Nd isotopic mapping of the Sri Lanka basement: update, and additional constraints from Sr isotopes

Precarnbrian Research, 66 (1994) 95-110

95

Elsevier Science B.V., Amsterdam

Nd isotopic mapping of the Sri Lanka basement: update, and additional constraints from Sr isotopes C.C. Milisenda *'a, T.C. Liew a, A.W. Hofmann a and H. K/Shlerb aMax Planck-Institut3~r Chemie, Postfach 3060, 6500 Mainz, Germany bMineralogisch-Petrographisches Institut, Universitdt Mfinchen, Theresienstrafle 41, 8000 Miinchen 2, Germany Received February 17, 1992; revised version accepted July 18, 1992

ABSTRACT The high-grade crystalline basement of Sri Lanka can be subdivided into three provinces distinguished by distinct isotopic model ages. This confirms the conclusions of Milisenda et al. ( 1988 ) made on the basis of much more limited data. High-grade ortho- and paragneisses from the Highland Complex in central Sri Lanka yield Palaeoproterozoic to Archaean Nd model ages from 2.0 to 3.4 Ga and reflect an extended premetamorphic crustal history. The Wanni Complex in the western and northwestern part of the island is characterized by Meso- to Neoproterozoic Nd model ages ranging from 1.0 to 2.0 Ga. The Vijayan Complex in the southeast is occupied by Neoproterozoic I-type granitoids with Nd model ages from 1.0 to 1.8 Ga. This distinct crustal province appears to fit best a detached continental margin plutonic terrane. The isotopic data preclude a common premetamorphic history of the Wanni and Vijayan Complexes with the ancient Highland Complex. The combined Nd-, St- and Pb-isotope data together with recent geochronological studies strongly suggest that the distinct crustal age provinces are separated by tectonic discontinuities and that they represent terranes that were accreted just prior to the high-grade metamorphic event in Neoproterozoic time.

1. Introduction

Precambrian high-grade terrains were commonly affected by tectonic processes at depth in the crust, and then uplifted to a higher crustal level followed by extensive erosion. The tectono-metamorphic events causing intense deformation and complex structures largely obliterated earlier relationships and are major hindrances to the understanding of the premetamorphic history. Extensive reworking and deformation of preexisting crust during metamorphism, accompanied and followed by synto post-tectonic low- and high-level intrusions, often inhibit or preclude analysis by conventional stratigraphic methods. An example of this may be found in the Precambrian basement of Sri Lanka, where the relationship be*Corresponding author.

0301-9268/94/$07.00

tween its different geological units and the geological significance of the boundaries between the major subdivisions has been the subject of considerable debate (e.g. Dissanayake and Van Riel, 1978; Dissanayake and Munasinghe, 1985; Kr6ner et al., 1987, 1991; Voll and Kleinschrodt, 1991 ). A powerful tool that can provide insights into the premetamorphic crustal history of a highgrade terrain is the use of Nd isotopes. Milisenda et al. (1988) mapped "Nd-isotopic provinces" within the complex high-grade basement of Sri Lanka. Crustal residence ages indicated by Nd model ages allowed a simple regional pattern to be recognized and provided a tool for distinguishing multiply deformed terranes that are otherwise difficult to separate by field observations alone. Milisenda et al. ( 1988 ) recognized three provinces with different Nd model ages, which were interpreted as

© 1994 Elsevier Science B.V. All rights reserved.

SSDI 0 3 0 1 - 9 2 6 8 ( 9 3 ) E 0 0 2 8 - B

96

v,(: MII.IS[ ND.\ Ei

TABLE 1 Rock types investigated and sample locations No. in Field No. Fig. 3 1 2 3 4 5

6 7

8 9 10 11 12

l3 14 15

16 17 18

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Unit

HC HC PR 41 HC SL 45 HC SL 57 HC SL 62 HC SL 82 HC SL 98 HC SL 110 HC SL 125 HC SL 137 HC K 200-10/200-2A HC K 320-1 HC 1038/1039 HC K 339 HC SL 349 HC SL 351 HC SL351-1C/351-2 HC SL 355 HC SL 357 HC SL 410 HC SL 669 HC SL 794 HC SL 402 HC SL 397 HC K 418 HC K 419 HC K 408 HC SL 38 VC SL 66 VC SL 164 VC SL 359/-359-3G VC SL 362 VC SL 391 VC SL 403 VC SL 584 VC SL 587 VC SL 611 VC SL 613 VC SL 616 VC SL 18 WC SL 29 WC SL 30 WC SL 56/414 WC SL 68 WC SL 71 WC SL 277-4a/542 WC SL 277-4c/544 WC SL 325 WC S L 3 3 0 / 3 3 2 / 3 3 3 WC SL 336 WC SL 339 WC SL 348 WC SL 8.3

