Int J Earth Sci (Geol Rundsch) (2004) 93:13–22DOI 10.1007/s00531-003-0376-1

O R I G I N A L P A P E R

Ingo Braun · Michael Br�cker

Monazite dating of granitic gneisses and leucogranitesfrom the Kerala Khondalite Belt, southern India:implications for Late Proterozoic crustal evolution in East Gondwana

Received: 14 February 2003 / Accepted: 30 August 2003 / Published online: 23 January 2004� Springer-Verlag 2004

Abstract Two stages of granitic magmatism occurredduring the Pan-African evolution of the Kerala Khon-dalite Belt (KKB) in southern India. Granitic gneisseswere derived from porphyritic granites, which intrudedprior to the main stage of deformation and peak-metamorphism. Subsequently, leucogranites and leuco-tonalites formed during fluid-absent melting and intrudedthe gneiss sequences. Monazites from granitic gneisses,leucogranites and a leucotonalite were investigated byconventional U-Pb and electron microprobe dating inorder to distinguish the different stages of magmaemplacement. U-Pb monazite dating yielded a wide rangeof ages between 590–520 Ma which are interpreted todate high-grade metamorphism rather than magma em-placement. The results of this study indicate that the KKBexperienced protracted heating (>50 Ma) at temperaturesabove 750–800 �C during the Pan-African orogeny. Thetectonometamorphic evolution of the study area iscomparable to southern Madagascar which underwent asimilar sequence of events earlier than the KKB. Theresults of this study further substantiate previous asser-tions that the timing of high-grade metamorphism in EastGondwana shifted from west to east during the LateProterozoic.

Keywords U-Pb dating · Monazite · Southern India ·Gondwana · Granulite-facies

Introduction

The Kerala Khondalite Belt (KKB) at the southern tip ofIndia is a fragment of East Gondwana. Based ongeochemical investigations, Chacko et al. (1992) con-cluded that the KKB represents a large supracrustal unitof predominantly high-grade metamorphic rocks, whichwere derived from sedimentary or volcanic protoliths.Geochronological studies have shown that the metamor-phic history is closely related to the Pan-African orogenyand that peak-metamorphism occurred ca. 550 Ma ago(Buhl 1987; Soman et al. 1995; Bartlett et al. 1998; Ghosh1999). It was long believed that the KKB was not affectedby major magmatic activity during this orogenic cycle,with the exception of some small and volumetricallysubordinate syenite, leucogranite or pegmatite intrusions(Miller et al. 1997; Kovach et al. 1998). However, in thelast years detailed field work showed that porphyriticgranites, now present as coarse-grained granitic garnet-biotite gneisses, represent a major lithological componentin the northern part of the KKB (Braun et al. 1998). Fieldobservations clearly indicate that the granitoids wereemplaced into the crystalline basement of the study areaprior or contemporaneous to the penetrative Pan-Africandeformation. The focus of this contribution is on thegeochronology of these magmatic rocks. Conventional U-Pb dating by means of thermal ionization mass spec-trometry (TIMS) and age determinations with the electronmicroprobe (EPMA) were carried out on monazites fromgranitic gneisses, leucogranites and a leucotonalite inorder to address the following questions: Is it possible toplace time constraints on the different stages of graniticmagmatism? Is there any regional variation in the timingof magmatism? Is it possible to correlate the timing ofmagmatic and/or metamorphic processes in the KKB withother fragments of East Gondwana?

I. Braun ())Mineralogisch-Petrologisches Institut,Universit�t Bonn,Poppelsdorfer Schloß, 53115 Bonn, Germanye-mail: ingo.braun@uni-bonn.deTel.: +49-228-733752Fax: +49-228-732763

M. Br�ckerInstitut f�r Mineralogie, Zentrallabor f�r Geochronologie,Universit�t M�nster,Corrensstr. 24, 48149 M�nster, Germany

Geological setting

The KKB in southern India (Fig. 1) comprises high-grademetamorphic rocks which were subjected to polyphaseductile deformation and intense migmatization during thePan-African orogeny ca. 550 Ma ago (Buhl 1987;Choudhary et al. 1992; Soman et al. 1995; Bartlett et al.1998; Ghosh 1999). Field, petrological, geochemical andgeochronological similarities to the lower crustal terranesin southern Madagascar (Nicollet 1990; Paquette et al.1994; Markl et al. 2000; de Wit et al. 2001), Sri Lanka(H�lzl et al. 1994; Kriegsman 1995) and East Antarctica(Shiraishi et al. 1994; Fitzsimons et al. 1997; Jacobs et al.1998; Fitzsimons 2000) suggest that all these areas oncebelonged to an internal mobile belt within East Gondwana(Unrug 1996).

