Rb - Sr Whole Rock-Mineral Isochron Ages of Plutonic Rocks from the Wanni Complex, Sri Lanka

Gondwana Research, 1/: 4, No. 1, p p . 113-120. 02001 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Rb - Sr Whole Rock-Mineral Isochron Ages of Plutonic Rocks from the Wanni Complex, Sri Lanka W.I.S. Fernando1and S. Iizumi2

' Department of Earth and Planetary System Science, Hiroshima LkzveTsify, Higashi Hiroshima 739-8526, Japan Department of Geoscience, Shirnane University, Matsue 690-8504, Japan (Manuscript received July 25, 2000; accepted September 16,2000)

Abstract Granitoid plutons in different East Gondwana fragments give evidence for vigorous felsic magmatism during the Pan-African period (800 to 500 Ma). The Sri Lankan basement, which is mainly composed of Proterozoic high-grade metamorphic rocks, was intruded by a few late- to post-tectonic syenitic and granitic plutons. Reliable geochronological data for these plutons are few, and some of the available data are inconsistent with the ages of the surrounding metamorphic country rocks. This report presents five Rb-Sr whole-rock-mineral isochron (WRMI) ages and initial Sr isotope ratios of three granitoid plutons from the Wanni Complex, Sri Lanka, namely the Ambagaspitiya, Tonigala and Kotadeniya Granites. Two samples from the Ambagaspitiya Granite yielded ages of 52025 and 502+15 Ma, with initial Sr ratios of 0.7103?0.0003 and 0.712520.0009 respectively, whereas ages of 4 6 7 t 2 7 and 497211 Ma and initial Sr ratios of 0.7070?0.0004 and 0.7085+0.0008 were determined for two Tonigala Granite plutons. A single sample from the Kotadeniya Granite gave an age of 533219 Ma and an initial Sr ratio of 0.7202+0.0018 These ages are consistent with the geological relations between the granitoids and their country rocks, which underwent granulite facies metamorphism between 650 and 550 Ma, and also with available U-Pb zircon ages (-550 Ma). The present study gives improved chronological correlation of the felsic magmatism in East Gondwana fragments in Sri Lanka, India, Madagascar, Antarctica and Western Australia. Each of the Sri Lankan plutons exhibits different initial Sr isotope ratios, indicating that their magmas were derived from distinct source materials.

Key words: Sri Lanka, Rb-Sr whole rock-mineral isochron ages, Pan-African granitoids, East Gondwana, Wanni complex.

Introduction Geological and geochronological data of metamorphic and igneous rocks in Sri Lanka continue to attract much attention from Earth scientists, especially those who study the Precambrian geology of the Gondwana supercontinent (e.g., Kroner et al., 1987; Milisenda et al., 1988; Hiroi et al., 1987; Yoshida andvitanage, 1993). Yoshida and Hiroi (1992) emphasized the importance of Sri Lanka for reconstructions of Gondwana, and described Sri Lanka as the pearl of the supercontinent. Based on Nd model ages, it is widely accepted that the Sri Lankan basement was formed by amalgamation of three different crustal blocks, the Wanni, Highland and Vijayan Complexes (Fig. la), during the regime of Pan-African collisional tectonics associated with the formation of the Gondwana continent (Milisenda et al., 1988,1994; Cooray, 1994; Kleinschrodt,

1994; Kroner, 1991; Mathavan et al., 1999). The Highland and Wanni Complexes are thought to have undergone granulite facies metamorphism together between 550 and 650 Ma (Baur et al., 1991; Kroner and Jaeckel, 1994; Kroner et al., 1991; Holzl et al., 1991, 1994). They were subsequently thrust over the Vijayan Complex (Faulhaber and Raith, 1991; Hiroi et al., 1994), which is mainly composed of amphibolite facies granitoid gneiss and migmatites. Sri Lanka is mainly underlain by Proterozoic high grade metamorphic rocks, and most petrological and geochronological studies so far had mainly focused on metamorphic rocks (e.g., Yoshida et al., 1991; Kagami et al., 1995; Yoshida and Santosh, 1994; Shiraishi et al., 1994: Hiroi et al., 1991; Kroner et al., 1996). As in other Gondwana fragments ( e.g., Santosh et al., 1989; Santosh and Drury, 1988; Nair and Santosh, 1984; Rajesh et al.,

