garnet 1.876 2.182 0.5198 0.512257 (06) 46.0 3.9

aSm/Nd ratio is considered precise to 0.5% (2c).b143Nd/144Nd normalised to 146Nd/144Nd = 0.7219. Errors are 2c and refer to the last two digits. Replicate analyses of La JollaNd standard yielded 0.511853 þ 0.000007 (2c).cAge of garnet using the whole rock as the low Sm/Nd phase. Errors are 2c and based upon a propagation of analytical uncer-tainties.

Fig. 5. A plot of 232Th^208Pb age of monazite inclusionswithin garnet-in samples 97g74, G90 and G57 against XMn

of adjacent garnet. Error bars on the 232Th^208Pb age are1c. Figs. 1a, 2a and 3a show that XMn decreases away fromthe core in all the garnets studied so that this parameter actsas a proxy for garnet growth stage within (but not between)each sample.

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5.3. Sm^Nd isotopic study

The Sm^Nd isotopic data for samples 97g97,97g97core, 97g97rim, 97g100U and 97g98 arepresented in Table 1. Nd concentrations of thegarnet separates vary from 0.2 to 2.2 ppm andare well within the range obtained by in situLA-ICP-MS analyses of garnet (e.g. [11,23]).This is consistent with the measured Nd and Smoriginating from the garnet lattice rather thanLREE-enriched inclusions. This assertion is fur-ther supported by the elevated Sm/Nd ratios ofall the separates. However, the variations in Sm/Nd and Nd concentrations between the variousseparates of sample 97g97 may well indicate theminor involvement of LREE phases, despite thefact that no such phases were observed during anSEM reconnaissance. Alternatively, it may re£ectREE zoning in the garnet. Nevertheless, the nearsynchronous ages of all the garnet separates ana-lysed in this study indicate that if any LREE-en-riched phases are involved they are the same ageas the occluding garnet. These robust age data inconjunction with the preservation of major ele-ment growth zoning indicate that garnet growthoccurred within the HHCS of the NPHM ataround 44^38 Ma. The slight variation in the tim-ing of garnet growth is probably real, resultingfrom post-peak and recent tectonic reorganisationof the NPHM (for example, see [12,24]).

6. Thermobarometry

In order to place the monazite age and chemicaldata presented above in a thermobarometric

framework, the pressure^temperature evolutionof the rocks needs to be established. Of particularinterest here is the thermal history of the rockduring the period of garnet growth and, hence,the occlusion of the dated monazite inclusions.In the ensuing thermobarometric calculations,the internally consistent dataset of Holland andPowell [25] is used throughout.

The pressure and temperature of garnet rimequilibration are the most straightforward partof the evolution to establish and this has beendone using the composition of the garnet innerrim (beyond the obvious reversals in compositionpresent at the extreme rim; see Figs. 1a, 2a and3a), those of the matrix phases in equilibrium withit and the average P^T mode of THERMOCALC[26] and well-established procedures (e.g. [26]).The resultant pressures and temperatures arelisted in Table 2 and show that garnet rim growthoccurred in the region of 650^750³C at 9^10 kbarfor samples 97g74, G57 and G90.

The pressure^temperature evolution prior torim growth is more di¤cult to determine but sev-eral recent contributions (e.g. [27,28]) have shownthat petrogenetic grids and, in particular, pseudo-sections that are speci¢c to the bulk compositionare particularly useful for this purpose. Since for agiven bulk composition the amounts of eachphase and their compositions are ¢xed at a spe-ci¢c point in P^T space, and provided the present,measured bulk composition of the rock re£ectsthat from which garnet grew, a pseudosectiongrid may be calculated that is speci¢c to that par-ticular rock. Hence the mineral assemblage andcompositions can be modelled in terms of a P^Tevolution. Note that, although garnet changes the

Table 2Summary of garnet rim-matrix pressure and temperature estimates

Sample Assemblagea P Error T Error Fitb

(kbar) (³C)

97g74 ga-bi-mu-ky-pl-q-ru 9.4 1.4 681 23 0.59 (1.49)G57 ga-bi-mu-pl-q-ru 10.5 1.5 771 37 0.72 (1.49)G90 ga-bi-pl-q-ru-ky 10.4 1.5 640 15 0.58 (1.46)aga = garnet; mu = muscovite; bi = biotite; q = quartz; ky = kyanite; pl = plagioclase; ru = rutile.bThis is a chi-squared type statistic and is a measure of the signi¢cance of the ¢t of the independent reactions to a single P^Tpoint. The cut-o¡ value for 95% signi¢cance is given in parentheses.

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e¡ective bulk composition as a result of itsgrowth, the P^T of the initialisation of garnetgrowth is still extractable from the pseudosection.