The KKB is subdivided into three different lithologicalunits (Srikantappa et al. 1985; Braun and Kriegsman2003) (Fig. 1). The Achankovil Unit forms the northern-most part of the study area. Its northern section, locatedbetween Chenganoor in the NW and Tenkasi in the SE(Fig. 1), is roughly identical with the Achankovil ShearZone (ASZ) of other workers, a tectonic lineament which,despite a great number of uncertainties, is commonlyinterpreted as the continuation of the Ranotsara ShearZone in Madagascar (Drury et al. 1984; Radhakrishna etal. 1990; Sacks et al. 1997; Rajesh et al. 1998). Itpredominantly consists of an intensely deformed andstrongly migmatised sequence of cordierite gneisses,garnet-biotite gneisses, quartzites and marbles, whichwere intruded syn- to postkinematically by (alkali-)granitic magmas (Rajesh and Santosh 1996; B�hm etal. 2003). The central part of the KKB (Ponmudi Unit) ismainly made up of garnet-biotite-sillimanite (-cordierite)-bearing metapelites (the so-called khondalites) andmigmatitic garnet-biotite gneisses, which are derivedfrom sediments (= leptynitic gneiss, Braun et al. 1996)and intrusive rocks (= granitic gneisses, Braun et al.

1998). Massive charnockites or enderbites, mafic gran-ulites and calc-silicate rocks occur in minor abundances.The Ponmudi Unit became well known as one of the typelocalities for in-situ charnockitization of garnet-biotitegneisses and the breakdown of charnockites (RavindraKumar and Chacko 1986; Santosh and Yoshida 1986;Hansen et al. 1987; Jackson et al. 1988; Santosh et al.1990; Raith and Srikantappa 1993; Harley and Santosh1995). Based on geochemical criteria, Chacko et al.(1992) classified the rocks of the Ponmudi Unit as entirelysupracrustal and suggested that the precursor sedimentsmost likely were derived from the Cardamom Hills northof the KKB (Fig. 1). However, the occurrence ofporphyritic gneisses of granitic composition indicatesthat rocks of magmatic origin also form a majorlithological component (Braun et al. 1998). The NagercoilUnit is composed of enderbites and quartz-norites withsubordinate abundances of gneisses and metapelites. Fieldobservations indicate a magmatic origin for the majorityof the Nagercoil granulites which most likely werederived from a differentiated calc-alkaline igneous rocksuite (Srikantappa et al. 1985).

Nd- and Sr-isotope studies indicate that the KKBconsists of reworked Proterozoic continental crust (Harriset al. 1994; Brandon and Meen 1995; Bartlett et al. 1998).Only the alkali syenite of Puttetti (Fig. 1), whosegeneration and emplacement at 572€2 Ma (Kovach etal. 1998) was placed in the context of extensionaltectonics (Rajesh and Santosh 1996), reflects a smallamount of crustal growth during the Late Proterozoic. U-Pb zircon geochronology revealed that the KKB wassubjected to high-grade metamorphism during the Pan-African orogeny at ca. 550 Ma (Buhl 1987; Soman et al.1995; Miller et al. 1997; Bartlett et al. 1998). Geother-mometry points to temperatures of ~900 �C and pressureconditions were estimated to have reached at least 5–6 kbar (Santosh 1987; Chacko et al. 1987, 1996; Braun etal. 1996; Satish-Kumar and Harley 1998; Nandakumar

Fig. 1 Schematic geologicalmap of the Kerala KhondaliteBelt with sample locations(from Braun and Kriegsman2003)

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and Harley 2000; Cenki et al. 2002). An upper pressurelimit of 8 kbar is indicated by the lack of coexistingorthopyroxene and sillimanite in cordierite-gneisses fromthe Achankovil Unit (Cenki et al. 2002). At these P–Tconditions intense migmatisation and fluid-absent meltingin metapelites and garnet-biotite gneisses occurred andled to the generation of leucogranitic melts (Braun et al.1996; Braun 1999). Segregation of the leucogranites fromtheir source region into higher crustal levels took place ina dynamic tectonic setting where they were emplaced asdikes with widths in the decimeter- to meter-scale.

