Centimetre- and Metre-scale Nd and Sr Isotopic Homogenization in Kadugannawa Complex, Sri Lanka

#Present Address: 2053-16 Ohmachi, Ohmachi-city, Nagano Prefecture, 398-0002 Japan.

Journal of the Geological Society of Sri Lanka Vol. 14 (2011): 129-141. C.B. Dissanayake Felicitation Volume

Centimetre- and Metre-scale Nd and Sr Isotopic Homogenization in Kadugannawa Complex, Sri Lanka

L. R. K. Perera1* and H. Kagami2#

1Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka.

2Division of Study of the Earth's Evolution, Institute for the Study of Earth's Interior, Misasa-city, Tottori 682-01, Japan.

(*Corresponding author, email: [email protected])

ABSTRACT

A charnockitic gneiss and a charnockitic to amphibolite-facies gneiss transition zone in the Kadugannawa Complex show isochron relationships indicating centimetre- and metre-scale isotopic homogenization in both Sm-Nd and Rb-Sr systems. Nine 2-cm thick slabs and five 7-cm cube samples of the charnockitic gneiss give a Sm-Nd whole-rock isochron age of 766±190 Ma which is indistinguishable from Rb-Sr isochron age of 666±72 Ma of the cube samples. The Rb-Sr whole-rock isochron age of the slabs of this first generation charnockit-ic gneiss is 483±38 Ma, and is younger than a previously reported 535±5 Ma Rb-Sr isochron age for an arrested second generation charnockite from the Wanni Complex. Ten whole-rock 7-cm cube samples from the charnockitic gneiss to amphibolite-facies gneiss transi-tion zone in the nearby quarry give a Sm-Nd isochron age of 722±180 Ma which is indistin-guishable from the Sm-Nd age of the cube samples and slabs of the charnockitic gneiss. The Rb-Sr data of six cube samples of amphibolite-facies gneiss combined with its nine 2-cm slabs give an age of 485±72 Ma which is indistinguishable from the Rb-Sr isochron age of 471±23 Ma and Sm-Nd age of 473±600 Ma of the nine slabs, suggesting severe resetting of both isotopic systems. An amphibolite-facies gneiss retrogressive after charnockite dated from another quarry employing 8-cm cube samples gives a Rb-Sr errochron of 1053±110 Ma which is similar to a previously reported zircon Pb-loss event at ~1100 Ma.

Our data suggest that, (i) the Sm-Nd system of the cube samples of the charnockitic- and amphibolite-facies gneisses of the first two quarries has been reset to the same de-gree as in the charnockitic gneiss slabs, (ii) Rb-Sr system has been extremely mobile at cen-timetre-scale irrespective of metamorphic grade, (iii) the probable presence of a hydrous fluid increased the mobility of Sr at metre-scale and Nd at centimetre-scale in the amphi-bolite-facies gneiss. Neither the small cube samples nor the slabs used in the first two qua-rries retain an isotopic memory of the tectono-thermal events suffered by these rocks. In-stead, they probably reflect cessation of isotopic exchange at low temperatures. Our data do not favour regional high-grade metamorphism at ~550 Ma, suggested on the basis of zircon work, but hints that this may be older than 1100 Ma. However, only Sm-Nd and Rb-Sr isotopic systematics of large whole-rock samples are likely to retain this record.

INTRODUCTION

The high-grade metamorphic rocks of Sri Lanka have been subdivided on the basis of Nd model ages into four litho-tectonic units name-ly; the Highland Complex (HC), the Wanni Com

plex (WC), the Vijayan Complex (VC) and the Kadugannawa Complex (KC) (Cooray, 1994)(Fig. 1). The Highland Complex is a granulite-facies belt with an Archaean to early Proterozoic dep-leted mantle model age signature (3.4-2.0 Ga); the Wanni, Vijayan and Kadugannawa Com-plexes are composed of rocks showing early to

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Perera and Kagami, Nd and Sr Isotopic Homogenization

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late Proterozoic model ages (2.0-1.0 Ga)(Milisenda et al., 1988, 1994). The Wanni Complex is a granulite- to amphibolite-facies terrain whereas the Vijayan Complex is domi-nated by amphibolite-facies rocks.

The geochronological framework of evolu-tion of the high-grade rocks of Sri Lanka mostly relies on U-Pb dating of zircon in ortho- and pa-ragneisses (Kröner et al., 1987, 1994; Baur et al., 1991; Hölzl et al., 1991, 1994; Kröner and Jaeckel, 1994). The main argument to support U-Pb dating has been that in both Rb-Sr and Sm-Nd systems isotopic homogenization oc-curred only over a few centimetres during high-grade metamorphism (Hölzl et al., 1991). How-ever, there is very little evidence to support this contention. The argument draws support main-ly from an oxygen isotope study (Fiorentini et al., 1990) which revealed preservation on a small scale, of original oxygen isotope hetero-geneities after high-grade metamorphism. On this basis, linear relationships between whole-rock analyses on Rb-Sr and Sm-Nd isochron plots indicating ages as old as ~2500 Ma from the HC have been interpreted as reflecting the protolith ages of precursors of ortho- and pa-ragneisses (Hölzl et al., 1994).