PR 22

Rock type

Locality

ga-hbl gneiss ga-sil-cor-bio gneiss ga-sil-cor-bio gneiss ga-bio gneiss ga-bio gneiss charnockite ga-bio gneiss charnockite ga-charnockite ga-charnockite charnockite ga-bio gneisses ga-bio gneiss ga-bio gneisses ga-bio gneiss ga-bio gneiss ga-sil-bio gneiss ga-sil-biogneisses ga gneiss ga gneiss ga-bio gneiss ga-sil-bio gneiss ga-bio gneiss ga-charnockite ga gneiss ga-bio gneiss ga-sil-bio gneiss ga-sil-bio gneiss hbl-bio gneiss calc silicate calc silicate ga-hbl-biogneisses hbl-bio gneiss hbl-bio gneiss hbl-bio gneiss bio gneiss hbl-bio gneiss hbl-bio gneiss hbl-bio gneiss hbl-bio gneiss ga-cor-bio gneiss charnockite charnockite ga-bio gneisses hbl-bio gneiss ga-bio gneiss hbl-bio gneisses charnockites hbl-bio gneiss pink granites hbl-bio gneiss pink granite charnockite

quarry at Digana outcrop S of Elpitiya on road to Ambalangoda quarry NE of Matugama on road to Kalutara quarry at 29/2 benchmark on road Kandy-Mahiyangana quarry at Inamalawa, road A6 Dambulla-Habarana quarry at ~ 51 mi. road post, A 11 Habarana-Polonnaruwa quarry ~ 2 mi. SE of Horawala outcrop ~ 1 mi. S ofKandadola, A17 Galle-Deniyaya outcrop at Lauderdale, A 17 Deniyaya-Madampe quarry at 151 km road post, A4 Balangoda-Belihul Oya quarry ~ 3.5 mi. N of Haputale, A 16 Haputale-Bandarawela quarry at Getambe, Peradeniya outcrop at ~ 14.3 mi. road post, A5 Gampola-Nuwara Eliya outcrop at 31.6 km road post, A5 Gampota-Nuwara Eliya outcrop 3 mi. N of Maturata, road Maturata-Rikillagaskada quarry at Habarana town West, A l 1 to Maradankadawala quarry at Habarana town East, A 11 to Polonnaruwa quarry at Habarana town West, A 11 to Maradankadawala quarry at Ambagaswewa, A 11 to Polonnaruwa outcrop at ~ 37 km road post, A 11 Habarana-Polonnaruwa quarry ~ 3.5 mi. E of Wellawaya, A4 to Buttala outcrop ~ 1.5 mi. NE of Paradeka, S of Gampola outcrop at Swami Rock, Trincomalee outcrop at Vadahitikanda, Kataragama Complex quarry at Bogahapalassa, Kataragama Complex outcrop at ~ 8.1 mi. road post, Tissamaharana-Kataragama outcrop at ~ 9.3 mi. road post, Tissamaharana-Kataragama outcrop at 192.4 km road post, A2 Wellawaya'Hambantota quarry ~ 7 mi. W of Mahiyangana on road to Kehelula outcrop at Gatlela train station, ~ 6.5 mi. SE of Polonnaruwa quarry at Kumbukkana, ~ 243 km road post, A4 to Wellawaya quarry at Mahiyangana quarry at Diggalvena, ~ 3.5 mi. N of Bibile quarry at Chethiyagiriya temple, Hungama, A2 to Tangalla outcrop at Sella Kataragama, ~ 3.5 mi. NW of Kataragama quarry at Godawa, SE of Ambalantota, A2 to Hambantota quarry at Kirinda, ~ 17 mi. NE of Hambantota outcrop ~ 1 mi. SE ofBakiella, ~ 19 mi. SSW of Batticaloa outcrop ~ 3.5 mi. SE ofMannambitiya, ~ 13 mi. SE of Polonnaruwa outcrop ~ 6.5 mi. W of coast and ~ 1 mi. S of A11 to Trikkandimadu quarry at Homagama, A4 to Avissawella quarry at Kurunegala, A 10 to Puttalam quarry at Udabadalawa, ~ 2.5 mi. NW of Kurunegala quarry at Dambulla quarry at ~ 7 km road post, A7 Galigomuwa-Avissawella outcrop ~ 2.5 mi. W of Ruwanwella, along bank of Gurugoda Oya quarry, at Udadigana, Kurunegala District same locality outcrop at Avulegama, Panavagedara quarry at Galpanowa, ~ 3 mi. S of Munnekulama quarry at Mahachchiya, ~ 5 mi. W of Galgamuwa quarry ~ 1 mi. N of Galgamuwa, A28 to Anuradhapura quarry ~ 1 mi. S of Tirappane, A9 to Dambulla