Field relations and petrography

Coarse-grained granitic gneisses predominantly occur inthe northern part of the Ponmudi Unit and samples for thisstudy were collected in sporadic outcrops over a largearea (ca. 500 km2) (Fig. 1). Own field work suggests thatthe granitic gneisses occur within the supracrustalsequence of the Ponmudi Unit which is dominated byfine-grained, so-called leptynitic garnet-biotite gneissesand garnet-biotite-sillimanite(-cordierite) gneisses. Intru-sive contacts with the country rocks have not yet beenobserved in the field and because detailed geologicalmaps are not available, we have no indication on thespatial relationship between the granitic gneisses and themetasedimentary rocks. Unknown is also the geneticrelationship between individual occurrences of graniticgneisses. Although it can not be ruled out that these rocksbelong to a single pluton we have absolutely no fieldevidence for a cogenetic relationship between the differ-ent sample locations and consider this possibility asunlikely.

Granitic gneisses are medium- to coarse-grained rocksand consist of quartz, alkalifeldspar, plagioclase, biotiteand garnet. The igneous origin of the gneiss protoliths isinferred from (1) the homogeneous magmatic fabric, (2)the occurrence of alkalifeldspar and/or plagioclase por-phyroclasts and (3) the presence of rounded or elongatedenclaves of fine-grained and extremely biotite-richgneisses (Fig. 2A). More common than the homogeneousfabric shown in Fig. 2A is a distinct stromatic texture(Fig. 2B) which is the result of ductile deformation andmigmatization subsequent to magma emplacement.Lense-shaped leucosomes or leucocratic layers predom-inantly consist of perthitic alkalifeldspar (Ab21–27Or70–77An02–03) and quartz and host large crystals or aggregatesof garnet which formed as an incongruent phase duringfluid-absent partial melting (Braun et al. 1996). Garnetcomposition is essentially that of an almandine-pyropesolid solution, generally without any major elementzoning (Alm78–86Pyr08–18Grs03–10Sps<02). The mesosomesare composed of biotite, plagioclase (Ab63–72Or01–02An27–35) and quartz. They display a weak gneissicfoliation which is defined by biotite. Accessory phasescomprise zircon, monazite, apatite and ilmenite andpredominantly occur in the mesosomes. Other U- and/or

Th-bearing mineral phases, such as allanite or xenotime,were not recognized in the studied rocks.

The leucogranites and the leucotonalite generally formveins and dikes of less than 5 m width which usuallycross-cut the gneissic foliation at high angles. The rocksare fine- to medium-grained and display a homogeneousand granoblastic magmatic texture. Most of theleucogranites contain garnet which occurs randomlydistributed in a matrix of quartz, perthitic alkalifeldspar(Ab20–29Or68–79An01-03) and plagioclase (Ab75–79Or01–02An19–23). Sample K13-2 contains cordierite, which eitherformed as a magmatic phase or from reaction between

Fig. 2 Outcrop photographs of a granitic gneiss (K15; Chadaya-mangalam). A Homogeneously distributed alkalifeldspar porphy-roblasts and the occurrence of enclaves of fine-grained grt-btgneisses (lower part right of the photo) point to the magmatic originof the gneiss; B Strongly foliated stromatic gneisses from the samelocality indicate an emplacement of the precursor magma prior toductile deformation and high-grade metamorphism

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garnet and the granitic melt and thus indicates rather low-pressure conditions for the crystallization of this sample.The leucotonalite (K1–3) consists of garnet, biotite,quartz and plagioclase. White mica, chlorite and an-dalusite occur as breakdown products of garnet andplagioclase and document intense post-magmatic interac-tion with a hydrous fluid phase.

Accessory phases are the same as in the gneisses butoccur in significantly lower modal abundances. Monaziteusually is the most common accessory and shows arounded or slightly elongated shape with curved orembayed grain boundaries. Grain size mostly rangesbetween 150 and 250 mm with maximum values up to800 mm. In the gneisses, monazites form aggregates withbiotite and/or garnet, or occur as inclusions in the majormineral phases. In the leucogranites, subhedral monazitegrains are randomly distributed in the quartzofeldspathicmatrix.