W.I.S. FERNANDO AND S. IIZUMI

114

79'30'

79045'

8OOOO'

80' 15'

B '00

000 '

7 OOC

"00 '

Granite Mesozoic sediments

....Hornblende gneiss, hornblende-biotite gneiss, .-.-.biotite ...... gneiss

a

Charnockite (hypersthene, diopside gneiss or granulite bearing hornblende, biotite, garnet); Charnockitic gneiss Biotite gneiss, hornblende-biotite gneiss, migmatitic and granitic in parts

###### # ##I## ####/#

Cenozoic cover

@ Granitic gneiss

0

Undifferentiated metasediments

Cordierite-garnet granulite Or gneiss

Fig. 1. a. Map of Sri Lanka showing major crustal units (after Kroner et al., 1991 and Cooray, 1994) and location of figure Ib (shaded area). WC: Wanni Complex; HC: Highland Complex; VC: Vijayan Complex. b. Geological map of the study area in the Wanni Complex and sample locations (modified from Geological Survey Department of Sri Lanka, 1982).

Gondwana Research, V. 4, No. 1,2001

GEOCHRONOLOGY OF PLUTONIC ROCKS, WANNI COMPLEX, SRI LANKA

1996; Nedelec et al., 1994, 1995; Sheraton and Black, 1988), some late- to post-tectonic granitoids also occur in Sri Lanka (Geological Survey Department, Sri Lanka, 1982). Although some geochronological data have been reported for these granites (Crawford and Oliver, 1969; Holzl et al., 1991, 1994), some of the data are not so precise. Crawford and Oliver (1969) reported Rb-Sr whole rock ages of 964-1089 Ma for the post tectonic granitoids, much older than recently published U-Pb zircon ages of -550 Ma (Holzl et al., 1991, 1994). The Rb-Sr ages are also much older than the U-Pb zircon ages of 550-650 Ma reported for the metamorphic country rocks (Baur et al., 1991; Kroner et al., 1991; Holzl et al., 1991, 1994). Reliable geochronological data for these granitoid rocks are required for understanding the tectonic evolution of Sri Lanka, as well as the timing of felsic magmatism which took place in Gondwana fragments during the Pan-African orogeny (Rajesh et al., 1996 and references therein). This report presents Rb-Sr whole rock-mineral isochron ages of granitoid rocks from the Wanni Complex of Sri Lanka. Our age data are similar to available U-Pb zircon ages of -550 Ma for these granitoids (Holzl et al., 1991, 1994), and are consistent with U-Pb ages for the metamorphic country rocks. Based on our new age data, we correlate this granite magmatism with Pan-African felsic magmatism in other East Gondwana fragments.

Geological Setting and Petrography The Highland Complex is composed mainly of granulite facies charnockites and metasediments that have Nd model ages of 2.0-3.0 Ga (Milisenda et al., 1988, 1994), and the Vijayan Complex consists mainly of amphibolite facies orthogneisses and migmatites with younger Nd model ages of 1.0-2.0 Ga (Milisenda et al., 1988, 1994). The Wanni Complex consists of granulite facies orthogneisses, migmatites, charnockites and subordinate metasediments with Nd model ages of 1.1-1.8 Ga (Milisenda et al., 1988, 1994). The boundary between the Highland and Wanni Complexes is still not well defined, and the nature and timing of their amalgamation is also not clearly understood. Although intrusive rocks are not common in Sri Lanka, five major granitic intrusions have been identified in the Highland and Wanni Complexes (Geological Survey Department, Sri Lanka, 1982). These are the Ambagaspitiya, Tonigala and Kotadeniya Granites (Wanni Complex), and the Balangoda and Arangala Granites (Highland Complex). The Ambagaspitiya Granite crops out in the western part of Sri Lanka, close to the boundary between the Wanni and Highland Complexes (Fig. l b ) . The Gondwana Research, V. 4, No. 1,2001