We present here the results of a detailed Rb-Sr and Sm-Nd whole-rock isochron study employing cube samples and thin slabs, in sev-eral fresh quarries of the Kadugannawa Com-plex - the arenas (Vitanage, 1970) where P-T-t stages of post-metamorphic peak evolution of the high-grade rocks are preserved in the form of prograde and retrograde changes in outcrop. The new data indicate that Sr and Nd isotopic homogenization occurred at metre- and centi-metre- scales, even during the later stages of post-metamorphic peak evolution of the high-grade rocks. The implications of the new data and their significance with respect to existing data are discussed. An Outline of the Geology of Sri Lanka

High-grade metamorphic rocks occupy nearly nine tenths of the island except in the northwest where a coastal belt of Miocene li-mestone is exposed (Fig. 1). The litho tectonic subdivision into HC, WC, VC and KC (Cooray, 1994) differs markedly, especially in the north-west, from previous subdivisions which were based on lithology, structure and metamorphic grade (Geol. Surv. Dept., 1982; Cooray, 1984).

The central HC belt lithologies include felsic to mafic charnockitic gneisses and typical meta-sediments such as quartzites, pelitic to semipe-litic gneisses and crystalline limestones or mar-ble. These have been metamorphosed at granu-lite grade and show evidence of relict mineral assemblages (Hiroi et al., 1994), and late stage reaction textures which post-date the main pe-riod of penetrative fabric formation (Perera, 1987; Schumacher et al., 1990). Peak metamor-

phic conditions (7-9 kb, 830°C) are nearly uni-form across the main HC outcrop, but these gradually decrease towards the West and

Southwest (5 kb, 600°C) exposing shallower levels of the crust (Schumacher and Faulhaber, 1994). The WC is a granulite- to amphibolite-facies terrain of charnockitic gneisses, granitic gneisses, and migmatitic gneisses containing rare metasediments. The VC is a dominantly amphibolite-facies terrain of granitic and mig-matitic gneisses, granites and rare metasedi-ments (Cooray, 1994). In all the lithotectonic units amphibolite-facies gneisses contain evi-dence for a second stage of charnockitization (Perera, 1987), but these are spectacularly de-veloped in the WC (Hansen et al., 1987).

Figure 1: Lithotectonic subdivision of the Precam-brian of Sri Lanka. HC-Highland Complex, VC-Vijayan Complex, WC-Wanni Complex, Shaded area - Kadu-gannawa Complex, Horizontal lines – Miocene limes-tone (modified after Perera, 1983).

Journal of the Geological Society of Sri Lanka, Vol. 14, 129-141

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The boundary between the HC and VC is often interpreted as a thrust contact, and two outliers of HC rocks are found in the southeastern part of the VC (Vitanage, 1985; Kröner, 1986)(Fig. 1). The boundary between the WC and HC is iso-topically defined, and there is no well-defined geological boundary. The KC is represented by a group of hornblende-biotite gneisses and mig-matites, metasediments and charnockitic gneisses mostly confined to the cores of some doubly plunging synforms (Fig. 2). Geological similarity between the KC and the VC and WC has been recognized by early workers (Vita-nage, 1970) and Kehelpannala (1997) also in-corporates the KC in the WC.

The HC rocks contain evidence for at least two phases of tight to isoclinal folding which often have been labeled as D1/D2 (Berger and Jayasinghe, 1976; Sandiford et al., 1988; Kriegsman, 1993, 1994). The main penetrative planar fabric of the rocks formed parallel to the compositional layering during these early stag-es, and is axial planar to the early folds. The layer parallel foliation is also observed in the WC, VC and KC. A N-NW trending mineral linea-tion that lies on this foliation is regionally ob-served in all the lithological subdivisions. The last major deformation (D3) folded the composi-tional layering and the early planar and linear fabrics to produce the prominent dome and basin-like structural features in the central HC and the KC, and is responsible for the present disposition of lithological layers (Berger and Jayasinghe, 1976). D3 is not associated with pe-netrative fabric formation, but ductile shear-zones axial planar to the tighter folds are com-mon in some hinge regions. Arrested, second generation charnockite formation is closely as-sociated with these shear-zones in many places (Perera, 1987; Hansen et al., 1987). There are arguments in favour of (Voll and Kleinschrodt, 1991) and against (Kreigsman, 1993, 1994; Tani, 1994) a common structural history for all four units. Previous Geochronology

Preliminary whole-rock Rb-Sr dating by Crawford and Oliver (1969) suggested that gra-nulite-facies metamorphism took place in the HC of Sri Lanka around 2000 Ma ago, and an amphibolite-facies event retrogressed the gra-nulites at 1150 Ma to form the Vijayan and Wanni Complexes. The derivation of the VC and WC by retrogression of the HC, and the notion

that the Sri Lankan granulites represent the continuation of Archaean granulites of peninsu-lar South India are long held beliefs (Cooray, 1961, 1984).

An ion-microprobe study of zircons in a HC quartzite revealed 2.45 - 3.2 Ga old concordant to discordant zircons which lost Pb at 1.1 Ga (Kröner et al., 1987). Two HC pelitic gneisses Kröner et al (1987) studied contain 2.0 Ga old zircons which lost Pb at 1.1 Ga. The ion-microprobe data combined with 2.2 - 3.2 Ga TDM model ages of the HC (Milisenda et al., 1988, 1994) suggested that the source rocks of the granulite grade metasediments are late Arc-haean to early Proterozoic. VC and WC gave 1.1 -2.2 Ga TDM model ages rejecting the idea that the HC has been retrogressed to form these terrains. In the HC granulite grade metamor-phism at 1100 Ma proposed by Kröner et al (1987) was subsequently retracted in the light of later U-Pb zircon data of Baur et al (1991) and Hölzl et al (1991, 1994), which failed to record a 1100 Ma Pb-loss event.