Analytical procedures

Conventional U-Pb TIMS and EPMA dating was carriedout on monazites from seven granitic gneisses, fourleucogranites and the leucotonalite sample. Mineralseparation and EPMA studies were carried out at theMineralogisch-Petrologisches Institut, Universit�t Bonn.For each sample, monazites were extracted from the grainsize fraction 125–250 mm, and if possible also from thefraction >250 mm. A few monazite grains of each sampleand size fraction were selected for backscatter electronimaging (BSE) and EPMA dating to check for compo-sitional complexities and age zoning. Microanalyses wereperformed with a Camebax electron microprobe at theMineralogisch-Petrologisches Institut, Universit�t Bonn.Monazite analysis were carried out at 20 kV and 50 nA,counting times were 30 (peak)/15 (background) s and100/50 s, respectively. Data processing was performedwith the PAP correction procedure (Pouchou and Pichoir1984).

Isotope analyses were carried out at the Zentrallabo-ratorium f�r Geochronologie, Institut f�r Mineralogie,Universit�t M�nster. Depending on grain size, 3–8 grainsper size fraction were hand-picked under a binocularmicroscope. To remove surface contamination, monazitefractions were ultrasonically cleaned with diluted highpurity HCl and deionised water. Decomposition ofmonazite followed the procedures suggested by Krogh(1973) for zircon, but using 6 N HCl instead of HF fordissolution. A 233U-205Pb mixed spike was added to allsamples for isotope dilution. For chemical separation of Ua HCl-HNO3 technique was applied. Purification of Pb isbased on HBr-HCl chemistry. U and Pb were loaded withphosphoric acid and silica gel on single Re filaments andmeasured on a VG Sector 54 multicollector massspectrometer in static mode, using Faraday cups and aDaly detector for 204Pb. Total procedural blanks were lessthan 140 pg for Pb. U blanks were not determined becausethe very low blank amounts commonly obtained are

negligible for the high U concentrations observed inmonazites. Isotopic ratios were corrected for massdiscrimination with a factor of 0.09% (Pb) and 0.1%(U) per a.m.u., based on analyses of standards NBS-SRM982 and NBS-SRM U-500, respectively. For initial leadcorrection, isotopic compositions were calculated accord-ing to the model of Stacey and Kramers (1975), assumingmonazite crystallization at 550 Ma. All ages and errorellipses were calculated using the Isoplot program,version 2.49 (Ludwig 2001).

Results

Microtextural and compositional featuresof investigated monazites

BSE images of monazites from both gneisses andleucogranites display complex zoning patterns (Fig. 3A,B). The internal structure of most monazites is charac-terized by irregularly shaped domains with sharp andstrongly curved boundaries, suggesting that monazitegrains record a complex history with multiple episodes ofdissolution and reprecipitation (Fig. 3A). A single mon-azite grain from a granitic gneiss displays a complexzoned core which is overgrown by a regularly zoned andeuhedral rim (Fig. 3B). Chemically, individual domainsof single monazite grains are distinct in their Th, U, Pband LREE concentrations. Th abundances generallydisplay a large spread and range between 2 and18 wt%. The variation in Th contents within single grainsas well as within individual samples is generally morepronounced in granitic gneisses (up to 10 wt%) than inleucogranites (up to 5 wt%) (Table 1). In all studiedsamples U and Pb concentrations mostly do not exceed 1and 0.5 wt%, respectively.

Results of EPMA dating

Irrespective of compositional heterogeneities, monazitesfrom granitic gneisses and leucogranites yielded almostexclusively Pan-African dates (Fig. 4A, B, Table 1). Formost of the granitic gneisses, a bimodal distribution ofLate Proterozoic (612–535 Ma) and Ordovician ages(517–423 Ma) is characteristic. The population is inperfect agreement with published U-Pb zircon ages (Buhl1987; Soman et al. 1995; Bartlett et al. 1998; Ghosh1999) and also overlaps with EPMA monazite dates ofKKB paragneisses (Braun et al. 1998). Only the core ofthe monazite grain shown in Fig. 3B yielded significantlyhigher dates (ca. 1,790–1,332 Ma; Fig. 4A).

The leucogranites yielded a single age population.Mean values for all but one sample (K15-2) rangebetween 515–463 Ma, which overlaps fairly well with theOrdovician age population of monazites in the graniticgneisses (Fig. 4B; Table 1). Five analyses on sample K15-2gave a poorly defined mean age of 561€25 Ma with asingle value of 672€61 Ma, which is probably related to

16

the incorporation of an inherited component. Early orMiddle Proterozoic ages have not been detected in thesesamples.