115

Ambagaspitiya Granite occurs within hornblende biotite gneiss, and occupies an area of 40 km2 (Fig. lb). Small scale patches, irregular layers and schlieren with gneissic appearance are frequently observed throughout the pluton. Globe-shaped inclusions of hornblende biotite schist -1m in diameter and with sharp boundaries frequently occur as enclaves in the northern part of the pluton. The boundary between the Ambagaspitiya Granite and its country rock is not clear due to lack of outcrop, but agmatic texture can be observed in the country rocks near the western boundary of the pluton. The Ambagaspitiya Granite mainly comprises four facies including medium-grained syenite, quartz syenite, monzonite and granite in the strict sense. These four facies are intergradational in the field. K-feldspar,plagioclase, quartz and biotite are the main constituent minerals, and apatite, zircon, magnetite, ilmenite and sphene are accessory phases. All the facies exhibit massive granular texture and show no preferred orientation. The K-feldspar is usually microcline, which occurs as an anhedral phase, ranging from 3 mm to 1 cm in size. Plagioclase generally occurs as a fine- to mediumgrained subhedral phase, which is sometimes weakly zoned. Fine apatite, zircon, biotite, plagioclase, K-feldspar and quartz inclusions occur within the feldspars. Quartz is anhedral, with grain size ranging from 1 mm to 3 mm. Biotite occurs as an interstitial phase to other minerals, and typically comprises 1 to 4 modal percent. More detailed petrographic descriptions are given by Mathavan and Hettiarachi (1989) and Mathavan (1991). The Tonigala Granite occurs within the Wanni Complex in the northwestern part of Sri Lanka as two parallel sheetlike bodies, Tonigala-A and Tonigala-B (Fig. lb). The Tonigala-Abody is about 1.5 km wide, and can be traced up to 36 km, whereas the Tonigala-B is about 1 km wide and extends some 13 km. Both bodies trend E-W. The contact between the Tonigala Granite and the granitic gneiss country rock is not clear due to paucity of outcrop. However, agmatite occurs near the southern boundary of Tonigala-A, indicating that it intrudes the granitic gneiss. Both Tonigala bodies are fine-to medium-grained granites. Angular shaped hornblende-biotite gneiss (1 m x 5 m) and several oval-shaped (1 m x 3 m) hornblende-biotite schist enclaves are found in the central part of TonigalaB. The oval enclaves are cross-cut by granitic veins (for more detailed petrographic description, see also Cooray, 1971). The main constituent minerals of Tonigala-A are Kfeldspar, plagioclase, quartz, biotite, hornblende and aegerine-augite, whereas Tonigala-B consists of Kfeldspar, plagioclase, quartz and biotite. Apatite, zircon, magnetite and ilmenite are accessory phases. The K-

116

W.I.S. FERNANDO AND S. IIZUMI

feldspar is anhedral perthite, averaging 1.5 mm in diameter. Plagioclase generally occurs as a fine-grained, weakly zoned anhedral phase. Ferromagnesian minerals are scarce in both bodies, and typically occur as interstitial phases lacking any preferred orientation. The Kotadeniya Granite occurs in the Wanni Complex in western Sri Lanka (Fig. lb), intruding both charnockite gneiss and cordierite gneiss. The pluton is roughly oval in shape (2 x 1 km2) and has sharp contacts with its country rocks. The granitoids range from granite to granodiorite, and are typically coarse-grained and mineralogically heterogeneous. K-feldspar, plagioclase, quartz, biotite and hornblende are the main constituent minerals, and apatite, zircon, magnetite and ilmenite are present as accessory phases. The K-feldspar is subhedral to anhedral perthite, with grain size ranging from 5 mm to 1 cm. Plagioclase (oligoclase) occurs as subhedral grains, which frequently shows myrmekitic texture with quartz along grain boundaries with K-feldspar. Hornblende and biotite (3 mm average size) occur as interstitial phases, and do not show any preferred orientation. Biotite and hornblende have similar modal contents, ranging from 3 to 5 To. Finegrained biotite, plagioclase, K-feldspar and quartz also occur as inclusions in coarse-grained feldspars. Microscopic and field observations show that all the constituent rocks of the Ambagaspitiya, Tonigala and Kotadeniya granites have quite different textures and mineralogy from the metamorphic rocks by which they are enclosed, and do not exhibit any deformation or metamorphism.