The pseudosections for samples 97g74 and G57are given in Fig. 6 and the bulk compositionsfrom which these have been calculated usingTHERMOCALC [25,29] in Table 3. These pseu-dosections show the stability ¢elds of various min-eral assemblages. An estimate of the pressure andtemperature of garnet core growth can be ob-tained simply from the position of the garnet-in

lines in Fig. 6. Given the possibility of ¢nite over-stepping of the isograd before actual garnetgrowth, such estimates for the conditions of gar-net growth will be minima.

Constraints on pressure are more equivocalthan those for temperature, but the pressure^tem-perature histories of both rocks clearly involveincreasing temperature and pressure during garnetgrowth, from 500^525³C at 4 kbar to 650^750³Cat 9^10 kbar. We note that this history is similarto that recently established for rocks from similarstructural levels within the Himalayan orogen inZanskar [30].

7. Discussion

7.1. Relative ages of porphyroblast phases

The general concordance between the U and Thisotopic systems of the majority of included mon-azites in these samples is consistent with monazitegrowth at around 45^38 Ma in 97g74 and 44^36Ma in G90 and G57. The main advantage of in

Fig. 6. Calculated pseudosections for samples (a) 97g74 and (b) G57. Several ¢elds have not been labelled on these plots forclarity. In (a) 97g74 these are: 1 = the divariant ¢eld g, bi, pl, chl, mu, st, sill ; 2 = the divariant ¢eld g, ctd, pl, chl, mu, st, and;3 = the divariant ¢eld g, bi, pl, chl, mu, st, sill, ky; 4 = the divariant ¢eld g, ctd, pl, chl, mu, st, ky; 5 = quadrivariant ¢eld g, chl,mu, ky, st; 6 = quadrivariant ¢eld g, pl, chl, mu, and; 7 = quadrivariant ¢eld pl, chl, mu, and, ctd; 8 = quadrivariant ¢eld chl,mu, st, ctd, pl; 9 = trivariant ¢eld g, chl, mu, st, ctd, pl; 10 = quadrivariant ¢eld g, chl, mu, st, ctd; 11 = quadrivariant ¢eld g,chl, mu, ky, ctd; 12 = trivariant ¢eld g, bi, chl, st, pl, mu; 13 = trivariant ¢eld g, pl, chl, mu, and, ctd; 14 = trivariant ¢eld g, bi,chl, and, pl, mu. In all cases with q and H2O. Abbreviations as Vance and Mahar [30] except ctd = chloritoid. These pseudosec-tions have also been contoured for garnet modal abundance labelled in italics. The shaded area shows the garnet absent ¢eld andhence the P^T conditions at which monazite grew (see text). Garnet rim-matrix P^T estimates and garnet core P^T estimates areshown as error ellipses, [1] is the high-temperature dehydration^melting solidus for a kyanite-zone metapelite from the HHCS ofLangtang, Nepal [43].

Table 3Major element composition of whole rocks (given as wt%)

97g74 G57

SiO2 63.39 70.82Al2O3 19.95 12.77Fe2Oa

3 9.52 5.87MnO 0.28 0.10MgO 1.74 2.76CaO 0.43 0.99Na2O 0.99 2.02K2O 2.14 2.83aAll Fe is reported as Fe2O3.

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situ isotopic analysis is that accessory phase tex-tural relationships are retained. In this study, wecan use this textural information to deduce thatmonazite included in garnet grew before or syn-chronously with this rock-forming mineral. If themonazite inclusions grew synchronously with gar-net, the enclosing mineral would determine theU^Th^Pb age of the monazite through occlusion,as this would e¡ectively isolate the monazite fromits nutrients and stop it growing. The most ob-vious result of such a process would be the gen-eration of an age gradient in the monazite inclu-sions from the core of the garnet to its rim. Fig. 5shows that no measurable relationship exists be-tween the U^Th^Pb age of monazite inclusionsand the MnO concentration (a proxy for thegrowth stage) of the adjacent garnet. Such a ran-dom distribution of monazite ages for all the gar-nets of this study indicates that either: (i) thegarnet grew fast enough such that the time reso-lution of the SHRIMP U^Th^Pb ages (2^3 Ma)does not allow an age gradient to be resolved or;(ii) that the monazites grew before garnet. Pre-vious estimates of the period of garnet growthduring regional metamorphism are generallygreater than 3 Ma (5^14 Ma; e.g. [31,32]), provid-ing support for the latter assertion. The importantpoint is that these data would predict that thegarnet porphyroblasts grew at the same time orafter the times de¢ned by U^Th^Pb ages of theincluded monazites ^ i.e. at or after 45^38 Ma in97g74 and 44^36 Ma in G90 and G57.