Results of U-Pb TIMS dating

All monazite ages (207Pb/235U) range from 589–515 Ma,but mostly cluster between 583–541 Ma. The new dataneither allow to make any distinction between graniticgneisses and leucogranites (Table 2; Fig. 5A, B) nor do

Table 1 Range of Th, U and Pbabundances and calculated agesin monazites from EPMA dat-ing of granitic gneisses andleucogranites

Th (wt%) U (wt%) Pb (wt%) T (Ma) Mean (Ma)

Granitic gneisses

K12-1 6.15–18.58 0.13–0.42 0.10–0.56 328–628 571€37 423€45K15-1 3.68–14.97 0.25–0.98 0.09–0.75 413–1790 567€10 481€14K18-1 7.05–10.44 0.29–0.56 0.16–0.32 346–637 605€37 487€29K27-1 4.85–15.86 0.20–0.71 0.16–0.40 439–585 541€19K28-1 5.32–15.00 0.32–0.99 0.18–0.49 451–684 612€13 517€30K40-1 5.70–9.71 0.17–0.55 0.12–0.28 418–638 586€22K43-1 2.10–12.00 0.15–0.58 0.04–0.30 345–642 535€27 438€34K44-1 2.94–39.04 0.16–0.59 0.10–1.09 493–618 584€16

Leucogranites

K1-3 6.74–9.67 0.15–0.51 0.12–0.24 340–613 475€21K13-2 5.48–6.79 0.13–2.06 0.10–0.28 346–584 463€30K15-2 9.59–14.15 0.33–0.51 0.26–0.40 506–672 561€25K18-2 8.13–12.89 0.21–0.85 0.23–0.34 436–560 485€28K35-5 4.87–17.49 0.13–0.62 0.09–0.44 331–615 515€20

Fig. 3 A BSE image of monazite from granitic gneiss sampleK28-1 displays a complex internal structure with a homogeneoushigh-Th core rimmed by zones of varying Th concentrations.Irrespective of this, the age distribution within this grain ishomogeneous and yields a single Pan-African population. B BSEimage of monazite from granitic gneiss sample K15-1 shows aeuhedral but complex zoned core with Mesoproterozoic ages. Therim shows oscillatory zoning of chemical composition andexclusively Pan-African ages

Fig. 4 A, B Histogram representations of mean EPMA monaziteages (Montel et al. 1996) from all investigated granitic gneiss andleucogranite samples

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they show any systematic regional distribution. Monazitesof the granitic gneisses are all concordant, except sampleK12-1, and yield ages between 589–519 Ma, furthersubstantiating similar EPMA dates of these samples(Table 2). They also are in agreement with published U-Pb zircon ages of garnet-biotite gneisses from thePonmudi Unit (Buhl 1987; Soman et al. 1995; Bartlettet al. 1998; Ghosh 1999), but show a previously notrecognized tendency towards higher ages (Table 2;Fig. 5A). A similar spread in ages is observed for theleucogranites. Most analysed samples yield values be-tween 573–541 Ma, whereas the results for sample K13-2are significantly younger (522.6 and 515.4 Ma). However,interpretation is much more complex due to the fact, thatsome are discordant (K1-3B) or reverse discordant (K18-2A). Reverse discordancy is quite common for monazitein young rocks (e.g. Himalaya; Sch�rer 1984) and hasbeen explained by the incorporation of 230Th which upondecay produces excess 206Pb. We have no indications foranalytical problems, which also may cause reversediscordancy and therefore suggest that the studied sam-ples are affected by excess 206Pb. In such cases only the207Pb/235U age provides a geologically meaningful ageinformation. In the following text only the 207Pb/235U agesare reported.

In the four samples, where two grain size fractions(125–250 mm and >250 mm) were analysed, the individualages of these fractions differ between 3–12 Ma and do notoverlap within error. In three of the samples the datadisplay a shift towards younger ages with decreasinggrain size whereas the leucotonalite (K1-3) shows anegative correlation between grain size and age (Table 2,Fig. 5B).

The interpretation of such patterns is not a straight-forward task because apparently concordant ages (e.g.K15-2B) may result from partial Pb-loss of previouslyreverse discordant ages. Thus, rather than giving precise

Table 2 U-Pb analytical results (TIMS) for monazite from augen gneisses and leucogranites of the Kerala Khondalite Belt

Sample Size(�m)

208Pb/206Pb

206Pb/204Pb

206Pb/238U

Error 2sigma

207Pb/235U

Error 2sigma

207Pb/206Pb

Error 2sigma

Corr.coef.