Most of the HC pelitic gneisses and orthog-neisses contain a very uniform population of concordant to discordant ~1.85-2.0 Ga old zir-cons which lost Pb around 611-550 Ma (Hölzl et al., 1991, 1994). Kröner et al (1994) reported single zircon 207Pb/206Pb evaporation ages of 1.8 - 2.5 Ga from another HC quartzite. Most of the WC and VC metasediments and orthogneisses have a uniform population of concordant to discordant 1.1 Ga old zircons which lost Pb at 611-550 Ma. Concordant zircons between 700-1100 Ma and single zircon 207Pb/206Pb evapora-tion ages between 750-790 Ma have also been recorded from WC orthogneisses (Kröner and Jaeckel, 1994; Kröner et al., 1994). 1.1 and ~0.75 Ga zircon ages have been attributed to the intrusive age of protoliths of the orthog-neisses. The Pb-loss event at 611-550 Ma is pre-sently interpreted as the age of peak of high-grade metamorphism in all geological units. However, in the WC there are metasediments and orthogneisses containing 1.1 Ga old multi-faceted metamorphic zircons (Kröner and Jaeckel, 1994; Kröner et al., 1994) which show no Pb-loss at the suggested metamorphic peak of 611-550 Ma.

In Kurunegala, in-situ charnockite and its host gneiss resulted in discordant ~771 Ma zir-cons showing Pb-loss at ~563 Ma. Sm-Nd and Rb-Sr whole-rock isochrons of ~535 Ma for the amphibolite- to granulite-facies transition has


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Figure 2: Kadugannawa Complex around Kandy showing the locations of the studied samples. GD-Gadaladeniya samples, Gh-Odillawe quarry, Dumbara synform (modified after Perera, 1983).

been interpreted as the age of in-situ charnock-itization (Burton and O'Nions, 1990a).

Despite the suggested 611-550 Ma meta-morphic age (Baur et al., 1991; Hölzl et al., 1991, 1994) there remains evidence of much earlier high-grade metamorphism in Sri Lanka. Rb-Sr and Sm-Nd isochron ages as old as 2500 Ma have been reported from both ortho- and paragneisses (Kagami et al., 1995; Hölzl et al., 1991) of the Highland Complex. DeMasschalck et al (1990) presented data to support the 2000 Ma granulite event suggested by Crawford and Oliver (1969). Evidence of Pb-loss from zircons and new zircon growth at 1100 Ma is also upheld by a combination of existing published U-Pb and Rb-Sr data (Kröner et al., 1987; Burton and O'Nions, 1990b), and present data.

SAMPLING

Most of the material used in this study comes from two quarries on the eastern limb of the Gadaladeniya synform of the Kadugannawa Complex (Figs. 1 & 2). One of the quarries con-tains charnockitic gneiss that is retrograded pa-rallel to the foliation. Whole-rock, 7-cm cube samples (GD 1-5) represent a banded charnock-itic gneiss away from the retrograded layers, collected over a distance of 2-3 metres perpen-dicular to the foliation. Nine 2-cm thick slabs

cut parallel to the foliation of GD-4 were also studied.

The other quarry which is just 200 metres to the West contains charnockitic gneiss which has been extensively retrograded parallel to the foliation to form amphibolite-facies gneiss. Large relict patches of the original charnockitic gneiss and small (2 -10 cm) patches of arrested second generation charnockite occur in the am-phibolite-facies gneiss. The arrested charnockit-ic rocks which in places occupy late-stage shear-zones are distinctly coarse grained compared to the relict, medium- to fine-grained, charnockitic gneiss which preserves original foliation. The whole-rock 7-cm cube samples of this quarry represent two adjacent lithological bands of charnockitic gneiss (GD-P to S) and amphibolite-facies gneiss (GD-A to E). The sample GD-X is a relict charnockitic patch in the gneiss. The sam-ples were collected from within a distance of 3-4 metres in a direction perpendicular to the fol-iation. Nine 2-cm thick amphibolite-facies gneiss slabs from GD-C were also studied.

Odillawe quarry on the eastern limb of the Dumbara synform in the KC (Fig. 2) exposes charnockitic gneisses, and charnockitic gneisses retrogressed to amphibolite-facies gneisses. The samples Gh 1-2 to 1-6 represent 8-cm cubes collected from a single lithological unit of amphibolite-facies granitic gneiss showing very


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small scale (2-4 cm) patches of arrested in situ charnockite. The samples were collected ran-domly from a quarry face of 2x4 metres.

The retrograde and prograde relationships between charnockitic rocks and amphibolite-facies gneisses in the sampled quarries of the (arenas) KC are consistent with the P-T-t rela-tions in the HC granulites deduced on the basis of garnet formation and breakdown reactions (cf. Fig. 2, Perera, 1994). These reaction tex-tures in the HC post-date the main penetrative fabric formation which has been attributed to a period of tight to isoclinal folding (D1/D2) under granulite-facies conditions. The garnet break-down reactions post-date a second generation of garnet formation during a period of near-isobaric cooling which probably led to the re-trogression of charnockitic gneisses in the sam-pled quarries of the KC (Perera, 1987). In some pelitic rocks of the HC, dehydration of biotite in the presence of quartz accompanies hyper-sthene-cordierite symplectite formation around garnet, suggesting that dehydration leading to arrested charnockitization along shear-zones was caused by decompression accompanying major D3 folding (Perera, 1987, 1994). It is be-lieved that the present sampling strategy sheds some light on the absolute timing of the pro-grade and retrograde relationships observed in the sampled quarries.