concentrations of the whole rock samples were also determined by XRE A maximum error of 5% is estimated for these Rb/Sr ratios, based on reproducibility of the data for the Geological Survey of Japan standards JG-la and JB-la. Rb and Sr concentrations of the mineral fractions were measured by isotope dilution method, using a 87Rb/ 86Srmixed spike. An error of 1%is estimated for the Rb/ Sr ratios determined by isotope dilution. All isotope analyses were carried out using a Finigan MAT 262 thermal ionization mass spectrometer equipped with five collectors. Rb-Sr extraction and mass spectrometric analysis were made following the procedures given in Kagami et al. (1987,1989) and Iizumi et al. (1994). Measured s7Sr/86Srratios were normalized to 86Sr/88Sr= 0.1194. The data were computed from a minimum of 150 measurements. Whole procedure blanks were 0.6 ng for Rb and 0.55 ng for Sr. Analyses of NBS 987 standard during this study gave 87Sr/86Srratios ranging from 0.710254 2 0.000009 to 0.710283 0.000009.

*

Results and Discussion Five samples were selected for Rb-Sr whole rockmineral isochron age dating. Those were a biotite monzonite (AG-8) and a biotite syenite (AG-13) from the Ambagaspitiya Granite, a biotite-hornblende-pyroxene granite (TG-1) from Tonigala-A, a biotite granite (TG-7) from Tonigala-B, and a biotite-hornblende granite (KG7) from the Kotadeniya Granite (Fig. 2). Three mineral

Q

Analytical Procedures Fresh samples were collected from working quarries in each pluton. Samples weighing 1-2 kg were finely chipped and crushed by hand, and split by coning and quartering. Whole-rock samples were subsequently ground in a tungsten carbide ball mill. Biotite-rich fractions were separated from the original crushed material using an isodynamic separator, and plagioclase-rich and K-feldsparrich fractions were separated by hand picking under a microscope. Major and trace element concentrations of whole rock samples were determined by X-ray fluorescence spectrometry (Rigaku RIX 2000) at Shimane University. Major element determinations were made on fused glass beads prepared with lithium tetraborate flux using a sample to flux ratio of 1:5, essentially following the method of Norrish and Hutton (1969). Trace elements were determined from the same beads by conventional peak over background techniques, with calibration against a suite of international standard rocks. Rb and Sr

\

K

\

P

Fig. 2. Modal compositions of the Wanni Complex granitoids analyzed for Rb-Sr age determinations. Classification after Streckeisen (1976). K Alkali feldspar, Q: Quartz, P: Plagioclase.

Gondwana Reseafch, V. 4, No. 1, 2001

117

GEOCHRONOLOGY OF PLUTONIC ROCKS, WANNI COMPLEX, SRI LANKA Table 1. Major and trace element compositions of whole rock samples. Ambagaspitiya Granite AG-8 AG-13

SampleNo.

60.03 0.86 19.08 1.86 1.45 0.06 0.99 2.08 3.98 7.86 0.40 98.65 1.28

SiO, (wt.%) TiO, A1203

FeP, FeO MnO MgO CaO Na,O K20

P A Total LO1 Ni (ppm) Nb Zr Y Sr Rb V

co Zn cu Ba

17 362 13 462 154 26 9 43 1 2545

61.04 0.92 18.72 1.99 1.54 0.05 1.01 2.16 3.97 6.97 0.25 98.62 0.59 2 11 494 12 1102 141 25 8 44 2194