206Pb/238U

207Pb/235U

207Pb/206Pb

Augen gneisses:

K12-1 B >125 24.8154 474 0.08342 0.00028 0.6679 0.0032 0.05806 0.00019 0.725 516.5 519.4 532.2K15-1 A >250 9.5993 4961 0.09131 0.00030 0.7418 0.0026 0.05892 0.00007 0.935 563.3 563.4 564.0K15-1 B >125 9.0312 2727 0.08967 0.00032 0.7257 0.0029 0.05870 0.00010 0.910 553.6 554.0 556.0K18-1 B >125 10.7995 5676 0.09053 0.00030 0.7332 0.0026 0.05874 0.00007 0.941 558.6 558.4 557.5K27-1 A >250 5.4533 5407 0.09569 0.00031 0.7855 0.0027 0.05954 0.00007 0.942 589.1 588.6 586.7K27-1 B >125 7.7199 2503 0.09454 0.00031 0.7751 0.0028 0.05946 0.00008 0.921 582.3 582.7 584.1K28-1 B >125 7.5847 7690 0.09278 0.00030 0.7549 0.0026 0.05902 0.00007 0.939 571.9 571.1 567.7K43-1 B >125 9.2585 6571 0.09192 0.00030 0.7462 0.0026 0.05887 0.00007 0.941 566.9 566.0 562.3K44-1 B >125 37.2013 658 0.08773 0.00028 0.7072 0.0032 0.05846 0.00017 0.750 542.1 543.1 547.2

Leucogranites:

K1-3 A >250 15.1176 717 0.08721 0.00030 0.7029 0.0028 0.05846 0.00011 0.875 539.0 540.5 546.9K1-3 B >125 14.4013 557 0.08903 0.00029 0.7236 0.0029 0.05895 0.00013 0.832 549.8 552.8 565.1K13-2 A >250 14.3538 8870 0.08468 0.00028 0.6730 0.0024 0.05764 0.00007 0.940 524.0 522.6 516.2K13-2 B >125 11.2543 2863 0.08344 0.00027 0.6613 0.0023 0.05748 0.00007 0.928 516.6 515.4 510.2K15-2 B >125 7.3277 6467 0.09039 0.00031 0.7315 0.0026 0.05870 0.00007 0.944 557.8 557.5 555.9K18-2 A >250 7.4459 6587 0.09348 0.00098 0.7576 0.0082 0.05878 0.00015 0.972 576.1 572.6 558.9K35-5 B >125 8.5710 6917 0.09183 0.00030 0.7458 0.0026 0.05891 0.00007 0.942 566.3 565.8 563.6

Corrected for fractionation, spike, blank and initial common lead

Fig. 5 A Concordia diagram showing the results of U-Pb dating ofmonazites from granitic gneisses. B Concordia diagram showingthe results of U-Pb dating of monazites from leucogranites

18

time constraints for magmatic or metamorphic processes,only minimum ages are provided by the monazites.

Discussion

By means of the EPMA and conventional U-Pb tech-niques we have dated monazites from granitic gneissesand leucogranites. The new data show a predominance ofLate Proterozoic or younger ages and an almost completelack of Early and Middle Proterozoic ages, which is instriking contrast to geochronological results reported formetasedimentary rocks of the KKB (Braun et al. 1998).

EPMA dating indicated a bimodal distribution of LateProterozoic and Ordovician dates in monazites from theaugen gneisses and the existence of an Ordovician agepopulation in leucogranites (Fig. 4A, B). In contrast,TIMS U-Pb dating exclusively yielded ages between 590to 515 Ma.

We suggest that the Ordovician dates most likelyreflect partial Pb loss. Almost all data points belonging tothe Ordovician population occur at the rims of monazitegrains or along fractures and yield similar Th and Uvalues, but significantly lower Pb concentrations whencompared to measurements that were carried out in morecentral parts of the grains. Such features have alreadybeen described by Braun et al. (1998) who suggested thatthey are related to localized fluid-rock interaction. Itshould be noted that those parts of the monazite grainsthat were affected by fluid-rock interaction are volumet-rically subordinate and thus either do not significantlyinfluence the age information obtained from conventionalU-Pb dating, or, more likely, were removed duringsample preparation for TIMS analysis by leaching withdiluted HCl.