ANALYTICAL TECHNIQUES

The chemical separation of Rb, Sr, Sm and Nd from the rock powders followed the proce-dures of Kagami et al (1987, 1989), and were carried out in the laboratories of the Institute for Study of the Earth's Interior, Misasa, Japan. Isotopic measurements were performed using a Ta-Re double filament assembly on a Finnigan MAT 260 mass-spectrometer modified to a mul-ti-collector arrangement. The measured 87Sr/86Sr ratios were normalized to a 88Sr/86Sr ratio of 8.37521 and the measured 143Nd/144Nd ratios were normalized to a 146Nd/144Nd of 0.7219. The measured 87Sr/86Sr ratio for NBS 987 during this study is 0.710221±13 (2-sigma mean, N=2). The 143Nd/144Nd ratios in Tables 1 to 5 are reported relative to 143Nd/144Nd = 0.512640 for BCR-1 (Wasserburg et al., 1981). The errors on the measured 87Sr/86Sr and 143Nd/144Nd ratios are 0.01% and the errors on the measured 87Rb/86Sr and 147Sm/144Nd ratios are 0.5% and 0.1%, respectively. Isotope dilu-

tion analysis was performed using a mixed 87Rb -86Sr spike to measure Rb and Sr concentrations and a mixed 149Sm -145Nd for Sm and Nd. Re-gression of Rb-Sr and Sm-Nd data was done us-ing the ISOPLOT 3.71 program of Ludwig (2008). All errors are at the 2-sigma level.

RESULTS

Tables 1 and 2 give whole rock Rb-Sr and Sm-Nd isotopic data for 7-cm cube samples (GD 1-5) and nine 2-cm thick slabs (GD 4A-I) of char-nockitic gneiss samples, collected in the Gada-ladeniya synform.

The five cube samples give a Sm-Nd whole-rock isochron age of 814±300 Ma (143Nd/144Ndi = 0.51148±0.00023, MSWD = 0.09)(not shown). The Rb-Sr data for these give a whole-rock isochron age of 666±72 Ma (87Sr/86Sri = 0.70547 ±0.00016, MSWD = 1.1)(Fig. 3), which is indis-tinguishable from the Sm-Nd isochron age. Sm-Nd data for the nine 2-cm thick slabs result in a whole-rock isochron age of 734±240 Ma (143Nd/144Ndi = 0.51154±0.00019 MSWD = 0.31) (not shown) which is indistinguishable from the result obtained for the cube samples. The data of the five cube samples and nine slabs com-bined together produce a Sm-Nd whole-rock isochron age of 766±190 Ma (143Nd/144Ndi = 0.51152±0.00015, MSWD = 0.22)(Fig. 4). The Rb-Sr data of the nine slabs give a much young-er whole-rock isochron age of 483±38 Ma (87Sr/86Sri = 0.705939±0.000089, MSWD = 0.85) (Fig. 5) compared to the whole-rock Rb-Sr age of the cube samples and Sm-Nd age of the cube samples and slabs. Although MSWD of the Sm-Nd isochrons indicate good statistical fits, the large errors are resulting from narrow ranges shown by parent ratios of the samples.

Tables 3 and 4 give whole-rock Rb-Sr and Sm-Nd isotopic data for 7-cm cube samples of charnockitic gneiss (GD - P to S and X) and am-phibolite-facies gneiss (GD A-E), and for nine 2-cm thick slabs (GD Ca-i) of amphibolite-facies gneiss, collected in the Gadaladeniya synform.

Sm-Nd data of the cube samples of char-nockitic gneiss and amphibolite-facies gneiss combined together give an isochron age of 722±180 Ma (143Nd/144Ndi= 0.51156±0.00013, MSWD = 0.76)(Fig. 6), which is indistinguishable from the result obtained from the previous quarry for the same isotopic system. The char-nockitic gneiss, amphibolites-facies gneiss cube samples and the 2-cm slabs from the latter


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combined together give an isochron age of 736±160 Ma (143Nd/144Ndi = 0.51156±0.00013, MSWD = 0.63) (not shown). Since the slab sam-ples show a narrow range of parent ratios com-pared to the cube samples, the combined isoch-ron does not deviate from that of the cube samples. The amphibolites-facies gneiss cube samples and slabs combined together indicate a younger isochron age of 638±240 Ma (143Nd/144Ndi =0.51164±0.00019, MSWD = 0.45) although indistinguishable from the former.

Figure 3: Rb-Sr isochron diagram for whole-rock 7-cm cube samples of charnockitic gneiss, Gadalade-niya synform.

Figure 4: Sm-Nd isochron diagram for 7-cm cube charnockitic gneiss whole-rock samples (open cir-cles) and 2-cm slabs (filled circles).

The Sm-Nd data of the slabs samples regressed alone produce a younger alignment of 473±600 Ma (143Nd/144Ndi = 0.51177±0.00047, MSWD = 0.30)(Fig. 6). Like in the previous quarry the larger errors in Sm-Nd ages result from very narrow ranges of parent ratios. Nine 2-cm slabs of the amphibolite-facies gneiss give a Rb-Sr isochron age (471±23 Ma, 87Sr/86Sri = 0.70655± 0.00012, MSWD = 3.5)(Fig. 7), and this together with Sm-Nd data alignment of ~473 Ma sug-gests that both isotopic systems of the slabs have been reset to identical extents.

Table 1: Rb-Sr and Sm-Nd whole-rock isotopic data for 7-cm cube samples of charnockitic gneiss, Gadaladeniya synform (KC).