Tonigala Kotadeniya Granite Granite TG-1 TG-7 KG-7 72.35 69.87 0.17 0.31 14.42 14.72 0.83 1.12 0.24 0.49 0.02 0.02 0.20 0.28 1.22 1.25 3.74 2.56 7.31 5.01 0.12 0.04 98.24 98.05 1.06 0.88 3 6 1 6 240 137 12 4 1493 497 185 163 15 6 4 20 30 15 1391 4633

72.74 0.47 13.10 0.93 1.74 0.02 0.75 2.50 2.64 3.04 0.01 97.94 0.81 3 8 143 171 133 10

9 57 747

fractions (i.e. biotite-rich, K-feldspar-rich and plagioclaserich fractions) and whole rock were analyzed in each sample for the age determinations. Major and trace element compositions of the whole rock samples are given in Table 1,and sample locations are shown in Fig.lb. The biotite-rich fraction of sample TG-1 contains some plagioclase, and consequently has a lower Rb/Sr ratio than the other biotite-rich-fractions measured in this study. Sr isotope ratios are listed in Table 2, and whole rock-mineral isochron diagrams are shown in figure 3. The isochron ages from this study are shown in Table 2, along with previously reported age data. Ambagaspitiya Granite samples AG-8 and AG-13 yield similar whole rock-mineral isochron (WRMI) ages of 520+5 Ma and 502+15 Ma, with initial Sr isotope ratios (IsJ of 0.7103+0.0003 (Fig. 3a) and 0.7125+0.0009 (Fig. 3b), respectively. Tonigala Granite samples have WRMI ages of 467+27 Ma with Isr of 0.7070+0.0004 (TG-1; Fig. 3c) and 497211 Ma with Isr of 0.708520.0008 (TG7; Fig. 3d), respectively. Although the age for TG-1 has a large error, both Tonigala ages agree within analytical error. Kotadeniya Granite (KG-7) has a whole rockmineral isochron age of 5 3 3 2 1 9 Ma with an Isr of 0.7202+0.0018 (Fig. 3e).

Table 2. Rb, Sr concentrations, Sr isotope ratios and ages with previously reported ages of granitoid rocks in WC . Sample No. Ambagaspitiya Granite AG-8

AG-13

Tonigala Granite TG-1

TG-7

Kotadeniya Granite KG-7

fraction

Sr (ppm)

87Rb/ %r

87Sr/86Sr

Age (Ma)

BRF WR KRF PRF

496 154 160 17

92 462 546 595

15.83 0.97 0.85 0.08

0.827616 0.717141 0.716817 0.710975

BRF WR KRF PRF

464 146 155 30

99 1149 662 835

13.74 0.37 0.68 0.10

0.810699 (09) 0.716052 (09) 0.716799 (10) 0.712834 (09)

502215

BRF WR KRF PRF

404 185 240 16

62 1 1493 1923 2179

1.88 0.36 0.36 0.02

0.719458 (09) 0.709575 (09) 0.709633 (09) 0.706870 (09)

467k27

BRF WR KRF PRF

819 497 235 53

111 163 625 249

21.66 0.95 1.09 0.62

0.861718 0.715550 0.716742 0.712168

(09) (09) (09) (09)

497211

BRF WR KRF PRF

72 1 133 298 24

27 171 308 295

83.65 2.25 2.81 0.24

1.355872 (11) 0.734844 (09) 0.742513 (09) 0.722282 (09)

533219

(10)

520rt5

(14) (09) (09)

BRF : biotite-rich fraction, WR : whole-rock sample, KRF : K-feldspar-rich fraction, PRF : plagioclase-rich fraction. * Crawford and Oliver (1969); ** Holzl et al. (1991, 1994).