In the gneiss sample K15-1 a relic core of a monaziteyielded Mid-Proterozoic EPMA dates, which indicate adiscordant minimum age for the protolith or an earliermetamorphic event. Similar dates were also reported formonazites from metasedimentary gneisses of the KKBand were interpreted as partly resetted Early Proterozoicages (Braun et al. 1998).

The monazite TIMS U-Pb ages of gneisses andleucogranites show no systematic difference, althoughthe field relationships clearly indicate that the leucogran-ites cross-cut the granitic gneisses. A gneiss-leucogranitepair from the same outcrop in the central part of thePonmudi Unit (K15, Fig. 1) yielded 207Pb/235U ages of554€2.2 and 557.4€2 Ma, respectively, for the same grainsize fraction (125–250 mm; Fig. 5A, B; Table 2). Acorresponding pair from the southernmost part of thePonmudi Unit (K18, Fig. 1) yielded a slightly reversediscordant 207Pb/235U age of 572.6€6.2 Ma for theleucogranite which is significantly older than the age ofthe host gneiss (558.4€2 Ma). However, due to the factthat different grain size fractions were analyzed(leucogranite: >250 mm; gneiss: 125–250 mm), the agesare not directly comparable if affected by diffusion duringcooling.

For most samples, only one grain-size fraction wasanalysed. However, for two granitic gneisses and twoleucogranites U-Pb data are available for two differentgrain-size fractions (Table 2). Three of these samplesshow a clear correlation between grain size and age,suggesting diffusive lead loss during cooling. In contrast,in the leucogranite K1-3 the smaller, discordant grain-sizefraction yielded a higher age, indicating inheritance. Thefact that there are older ages preserved (K1-3 and K15-1)suggests that these monazites have not cooled from abovethe closure temperature, because in this case inheritanceshould have been erased.

What is the geological significance of the monazitedata? Experimental investigations showed that volumediffusion of Pb in monazite alone is rather inefficient inresetting the U-Th-Pb system even at high-temperatureconditions (Poitrasson et al. 1996; Teufel and Heinrich1997; Seydoux-Guillaume et al. 2002). Furthermore,EPMA dating of monazites from lower crustal rocksoften reveals the presence of relic domains which arerelated to earlier stages of monazite growth or resetting/recrystallization during metamorphism or magmatism(Braun et al. 1998; Cocherie et al. 1998; Montel et al.2000; Catlos et al. 2002). The occurrence of a Mid-Proterozoic core in a monazite from the gneiss sampleK15-1 represents such a relic domain. Although thegeological significance of the Mid-Proterozoic dates isunclear, they indicate that at least some of the monazitesalready formed prior to the Pan-African high-grademetamorphism. However, the vast majority of our U-PbTIMS data from granitic gneisses and leucogranites yieldthe same range of Pan-African ages. Interpretation of allU-Pb data as magmatic ages would indicate that (1)intrusion of the precursor granites, (2) high-grade meta-morphism with partial melting and leucogranite forma-tion, and (3) crystallization of the leucogranites must haveoccurred within a very short time period which cannot beresolved with the dating technique used in this study. Weconsider this interpretation as unlikely.

More reasonable is the assumption that the two typesof magmas were emplaced at distinctly different timesduring the Pan-African orogeny. The conformity in agesbetween granitic gneisses and leucogranites is consideredhere as the result of metamorphic processes. In thismodel, the originally magmatic monazite of the granitesand leucogranites experienced partial to complete reset-ting (recrystallization/reprecipitation) at various stagesand temperatures during high-T metamorphism. Theseprocesses might have been related to enhanced mineralreactivity and reaction progress due to local thermaldisturbances and/or increased fluid-rock interaction.Some of the monazites of the leucogranites still mayrepresent magmatic phases, whereas monazites of mostgranitic gneisses (except sample K15-1) were completelyreset during metamorphic overprinting. As indicated bythe considerable range in ages, high-T metamorphism wasnot a short-lived event, but occurred over a long time span(up to 60 Ma) with local variations in the timing ofmetamorphic resetting.