Nd ISOTOPIC MAPPINGOF THE SRI LANKABASEMENT

97

No. in Field No. Fig. 3

Unit

Rock type

Locality

49 50 51 52 53 54 55 56 57 58 59

WC WC WC WC WC WC WC WC AN AN AN

charnockite ga-hbl-bio gneiss ga-bio gneiss biogneiss ga-biogneiss charnockite charnockite ga-biogneiss ga-sil-bio gneiss pink granite pink granite

quarry at Ratmale, A28 to Padeniya outcrop ~ 1.5 mi. SW of Danowita, A1 to Kegalla quarry ~ 1.5 mi. NE of Ragama, Ragama Harbour Work Company outcrop ~ 16 mi. NW of Kurunegala, A10 to Puttalam outcrop at Yayarawate, E of Kuliyapitiya quarry at Iratperiyakulama, A9 to Vavuniya quarry ~ 3.5 mi. NW of Harowupotana on road to Ratmalagahawewa quarry below Issenbettawe temple, A9 to Vavuniya quarry at Kadugannawa, W of Kandy outcrop ~ 1.5 mi. from Welamboda junction towards Peradeniya outcrop at south shore of new Victoria Dam, ~ 1.5 mi. SW of Kandy

SL 417 K 442 K 460 SL 622 SL648 SL 706 SL 708 SL 789 SL 1 SL 2.1 SL 5.1

HC: Highland Complex, VC: Vijayan Complex, WC: Wanni Complex, AN: Arena structures.

distinct crustal provinces, probably separated by major tectonic breaks. This finding contributed to the recent modifications in the nomenclature of the Sri Lankan basement (for discussion see Kr6ner et al., 1994; Cooray, 1994) which can be summarized as follows: The Highland Complex (HC; see Fig. 1 of Liew et al., 1994) forms a meridional-trending, central belt of granulite-grade supracrustal rocks and tectonically intercalated orthogneisses consisting of recycled Palaeoproterozoic to Archaean material. This complex is flanked in the west and northwest by a wide spectrum of rock types (including metapelites, charnockites and granitoids) showing Meso- to Neoproterozoic model ages, now termed the Wanni Complex (WC), and in the east and southeast by predominantly Neoproterozoic granitoid gneisses of the Vijayan Complex (VC). The use of Nd model ages as a tool for isotopic mapping was recently questioned by Burton and O'Nions (1990), who argued that granulite-grade metamorphism is accompanied by S m - N d fractionation, and therefore the significance of a crustal residence age, calculated by assuming a simple two-stage SmNd isotope evolution, is enigmatic. Voll and Kleinschrodt ( 1991 ) continued to question the significance of Nd model age data and rejected the revised lithotectonic subdivision of Sri Lanka because they considered the isotopic and

field-related data base to be insufficient. Instead, they argued on the basis of their own field observations that the Wanni and the Highland Complex probably belong to the same crustal sequence. During the past three years, some 40 additional samples have been analyzed for Nd isotopes, and a number of Sr analyses is now also available for comparison. In addition, Liew et al. (1994) present Pb isotope data that provide additional important constraints on the evolution of the Sri Lankan crust. We therefore consider it useful to present an updated version of the Nd isotope map of Sri Lanka previously discussed in Milisenda et al. (1988). These data can now be better integrated and reviewed within the framework of new geochronological results of other members of the German-Sri Lankan Consortium (e.g. H/51zl et al., 1994; Kr6ner et al., 1994). In particular, the present study provides extensive new isotopic data that characterize the Wanni Complex, because the geographical extent of this Meso- to Neoproterozoic model age province and the nature of the boundary separating it from the ancient Highland Complex is particularly controversial.