Gondwana Research, V. 4, No. 1,2001

Previously reported Ages (Ma)

1050 +: 1089 * 1079 '"

964230 * 556217

W.I.S. FERNANDO AND S. IIZUMI

118

/

Ambagaspitiya Granite AG-8

PE3:

o.8000.780-

0'7601 /

Age: 520 f 5 Ma SrI: 0.7103+0.0003 MSWD: 0.15

WR

0.740

'

'

'

6

9

"

I

12

'

~

15

'

MSWD: 0.86

5

0

18

15

10

20

25

87Rb/n6Sr

87Rb/a4Sr

1.300

P

'a

0.780:

Kotadeniya Granite

Ambagaspitiya Granite AG-13 $1.100: b

Em 0.7600.740 0.720

m

y,

0.700

0

6

3

,

,

9

12

Age: 533 i 1 9 Ma SrI: 0.7202 f0.0018 MSWD: 3.20

6

Age: 502f15 Ma SrI: 0.7125+0.0009 MSWD 1.22

0.900

KRF 1

"Rb/%3 r

0 . 7 a0 0 w R F15.

.

30

4s 87Rb/86Sr

60

75

Tonigala Granite 0.715-

i? b

Age: 467 i 2 7 Ma SrI: 0.7070 fO.OoO4 MSWD 0.16

m

c

0.710-

-PRF 0.705 T 0.0

0.5

1.o

1.5

87Rb/86Sr

Crawford and Oliver (1969) analysed two whole rock samples and one K-feldspar sample from the Ambagaspitiya Granite. They determined Rb-Sr whole rock ages of 1050 and 1089 Ma and a Rb-Sr mineral age of 1079 Ma, assuming an initial Sr isotope ratio of 0.7000. They also reported a whole rock isochron age of 964+30 Ma with an ISrof 0.7033?0.0026 for the Tonigala Granite by analyzing five whole-rock samples from both the Tonigala-A and -B bodies. The above ages were recalculated here using a decay constant of 1.42x1O-'ly1 for 87Rb. According to our field and microscopic observations, however, these plutons post-date the peak metamorphic event (650-550 Ma) of the country rocks in the Wanni Complex. Therefore, the ages reported by

Fig. 3. Rb-Sr whole-rock- mineral isochron diagrams for granitoids from the Wanni Complex. a. Ambagaspitiya Granite (sample AG-8); b. Ambagaspitiya Granite (AG-13); c. Tonigala Granite-A (TG1); d. Tonigala Granite-B (TG-7); e. Kotadeniya Granite (KG7). BRF=biotite-rich fraction; KRF=K-feldspar rich fraction; PRF=plagioclase-rich fraction; WR=whole rock sample.

Crawford and Oliver (1969) are not acceptable (e.g. Holzl et al., 1991, 1994; Mathavan et al., 1999). Holzl et al. (1991) reported U-Pb zircon ages from Tonigala-B, and interpreted the upper intercept age of 556-c 17 Ma as the intrusive age. This age is about 60 million years older than our Rb-Sr whole rock-mineral isochron age for that body. No geochronological data have been reported for the Kotadeniya Granite to date. The ages we report here are consistent with the field relations between the granitoids and the metamorphic country rocks in the Wanni Complex. The above data and other published dates indicate that wide spread magmatism took place in Sri Lanka around 500 to 550 Ma years ago, postdating the peak Gondwana Research, K 4, No. 1,2001

GEOCHRONOLOGY OF PLUTONIC ROCKS, WANNI COMPLEX, SRI LANKA

119

~

metamorphic events in the basement. Baur et al. (1991) reported a U-Pb zircon age of 550k2 Ma for a small posttectonic granite a t Tangala, within the Highland Complex on the south coast of Sri Lanka. Holzl et al. (1991,1994) reported a U-Pb zircon age of 552+8 Ma and a Rb-Sr whole rock isochron age of 492+10 Ma for a small posttectonic granite in the Wanni Complex at Galgamuwa (near the Tonigala Granite). Ages of -550 Ma have also been reported for Sri Lankan pegmatites and gemstone zircons by other workers ( e g , Tilton and Aldrich, 1955; Gottfried et al., 1956). Available chronological data (Fig. 4) for plutonic rocks in other East Gondwana fragments also suggest that strong felsic magmatism took place in East Gondwana between

SL

SI EI MD

EA

WA

to post-tectonic granite magmatism in the Sri Lankan basement. The high initial Sr isotope ratios of the granitoids, especially the Ambagaspitiya and Kotadeniya Granites, suggest that they were not formed by partial melting of lower crustal materials with basic to intermediate compositions, but were generated by reworking of older crustal rocks. Taking into account the fact that the granitoids have differing ISr,it also seems that each pluton w a s derived from distinct source materials with independent pre-crustal histories.