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The KKB as part of the South Indian Granulite Terrainplays an important role for understanding the tec-tonometamorphic evolution of East Gondwana at theend of the Proterozoic. This study indicates that repeatedintrusion of granitic magmas occurred before, during andsubsequent to the peak stage of Pan-African high-grademetamorphism. Available petrological and geochronolog-ical data, including those presented in this study, point tofairly long-lasting high-T conditions. The highest age ofour dataset (588€2 Ma; K27-1) provides a minimum agefor the onset of magmatic activity. A biotite-bearinggranite dike where biotite broke down to form garnet(K35-5) yielded an age of 564€2 Ma. It is in goodagreement with available U-Pb zircon ages which clusteraround 550 Ma (Soman et al. 1995; Bartlett et al. 1998;Ghosh 1999) and thus could be interpreted as the bestapproximation for the peak of Pan-African metamorphismin the study area. From this it follows that the high-temperature event in the KKB lasted from ca. 590–540 Ma which gives a minimum duration of ca. 50 Ma.However, it is reasonable to suggest that the KKBexperienced high-T conditions for even longer time. Thegeneration of garnet-bearing leucogranites is related todehydration-melting of psammitic and pelitic gneisseswhich most likely occurred at temperatures exceeding850 �C at a pressure between 5–7 kbar (Braun et al.1996). The waning stage of this high-T segment of thePan-African orogeny in the KKB might be represented bythe sample K13-2 (522€1.9 and 515€1.8 Ma) wherecordierite formed at the expense of garnet, pointing to apressure decrease along the retrograde P–T path. Theseages are in perfect agreement with U-Pb zircon ages of513€2, 526€3 and 526€1 Ma for garnet-bearing granitesfrom the northwestern part of the KKB (Miller et al. 1997;Ghosh 1999). Thus, the high-T stage of the Pan-Africanorogeny in the KKB might have lasted from 590–520 Ma.

This protracted tectonometamorphic evolution at con-ditions of the lower continental crust is in accord withconclusions of De Wit et al. (2001), based on U-Pb zircondating of gneisses and granites from the lower crustalbasement of southern Madagascar. These authors subdi-vided the long-lasting metamorphic history into threestages, that include a collisional episode of ca. 40 Maduration (>605 Ma), granulite-facies metamorphism(605–550 Ma) and an amphibolite- to greenschist-faciesepisode (520–490 Ma).

Petrological studies of cordierite and grt-bt gneissesfrom the northern part of the KKB point to a complex andmultistage metamorphic evolution along a clockwise P-Tpath that involves stages of isothermal decompression andisobaric cooling (Satish-Kumar and Harley 1998; Nan-dakumar and Harley 2000; Cenki et al. 2002). This showsthat the prograde stages and peak-stage metamorphismare genetically related to collisional tectonics. This is instriking contrast to the interpretation of Miller et al.(1997) who suggested that the generation and intrusion ofpegmatites took place in an extensional tectonic regime.Unfortunately, no information on the composition of the

pegmatite and the presumed P–T conditions of emplace-ment are given by the authors.

Conclusions

The results of this study provide the most comprehensivegeochronological dataset for gneisses and granites fromthe Kerala Khondalite Belt. Granitic magmatism in theKKB most likely occurred during a protracted periodalong the prograde clockwise P–T path during whichtemperature of the upper amphibolite- and the granulite-facies were attained. This led to the emplacement ofbiotite-bearing granites and, subsequently, the generationof garnet- and/or biotite-bearing leucogranites and peg-matites which experienced ductile deformation. High-temperature conditions (>600 �C) prevailed and probablylasted for more than 60 Ma. This was followed by slowcooling in the upper crust down to temperatures below theclosure temperature for Rb-Sr in biotite at ca. 460 Ma ago(Choudhary et al. 1992, Unnikrishnan-Warrier et al. 1995,Unnikrishnan-Warrier 1997). This scenario is comparableto what has been identified for the crustal basement ofsouthern Madagascar (De Wit et al. 2001). However, thetectonometamorphic evolution in both areas is not asynchronous process, since southern Madagascar under-went this sequence of events earlier than the KKB. Yet,the emerging picture is in good agreement with previousobservations indicating that the timing of high-grademetamorphism in different fragments of East Gondwanashifted from west to east during the Late Proterozoic(M�ller et al. 1996).

Acknowledgements Thanks are due to H. Baier (M�nster) forlaboratory assistance. Financial support from the DeutscheForschungsgemeinschaft (Br1909/1-1) is greatly acknowledged.The careful and constructive reviews of Anne N�d�lec and Jean-Louis Paquette provided valuable help in clarifying our views andare greatly acknowledged.

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