2. Analytical techniques Nd and Sr isotope dilution (ID) analyses and isotopic analyses were performed on two

98 '[ABLE 2 S m - N d data for rocks of the Highland C o m p l e x (Nos. 1 - 2 0 ) a n d the tectonic ldippen (Nos. 2 1 - 2 4 ) No. in Fig. 3 1 2 3 4 5 6

7 8 9 10 11 12 13

Field No. SL 8.3 PR 22 PR 41 SL 45 SL 57 SL 62 SL 82 SL 98 SL 110 SL 125 SL 137 K 200-10 K 200.2A K 320-1

1038 1039 14 15

16 17 18 19 20 21 22 23 24

K 339 SL 349 SL 351 SL 3 S l - I C SL 351-2 SL 355 SL 357 SL 410

Rock type

b~l (ppm)

Sm (ppm)

147Sm/144N d

143Nd/144Nda

8Ndb (0.6 Ga)

ga-hbl gneiss ga-sil-cor-bio gneiss ga-sil-cot-biogneiss ga-bio gneiss ga-bio gneiss

61.84 42.27 95.83 37.08 21.30 55.51

11.56 8.30 17.98 6.56 3.83 10.04

0.1130 0,I 186 0,1t34 0.1069 0.1086 0.1093

33.62

6.13

0.1102

9.09 35.04 29.61 73,58 54.33 43.47 51.52 53.18 61.27 42.20 61.93 58.07 18.59 49.56 49.83 35.06 127.7 81.67 45.98 47.34 27.37 82.5 47.5 15.25

1.62 9.86 5.43 16.72 9.89 6.80 9.28 7.57 8.76 9.06 9.40 10.83 2.64 7.87 7.62 5.65 21.64 12.23 8.29 9.01 8.53 20.6 8.91 2.65

0.1079 0.1702 0.1109 0.1373 0,1110 0,0945 0.1090 0.0860 0.0864 0.1300 0.0824 0.1130 0.0843 0.0959 0.0925 0.0974 0.1020 0.0905 0.1089 0.1150 0.1885 0.1510 0,1140 0,1050

0.511217_+27 0.511283+-27 0.511285-+20 0,511521+13 0.511561-+35 0.511165-+ 7 0.511316-+11 0.511341_+16 0.511933_+18 0,511196_+11 0.511302_+12 0.511272~10 0.511263_+13 0.511292~11 0.511089-2--11 0.511095_+ 9 0.512004_+11 0.511348+14 0.511302-+14 0.511285+tl 0.511300-+12 0.511083-+13 0,511233+15 0.510981_+15 0.511116+10 0.511165-+ 9 0.511245+15 0.512023+16 0.511930-~-_13 0.511240-+17 0.511468-+20

-21.3 -20,4 -20,0 -14.9 -14.3 -22.0 -19.2 -18.5 -11.7 -21,5 -21.5 -20.1 -19,0 -19.5 -21.7 -21.6 -7.3 -16,4 -19.6 -17.8 -18.4 -22.3 -19.8 -25.1 -21.5 -22.0 -20.9 -11.4 -t0.3 -20.9 -15.8

chamockite ga-bio gneiss chamockite ga-chamockite ga-chamockite clmmockite

ga-bio gneiss ga-bio gneiss

ga-bio gneiss 8a-bio gneiss ga-bio gneiss ga-bio gneiss ga-bio gneiss

ga-sil-bio gneiss ga-sil-bio gneiss ga-sil-bio gneiss ga gneiss ga gneiss ga-bio gneiss

SL 669

ga-sil-bio gneiss

SL 794 SL 397 SL 402 K 418 K 419

ga-bio gneiss ga gneiss ga chamockite ga-bio gneiss ga-sil-bio gneiss

K 408

ga-sil-bio gneiss

TDMc ..........[~Ga) 2.8 2.8 2.7 2.2 2.2 2,7 2.6 2,5 2.2* 2,7 3,4 2.6 2.3 2.6 2.4 2.4 2.0 2.0 2.6 2.1 2.3 2.5 2.4 2.8 2.4 2.7 2,8 2,1" 2.1" 2.8 2.2

For notes see Table 3.