Sample Rb

(ppm) Sr

(ppm) 87

Rb/86

Sr 87

Sr/86

Sr Sm

(ppm) Nd

(ppm) 147

Sm/144

Nd 143

Nd/144

Nd

GD - 1 42.5 635 0.1941 0.707319±13 3.76 18.4 0.1237 0.512142±12 GD - 2 33.2 660 0.1458 0.706820±16 2.70 14.4 0.1132 0.512089± 6 GD - 3 28.6 757 0.1096 0.706491±14 0.86 5.3 0.0980 0.512002±14 GD - 4 32.6 535 0.1767 0.707131±14 3.10 14.5 0.1294 0.512178± 6 GD - 5 29.1 625 0.1348 0.706799±09 4.59 21.9 0.1267 0.512146± 5

Table 2: Rb-Sr and Sm-Nd isotopic data for nine 2-cm thick slabs of charnockitic gneiss sample GD - 4.

Sample Rb

(ppm) Sr

(ppm) 87

Rb/86

Sr 87

Sr/86

Sr Sm

(ppm) Nd

(ppm) 147

Sm/144

Nd 143

Nd/144

Nd

GD - 4A 36.8 563 0.1895 0.707248± 9 3.16 15.3 0.1255 0.512164± 9 GD - 4B 53.7 627 0.2481 0.707657± 9 1.89 9.64 0.1188 0.512119±11 GD - 4C 15.7 438 0.1040 0.706693± 9 4.30 18.5 0.1402 0.512212± 8 GD - 4D 27.2 526 0.1495 0.706952± 9 2.83 13.8 0.1245 0.512149± 9 GD - 4E 23.6 654 0.1044 0.706635± 9 0.91 5.41 0.1017 0.512053±12 GD - 4F 31.1 683 0.1317 0.706863±10 1.84 9.45 0.1175 0.512113±12 GD - 4G 43.0 691 0.1801 0.707213± 9 3.46 17.9 0.1166 0.512100± 9 GD - 4H 35.0 669 0.1516 0.706955± 8 4.07 22.6 0.1088 0.512056± 8 GD - 4I 40.6 667 0.1761 0.707097± 8 3.07 16.9 0.1094 0.512048± 9


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Table 3: Rb-Sr and Sm-Nd whole-rock isotopic data for 7-cm cube samples of charnockitic gneiss (GD-P to S and X) to amphibolite-facies gneiss (GD-A to E) transition zone, Gadaladeniya synform (KC).

Sample Rb

(ppm) Sr

(ppm) 87

Rb/86

Sr 87

Sr/86

Sr Sm

(ppm) Nd

(ppm) 147

Sm/144

Nd 143

Nd/144

Nd

GD - P 58.5 657 0.2578 0.708940±14 2.46 14.7 0.1014 0.512008± 5 GD - Q 61.8 641 0.2790 0.709180±12 1.84 11.1 0.0996 0.512012± 6 GD - R 41.8 591 0.2050 0.708634±10 3.70 16.5 0.1357 0.512194± 7 GD - S 69.5 517 0.3895 0.710750±14 4.98 30.3 0.0995 0.512042±17 GD - A 64.2 503 0.3697 0.709217±13 6.91 30.8 0.1354 0.512213± 8 GD - B 43.1 640 0.1949 0.707860±17 4.36 23.3 0.1131 0.512076±12 GD - C 70.1 645 0.3143 0.708681±12 3.61 18.6 0.1171 0.512104±11 GD - D 75.8 577 0.3801 0.709974±17 4.56 23.2 0.1186 0.512121± 7 GD - E 68.6 622 0.3192 0.708928±10 4.18 22.5 0.1123 0.512121±11 GD - X 71.2 625 0.3298 0.708910±14 2.13 13.4 0.0964 0.512046± 6

Table 4: Rb-Sr and Sm-Nd whole-rock isotopic data for nine 2-cm thick slabs of amphibolite-facies gneiss sam-ple GD - C.

Sample Rb

(ppm) Sr

(ppm) 87

Rb/86

Sr 87

Sr/86

Sr Sm

(ppm) Nd

(ppm) 147

Sm/144

Nd 143

Nd/144

Nd

Gd - Ca 74.2 641 0.3348 0.708909±12 3.11 15.9 0.1184 0.512130±15 Gd - Cb 76.1 651 0.3378 0.708862±11 2.31 11.7 0.1189 0.512141±18 Gd - Cc 65.9 639 0.2985 0.708582±12 4.30 22.2 0.1174 0.512139± 9 Gd - Cd 75.2 696 0.3124 0.708636± 9 4.35 23.1 0.1140 0.512126±11 Gd - Ce 69.1 641 0.3117 0.708662±12 3.15 17.2 0.1111 0.512116± 9 Gd - Cf 61.8 605 0.2957 0.708535±11 6.53 31.6 0.1250 0.512171± 7 Gd - Cg 38.8 592 0.1895 0.707734± 9 5.14 25.1 0.1239 0.512163± 6 Gd - Ch 130.9 496 0.7647 0.711630±10 3.46 17.2 0.1220 0.512116± 9 Gd - Ci 59.9 944 0.1838 0.707709±12 5.68 28.6 0.1199 0.512143± 7

Table 5: Rb-Sr and Sm-Nd whole-rock isotopic data for 8-cm cube samples of amphibolite-facies gneiss, Dum-bara synform (KC).