Acknowledgments Our thanks to Prof. Y. Sawada of Shimane University for his help with XRF analysis; to Mrs. C. Akasaka for help with laboratory work; to Dr. V. Mathavan, Dr. N.P. Wijayananda and Dr. W.K.B.N. Prame for their guidance during field work in Sri Lanka; to Dr. B.P. Roser for comments on the manuscript, and to Dr. M. Jayananda for his helpful review. The first author also thanks the Japanese Ministry of Education, Science, Culture and Sports (MONBUSHO) a n d t h e Rotary Yoneyama Scholarship Foundation for providing fellowship support during his study in Japan.

References

I.-

900

SL- Sri Lanka SI - Southwestern India EI - Eastern Peninsular India MD - Madagascar EA- East Antarctica WA - Western Australia Fig. 4. Comparison of available geochronological data for granitoid plutons from East Gondwana fragments. Data sources for EI, SI, MD and WA after Rajesh et al. (1996), and references therein. Data sources and symbols for the Sri Lankan panel are *Rajesh et al. (1996); ""Holzl et al. (1991, 1994) and "**this study.

550 Ma to 770 Ma (Rajesh et al., 1996 and references therein), although two younger ages of around 400 Ma have been reported for plutonic rocks in East Antarctica. This magmatic activity is thought to have taken place in relation to a major tectonothermal event which was widespread throughout the Gondwana continent during the Pan-African orogeny (e.g. Santosh and Drury, 1988; Yoshida and Vitanage, 1993; Windley et al., 1994; Rajesh et al., 1996). Our geochronological data are consistent with these suggestions. We suggest that decompressional melting triggered by extensional movements induced lateGondwana Research, K 4, No. 1,2001

Baur, N., Kroner, A., Todt, W., Liew, T.C. and Hofmann, A.W.C. (1991) U-Pb isotopic systematics of zircons from prograde and retrograde transition zones in high-grade orthogneisses in Sri Lanka. J. Geol., v. 99, pp. 527-545. Cooray, P.G. (1971) The Tonigala granite, NW Ceylon. Bull. Geol. SOC.Finland, v. 43, pp. 19-37. Cooray, P.G. (1994) The Precambrian of Sri Lanka : a historical review. Precamb. Res., v. 66, pp. 3-18. Crawford, A.R. and Oliver, R.C. (1969) The Precambrian geochronology of Ceylon. Geol. SOC.Australia, Sp. Publ. NO.2, pp. 288-316. Faulhaber, S. and Raith, M. (1991) Geothermometry and geobarometry of high-grade rocks : a case study of garnetpyroxene granulites in southern Sri Lanka. Mineral Mag., v. 55, pp. 33-56. Geological Survey Department, Colombo, Sri Lanka (1982) Geological Map of Sri Lanka. Scale: 8 miles to one inch. Gottfried, D., Senftle, F.E. and Waring, C.L. (1956) Age determinations of zircon crystal from Ceylon. Amer. Mineral., V.41, pp. 157-161. Hiroi, Y., Ogo, Y. and Namba, K. (1994) Evidence for prograde metamorphic evolution of Sri Lankan pelitic granulites and implications for the development of continental crust. Precamb. Res., v. 66, pp. 254-263. Hiroi, Y., Shiraishi,K. and Motoyoshi, Y. (1991) Late Proterozoic paired metamorphic complexes in East Antarctica, with special reference to the tectonic significance of ultramafic rocks. In: Thompson, M.R.A., Crame, J.A. and Thompson, J.W. (Eds.), Geological Evolution of Antarctica. Cambridge University Press, pp. 83-87.

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