Monazites in G57 provide further constraintson the timing of garnet growth because monazitesin the matrix of this rock record younger agesthan those in garnet (see Fig. 3c). The cause ofthe younger ages is discussed below but, impor-tantly, monazites within garnet are consistentlyolder than matrix monazites, suggesting that gar-net protected the inclusions from later re-equili-bration, recrystallisation or overgrowth. This be-haviour of garnet is not unexpected given itsrobustness to isotopic resetting and has beendocumented before (e.g. [8,9]). Therefore, if gar-net is present to armour the inclusions, then itmust have grown before the new growth or Pbloss event recorded by the matrix monazites.

This allows us to conclude that garnets in G57grew sometime between V36 Ma and V30 Ma.

Crucially, the timing of garnet growth withinsurrounding rocks has been determined so thatthese predictions from the monazite data can betested. In this study, we document using Sm^Ndsystematics that garnet growth in 5 km of sample97g74 occurred around 44^38 Ma. Prince et al.[11] indicate that, within 2 km of G90, garnetswere growing as early as 37 þ 2 Ma and 40 þ 2Ma and continued until as late as 29 þ 3 Ma.The base of the HHCS slab in Garhwal, however,records a more complicated, polyphase metamor-phic history and no estimates exist for the time atwhich prograde garnet growth began in the rockssurrounding G57. However, garnets from the baseof the HHCS with homogenised major elementpro¢les (indicating that the Sm^Nd systematicshave probably re-equilibrated since growth) rec-ord Sm^Nd ages of 22 þ 3 Ma [11].

The ages of monazite inclusions in garnet-insamples 97g74 and G90 are, therefore, all greaterthan or within error of estimates of the timing oflocal garnet growth. This approach, therefore,clearly o¡ers the opportunity to place relativeconstraints on the age of the occluding porphyr-oblast. For instance, the youngest monazite in-cluded within the garnet of sample 97g74 is40.2 þ 1.4 Ma. The garnet must, therefore, havegrown after V40 Ma, an inference that is invery good agreement with the 44^38 Ma Sm^Ndage for garnet growth near 97g74 presented above(Section 5.3). Similarly, the age of the youngestmonazite inclusion in G90 of 36.3 þ 2 Ma is sim-ilar to the age estimates for the initiation of gar-net growth within 2 km of G90 of 37 þ 2 Ma and40 þ 2 Ma.

Sample G57 is more equivocal in that no con-straints are available on the timing of garnetgrowth from Sm^Nd. We note below, however,that matrix monazites from this sample appearto be overgrown by new monazite at V25 Ma,which is similar to re-equilibrated ages for garnetfrom the same structural levels [11]. The monaziteinclusion data from this sample, however, implythe initiation of garnet growth much earlier thanthis, sometime between 36 Ma and 30 Ma.

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7.2. Conditions of early monazite growth:included monazites

Now that we have established that monazitegrew in these rocks prior to the growth of garnet,we can estimate the conditions necessary for thisto occur. Fig. 6 shows that garnets in samplesG57 and 97g74 grew during increasing pressureand temperature although, as we have outlinedabove, there are no unequivocal estimates ofcore P^T conditions. However, the position ofthe garnet-in line on these pseudosections pro-vides us with a minimum estimate. In both cases,this indicates that garnet grew shortly after 520^550³C and above 6.5 kbar (assuming a smallamount of reaction overstepping). It has beensuggested that monazite growth in pelitic rocksoccurs at staurolite grade at V525³C [33,34].The data presented here are in good agreementwith this estimate, although given the possibilityof a protracted period of monazite growth withinthese samples, it may be slightly too high. Never-theless, in these pelitic assemblages, monazitegrowth begins prior to the growth of metamor-phic garnet, at upper greenschist-facies conditions(the shaded areas of Fig. 6).

It is clear from the chemistry presented in Figs.1b, 2b and 3b, and from the U^Th^Pb isotopicdata, that matrix monazites generally di¡er incomposition from the included population.Equally important, however, is the fact thatsome parts of matrix monazites do show similarage and chemical characteristics to the includedgrains (e.g. 97g74Mat1-3). This observation indi-cates that even when monazites are not armouredby garnet, they can retain their greenschist gradechemical and isotopic characteristics up to at leastupper amphibolite-facies conditions. The assump-tion that monazite in such rocks will date thetiming of peak metamorphism, or cooling through700³C would lead to erroneous tectonic interpre-tations, further illustrating the importance of tex-turally controlled isotopic sampling.