Sample Rb

(ppm) Sr

(ppm) 87

Rb/86

Sr 87

Sr/86

Sr Sm

(ppm) Nd

(ppm) 147

Sm/144

Nd 143

Nd/144

Nd

Gh 1-2 81.9 313 0.7581 0.714622±18 7.86 37.3 0.1275 0.512058±11 Gh 1-3 92.8 147 1.8344 0.730433±15 5.94 30.9 0.1162 0.512160±11 Gh 1-4 96.3 261 1.0665 0.718513±10 9.02 46.4 0.1176 0.512064± 8 Gh 1-5 86.1 163 1.5354 0.726545±13 8.20 42.7 0.1161 0.512135±10 Gh 1-6 97.3 285 0.9898 0.717916±12 5.83 25.6 0.1378 0.512238±13

Moreover, the Rb-Sr isochron age of 483±38 Ma for the charnockitic gneiss slabs in the previous quarry is indistinguishable from the 471±23 Ma age. The amphibolite-facies gneiss cube sam-ples and slabs and the relict charnockite sample GD-X combined together give an errochron of 485±72 Ma (MSWD = 44). The sample GD-D causing the large MSWD if eliminated from re-gression a better statistical fit (MSWD =7.7) of 473±31 Ma is produced. We cannot justify such elimination, but it suggests that GD-D possesses some older radiogenic component, though the other cube samples from the amphibolite-facies gneiss hosting GD-X have been strongly reset to an extent shown by the gneiss slabs. The char-nockitic gneiss samples GD-P to S give only a Rb-Sr errochron of 836±460 Ma (87Sr/86Sri = 0.7060±0.0019, MSWD = 33). The Rb-Sr data of

amphibolite-facies gneiss cube samples give an errochron of 681±370 Ma (MSWD = 60). These errochrons are indistinguishable from the Rb-Sr isochron age of 666±72 Ma for the cube sam-ples of charnockitic gneiss in the other quarry.

Table 5 contains whole-rock Rb-Sr and Sm-Nd data for the five 8-cm cube samples col-lected from amphibolite-facies gneiss in the Dumbara synform. The Rb-Sr data produce an errochron age of 1053±110 Ma (87Sr/86Sri = 0.7030±0.0020, MSWD = 56) (Fig. 8). However, the Sm-Nd data failed to produce any linearity.

DISCUSSION

The Rb-Sr and Sm-Nd whole-rock isochron relationships for the cube samples collected


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Figure 5: Rb-Sr isochron for 2-cm charnockitic gneiss slabs (GD 4 A-I), Gadaladeniya synform.

Figure 6: Sm-Nd isochrons for charnockitic gneiss and amphibolite-facies gneiss whole-rock cube sam-ples (open circles) and amphibolite-facies gneiss slabs (filled circles).

within a distance of 2-3 metres in the first quar-ry clearly indicate that isotopic homogenization in both isotopic systems had occurred at some stage of their geologic evolution. In particular,

Figure 7: Rb-Sr isochron for amphibolite-facies gneiss 2-cm slabs (filled circles). Amphibolite-facies gneiss whole-rock cube samples and relict charnock-itic gneiss sample GD-X (open circles) suggest isotop-ic homogenization with slabs.

Figure 8: Rb-Sr errochron for large cube samples of amphibolite-facies gneiss from Oodillawe quarry, Dumbara synform.

the inability to distinguish the Sm-Nd age (814±300 Ma) of the cube samples from that of the slabs (734±240 Ma) indicates preservation of the metre-scale isotopic homogenization.

The isochron age of 766±190 Ma (Fig. 4) for cube samples and slabs combined further em-phasizes it. Although the three ages are mutual-ly indistinguishable, preservation of an older isotopic component in the cube samples is evi-dent, indicating that those and the slabs have been reset to different degrees. The disparity between the Rb-Sr result for the cube samples (666±72 Ma) and slabs (483±38 Ma)(Figs. 3 & 5) indicate that the mobility of Sr continued on a centimetre scale long after the whole-rock cube samples became closed for Sr and Nd exchange. The Sm-Nd isochron age of the slabs (734±240 Ma), and cube samples and slabs (766±190 Ma) (Fig. 4) indicate that Nd exchange among the cube samples and slabs ceased earlier than Sr exchange closed at centimetre scale.

The Sm-Nd and Rb-Sr data of the second quarry are consistent with the observations on the isotopic behavior of the two systems recog-nized in the previous quarry. However, more extensive isotopic resetting in both systems evident in the second quarry probably has been caused by the presence of a hydrous fluid which led to the retrogression of the charnockitic gneiss under amphibolite-facies conditions. A clear manifestation of the enhanced mobility of daughter isotopes due to the probable pres-ence of a fluid is shown by both Sm-Nd and Rb-Sr systems. The cube samples of charnockitic gneiss, amphibolite-facies gneiss, and amphibo-lite-facies gneiss slabs, irrespective of meta-morphic grade, produced a Sm-Nd isochron age


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(736±160 Ma) which is indistinguishable from the Sm-Nd isochron for the cube samples and slabs (766±190 Ma) in the previous quarry. Thus, one might argue that the fluid in the am-phibolite-facies gneiss apparently had limited influence on the Nd isotopic system and the retrogressive event did not reset Nd on a me-tre-scale. However, unlike in the previous quar-ry where the slabs produced a Sm-Nd isochron of (734±240 Ma), Nd continued to be mobile at centimetre-scale in the amphibolite-facies gneiss and produced a Sm-Nd isochron age of (473±600 Ma) (Fig. 6) suggesting that the Nd in the slabs has been strongly reset at a later age. The extent to which the Sm-Nd of slabs was re-set is comparable to that of their Rb-Sr system producing an age of 471±23 Ma (Fig. 7). The Rb-Sr errochron age of 485±72 Ma of the amphibo-lite-facies gneiss for 7-cm cube samples and 2-cm slabs combined also suggests that Sr isotop-ic resetting severely affected both to similar extent, and Sr isotopic mobility occurred at me-tre-scale under the influence of a fluid till a later date. However, the gradually older Sm-Nd isochron ages given by combined amphibolite-facies gneiss cube samples and slabs (638±240 Ma) and charnockitic gneiss, amphibolites-facies gneiss cube samples and slabs (736±160 Ma) (both not shown) indicate that charnockitic gneiss and amphibolites-facies gneiss cube samples retain older isotopic components, and they have been reset to different degrees. On the other hand, these ages are indistinguishable from the Rb-Sr isochron age (666±72 Ma) (Figs. 3, 4) and Sm-Nd isochron age (766±190 Ma) of the charnockitic gneiss in the previous quarry also suggests that the true age of granulite-facies metamorphism, and granulite- to amphi-bolite-facies retrogression are presumably old-er. The datum for the relict charnockitic gneiss sample (GD-X) plotting on the isochron relation-ship of the host gneiss samples upholds the lat-ter point and favours the argument that Sr con-tinued to be mobile in the hydrous system. The identical Rb-Sr whole-rock isochrons for thin slabs of both charnockitic gneiss and amphibo-lite-facies gneiss (Figs. 5,7) suggest that the Rb-Sr system was highly mobile on a centimetre scale till ~470 Ma irrespective of the metamor-phic grade.

According to the present data there is com-pelling evidence that metre-scale and centime-tre-scale isotopic homogenization and resetting took place both in Rb-Sr and Sm-Nd isotopic

systems at some stage during the geological evolution of the sampled rocks. However, the isotopic data of the present study cannot re-solve the absolute ages of the observed meta-morphic changes in outcrop. Instead, the two quarries which are 200 m apart contain very similar isotopic records, the only difference be-ing the extensive resetting of Rb-Sr system of both cube and slab samples of amphibolite-facies gneiss to similar extents in the second quarry, and resetting of the Sm-Nd in the am-phibolite-facies gneiss slabs to an extent com-parable to their Rb-Sr system. Despite these observations, one could argue that the Sm-Nd system of cube samples at both localities is pre-serving the age of the protoliths of the gneisses. This is more so because 791±6 Ma to 755±7 Ma single zircon 207Pb/206Pb evaporation ages and 771+17/-14 Ma discordant zircon upper inter-cept ages have been interpreted as intrusive ages of WC orthogneisses (Baur et al., 1991; Kröner and Jaeckel, 1994). Any interpretation of our 766±190 Ma and 736±160 Ma Sm-Nd isoch-ron ages as protolith ages implies that there had been no metre-scale isotopic homogeniza-tion during subsequent high-grade metamor-phism of the Sri Lankan granulites. The only possibility then is that peak granulite grade me-tamorphism and the main period of charnock-itization are probably represented by the Rb-Sr whole-rock isochron age of 483±38 Ma shown by the thin slabs of the charnockitic gneiss.

However, Burton and O'Nions (1990a) 535±5 Ma Rb-Sr isochron for whole-rock sam-ples across an arrested second generation charnockite to amphibolite-facies gneiss transi-tion zone, is older than the Rb-Sr age of 483±38 Ma for slabs of earlier generation charnockitic rocks of this study. Burton and O'Nions' (1990a) age is also older than the 471±23 Ma Rb-Sr whole-rock isochron age for thin slabs of am-phibolite-facies gneiss hosting arrested char-nockites in the quarry we studied. Similarly, their Sm-Nd isochron age of 535±36 Ma for ar-rested charnockitization is apparently older than the Sm-Nd whole rock errochron age of 473±600 Ma for the thin amphibolite-facies gneiss slabs and the 473±31 Ma Rb-Sr whole-rock isochron for cube samples of the same gneiss and relic charnockite. Clearly, an inter-pretation our Rb-Sr whole-rock isochron age of 483±38 Ma as representing granulite-facies me-tamorphism contradicts with Burton and O'Nions' (1990a) data. We thus strongly believe


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that our Sm-Nd and Rb-Sr data neither contain any protolith history nor any event of meta-morphism. Our data suggest that Burton and O'Nions' (1990a) data do not necessarily represent arrested in situ charnockitization, instead, metre- to centimetre-scale isotopic homogenization preserved in the gneiss, has been picked up by the small whole-rock sam-ples they employed. Contrary to our interpreta-tion, Unnikrishnan-Warrier et al (1995) re-ported Sm-Nd and Rb-Sr whole rock-mineral isochrons of 517±26 Ma and 484±15 Ma for a `massive' or first generation charnockitic rock from Kottaram at the southern tip of peninsular India. Since, these ages are similar to the age of arrested in situ charnockitization from the local-ities further to the North at Ponmudi, and also from Sri Lanka, those have been attributed to a regional Pan-African event which affected parts of India and Sri Lanka. On the other hand, Ka-gami et al (1995) reported a Sm-Nd whole-rock-mineral isochron age of 525±6 Ma for a `mas-sive' or first generation charnockitic rock in the HC, which lies on a Sm-Nd whole-rock isochron of 2330±30 Ma for bulk samples. Though the whole rock-mineral age is identical to the Sm-Nd mineral isochron age of 524±9 Ma of the arrested charnockite at Kurunegala (Burton and O'Nions, 1990a) we doubt that there is only a single period of charnockitization since field and petrographic data suggest that these represent two extreme stages of tectono-metamorphic evolution of HC & WC; i.e. early tight to isoclinal folding and shear-zones post-dating major D3 folding, respectively. Also the early charnockitic

rocks formed at 850-900°C (Schumacher and Faulhaber, 1994) and in situ charnockites at

738±60 °C (Burton and O'Nions, 1990a). Thus, as discussed above these Pan-African mineral-whole rock ages probably represent cessation of isotopic exchange at mineral or small whole-rock scale rather than high-grade metamor-phism. Recent, garnet-biotite thermometric mapping (Sajeev and Osanai, 2005) shows that the dated samples of the present study come from an area that has recorded the highest me-tamorphic temperatures. Thus, those probably cooled for a longer period closing down isotopic exchanges very late.