7.3. Conditions of late monazite growth: matrixmonazites

Notwithstanding the above observations, it is

also clear from the chemical and isotopic datathat in samples G90 and G57, the matrix andincluded monazites show contrasting chemicaland isotopic signatures (see Figs. 2, 3 and 4).The evolving chemistry of any mineral is a recordof changes in the temperature, pressure and e¡ec-tive bulk composition experienced by the rockduring the growth period of the mineral in ques-tion. Although element zonation within metamor-phic monazites has recently been interpreted to beindicative of multiphase monazite growth in re-sponse to polymetamorphism (e.g. [9,35]), duringprogressive prograde metamorphism the afore-mentioned parameters are also constantly chang-ing. Therefore, monazite growth at di¡erent timesduring the same metamorphic event may also bechemically distinct. Fig. 4a,c shows that in sampleG57 where matrix monazites yield ages of V30Ma, they are relatively HREE-depleted comparedto the included population. This chemical charac-teristic could arise during prograde metamor-phism. For example, metamorphic garnet typi-cally contains up to V1000 ppm Y [23,36] anda large proportion of the bulk rock's HREEbudget [23,37^39]. Garnet crystallisation wouldtherefore deplete the whole rock reservoir ofHREEs and Y, restricting the amount of theseelements available to monazite that grew aftergarnet formation. Therefore, the distinct chemis-try of the younger monazites, in addition to theconcordancy of the analyses in U^Pb vs. Th^Pbspace, suggests that in these samples monazitegrowth was episodic during continuous progrademetamorphism before and after garnet growth.

This hypothesis also allows the conditions re-sponsible for this later growth to be constrainedas garnet ceased to grow in these samples ataround upper amphibolite grade (see Table 2).Monazite growth with distinct chemical character-istics at these conditions is entirely consistent withthe detailed study of monazite behaviour duringprogressive regional metamorphism by Bea andMontero [7]. Whether this new monazite resultsfrom prograde mineral growth, melt/£uid in¢ltra-tion or deformation is unknown at present. How-ever, analysis of G57mat1c gives an age of V25Ma, and as Fig. 4 shows, this ion probe pit issampling both the low Y outer zone of matrix

EPSL 5568 16-8-00


monazite Mat1 and its high Y rim. This analysistherefore places an upper limit on the age of thehigh Y rim, i.e. less than 25 Ma. Assuming garnetis controlling the HREE budget of this sample, anincrease in the Y available to monazite at 6 25Ma may coincide with the time of garnet break-down. Intriguingly, V22 Ma coincides with a pe-riod of decompression in this region of the Hima-laya [40]. Garnet breakdown is evident in thinsection and garnet adsorption during decompres-sion is consistent with the pseudosection of thissample presented in Fig. 6.

If one assumes the above hypothesis is correct,this approach allows us to determine the durationof the metamorphic event from greenschist gradeto the time of post-peak decompression that af-fected sample G57. Moreover, given the availableconstraints on the P^T evolution of this sampleduring this period (Table 2, Fig. 6), minimumestimates of the burial and heating rate duringthe metamorphic episode can be calculated. Forexample, the P^T constraints for sample G57 in-dicate that, initially, P increased by V4 kbar andT by V200³C. This was followed at some latertime by decompression. The chronological datasuggest that all this occurred in V10 Ma. Thus,minimum heating and burial rates for the P and Tincrease can be calculated as 20³C/Ma and 1 mm/yr. This heating rate is somewhat faster thanthose derived from previous chronological studiesof garnet (e.g. [27,31,32,41,42]) and the burial rateis signi¢cantly slower (V4 mm/yr; [32]). How-ever, this estimate of burial rate and that of [32]are both radically slower than might be suggestedby the rate of convergence of India and Asia (5cm/yr; [15]).

8. Conclusions

This study illustrates that metamorphic mona-zite, much like any other metamorphic mineral,undergoes a complex growth history during re-gional metamorphism. Until a full understandingof the reactions responsible for its growth andbreakdown is recognised, the true power of meta-morphic monazite U^Th^Pb chronometry will notbe realised. However, in this contribution, we

demonstrate that, by retaining the textural rela-tionships of monazites relative to major fabric-forming phases through the application of insitu analytical techniques, metamorphic monaziteis able to provide temporal information on signif-icant portions of the prograde P^T path of meta-morphic pelites. Future geochronological work ofthis sort, incorporating more detailed studies of Yand HREE distribution in both the dated mona-zite and major porphyroblast phases (e.g. [38]),will no doubt further the utility of metamorphicmonazite.

Acknowledgements

This research was supported by the NERC.Andy Tindle is thanked for his help with theEMP analyses and Felix Oberli for assistancewith Sm^Nd mass spectrometry. Two highly con-structive reviews by Frank Spear and Randy Par-rish signi¢cantly improved this manuscript.[RV]

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Documents

The significance of monazite U–Th–Pb age data in metamorphic assemblages; a combined study of monazite and garnet chronometry