Since the Sm-Nd system of our charnockitic cube samples in the first quarry has been more or less reset to the same extent as in 2-cm thick charnockitic gneiss slabs, and the Rb-Sr system of amphibolite-facies gneiss cube samples in

the second quarry has been more or less reset to the same extent as in 2-cm thick amphibo-lite-facies gneiss slabs, we believe that the 7-cm cube samples from the Gadaladeniya synform do not retain any record of previous isotopic homogenization. Our data also suggest that Sm-Nd system of thin-slabs of amphibolite-facies gneiss has been strongly reset to an extent simi-lar to their Rb-Sr system in the presence of a fluid. Thus, the resulting isochron ages suggest the time Nd-isotopic exchange ceased for the selected cube samples/slabs rather than any metamorphic or structural event. This becomes clear if our data from the Gadaladeniya synform are compared with those from the Dumbara synform where 8-cm cube whole-rock samples were dated.

The presence of very small patches of ar-rested second generation charnockite in the dated amphibolite-facies gneiss from the Dum-bara synform suggests that the same sequence of P-T-t events depicted in (cf. Fig. 2, Perera, 1994) occurred both in the Dumbara and Gada-ladeniya synforms. The 1053±110 Ma Rb-Sr whole-rock errochron for the retrogressed gneiss from the Dumbara synform is distinctly older than the 666±72 Ma Rb-Sr isochron for the charnockitic gneiss from the Gadaladeniya synform. We employed large (8-cm cubes) whole-rock samples here compared to 7-cm cubes used for Gadaladeniya samples. The hint of an older age for Dumbara synform samples supports our argument that the smaller whole-rock cube samples do not retain an earlier iso-topic record due to late isotopic exchange. The Sm-Nd data for the gneiss in the Odillawe quar-ry did not produce an isochron. The 1053±110 Ma errochron could be suggesting that the am-phibolite-facies retrogression in this quarry is close to ~1100 Ma. Concordant to discordant ~1100 Ma U-Pb zircon ages are wide-spread in the ortho and paragneisses of the WC and VC (Hölzl et al., 1991; 1994; Kröner and Jaeckel, 1994; Kröner et al., 1994) and Kröner et al (1987) noted Pb-loss from zircons in metasedi-ments in the HC at this age. However, this age has been attributed to initial crust formation in the WC and VC areas, though multifaceted ~1100 Ma metamorphic zircons showing no subsequent Pb-loss are found both in the ortho- and paragneisses (Kröner and Jaeckel, 1994; Kröner et al., 1994).


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CONCLUSION

Successful application of Rb-Sr and Sm-Nd isochron methods to obtain isotopic informa-tion on the tectono-metamorphic evolution of high-grade terrains is often confronted by the sampling techniques employed and ignorance on the length-scales of isotopic resetting per-taining to an envisaged event. Isochron rela-tionships neither prove the success of the sam-pling technique nor the correct targeting of the event and length scale of isotopic resetting. On-ly detailed investigations will reveal the signific-ance of the isotopic ages obtained.

The detailed Rb-Sr and Sm-Nd isotopic study of the granulite- to amphibolite-facies grade rocks of the Kadugannawa Complex of Sri Lanka, employing cube samples and thin slabs, illustrates the point mentioned in the previous paragraph. Our data indicate metre- to centi-metre-scale isotopic homogenization in both Rb-Sr and Sm-Nd systems during the geological evolution of the high-grade rocks, a view that has been previously rejected in favour of a U-Pb zircon age of 600-550 Ma for high-grade meta-morphism. Our data demonstrate that small whole-rock samples could be extensively reset that those may not contain meaningful ages. This resetting could be severe in the presence of a fluid, so that, both Sm-Nd and Rb-Sr sys-tems of whole-rock cube samples and slabs could be reset to very similar extents. There-fore, the isochron relationships shown by the samples may not indicate any geological event, instead the time of closure of whole-rock sys-tems for isotopic exchange. However, large whole-rock samples may contain useful age in-formation. Thus, present Rb-Sr and Sm-Nd data suggest that neither the protolith ages of rocks nor any evidence of geological events have been preserved in the dated samples. Acknowledgements

LRKP is grateful to the Japanese members of the Japan-Sri Lanka Joint Geological Research for providing financial support to carry out this study. The Director of ISEI, Misasa, Japan is thanked for granting permission to use the facil-ities of the Division of Study of the Earth’s Evo-lution of the institute for this research. The Di-rector of IFS, Kandy, Sri Lanka is thanked for permission to use rock crushing facilities. Dr. K. Shiraishi of the NIPR, Tokyo is thanked for kind-ly making all the arrangements to make this

study a success. Drs. L.P. Black, and G. Rogers are thanked for their comments to improve the paper. Mr. G.N. Wijeratne and Mrs. Moriguchi are thanked for their assistance. Keshan Perera is thanked for his assistance in drafting the fig-ures.

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Centimetre- and Metre-scale Nd and Sr Isotopic Homogenization in Kadugannawa Complex, Sri Lanka

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