Finnigan Mat 261 mass spectrometers at MPI Mainz. The chemical techniques were slightly modified from White and Patchett (1984) and Patchett and Bridgwater (1984). Sr ID analyses were made on separate dissolutions using a mixed 84Sr-SSRb spike. Sr isotopic composition analyses were performed using a doublecollector, peak-jumping mode that involves simultaneous measurements of 87St and 8SSr alternating with 86Sr and 87Sr. The isotope ratios were then normalized to 86Sr/SaSr = 0.1194. On 54 runs of the NBS 987 standard a mean S7Sr/ 86Sr ratio of 0.710236+ 19 (+_ lcr, one standard deviation) was obtained during the study. Nd ID analyses were done either by using a mixed 145Nd-149Sm spike or a 149Sm-15°Nd enriched tracer. The latter became available only recently and was introduced because it al-

lows measurement of both isotope composition and Nd concentration on totally spiked samples. Nd isotopic compositions were made either by a dynamic, triple-collection procedure involving simultaneous measurement of masses 143, 144, 145 alternating with masses 144, 145 and 146, or performed by static multiple collector analyses of masses 143, 144, 145, 146 and 150. The mean 143Nd/144Nd ratios for the La Jolla standard were 0.511834+ 13 ( + 1~, peak jumping mode, n = 31 ) and 0.511836_+ 13 (_+ lo;, static mode, n = 2 4 ) , both fractionation-corrected to 146Nd/ 144Nd = 0.7219. Rock types and sample locations are listed in Table I. The S m - N d data are presented in Tables 2, 3 and 4. Bold sample numbers represent new analyses, all others are taken from

99

Nd ISOTOPIC MAPPING OF THE SRI LANKABASEMENT TABLE 3

S m - N d data for rocks of the Vijayan Complex Field

No. in Fig. 3

No.

25 26 27 28

SL 38 SL 66 SL 164

29 30 31 32 33 34 35 36

SL 362 SL 391 SL 403 SL 584 SL 587 SL 611 SL 613 SL 616

SL SL SL SL SL SL SL SL

359 359-3A 359-3B 359-3C 359-3D 359-3E 359-3F 359-3G

Rock type hbl-bio gneiss calc-silicate calc -silicate ga-hbl-bio gneiss ga-hbl-bio gneiss ga-hbl-bio gneiss ga-hbl-bio gneiss ga-hbl-bio gneiss ga-hbl-bio gneiss ga-hbl-bio gneiss ga-hbl-bio gneiss hbi-bio gneiss hbl-bio gneiss hbl-bio gneiss

bio gneiss hbl-bio gneiss hbl-bio gneiss hbl-bio gneiss hbl-bio gneiss

(ppm)

Nd

(ppm)

Sm

147Sm/144N d

143Nd/144Nda

31.93 25.72 29.77 20.18 25.15 33.66 21.76 12.89 16.79 19.12 5.06 26.45 9.04 6.11 10.03 22.20 51.22 18.01 42.78

5.60 5.31 5.62 4.26 3.96 4.48 4.64 2.84 3.07 4.20 1.14 5.26 2.33 1.32 2.28 4.87 9.59 3.86 8.49

0.1060 0.1248 0.1141 0.1270 0.0953 0.0804 0.1290 0.1330 0.1110 0.1330 0.1360 0.1203 0.1555 0.1307 0.1324 0.1327 0.1131 0.1295 0.1199

0.512224_+19 0.512289-2-_20 0.512247_+22 0.512261_+16 0.512222_+9 0.512190-+9 0.512459+-12 0.512603_+14 0.512439-2-_12 0.512527+_19 0.512562+-16 0.512281_+20 0.512668_+24 0.512126+21 0.512239-2-_23 0.512498+_16 0.512156+--_12 0.512421+_11 0.512320-2-_20

ENdb

TDMc

(I.0Ga)

(Ca)

+3.5 +2.4 +2.9 +1.6 +4.9 +6.1 +5.2 +7.5 +7.1 +6.0 +6.3 +2.8 +5.9 -1.5 +0.4 +5.5 +1.3 +4.4 +3.6

1.3 1.4 1.3 -1.5 1.1 1.1 1.2 1.0 1.0 1.1 1.1 1.3 1.0" 1.8 1.6 1.2 1.4 1.2 1.3

Notes to Table 2-4: Bold samples are new analyses, all others adopted from Milisenda et al. (1988). ga: garnet, cor: cordierite, bio: biotite, hbl: hornblende, sil: sillimanite. aErrors on 143Nd/144Nd are 2o m. beNd values corrected back to assumed ages inferred from recent geochronological studies as summarized in Holzl et al. (this volume) and Krtner et al. (this volume). c TDM calculated following: 0"513151 "(143Nd/144Nd)meas] TDM = 1 In 1÷ " k O.~i~ - 047Srnll~4Nd)m~.J

* Corrected TDM calculated according to the equation given in the text.

Milisenda et al. (1988) and are reprinted for the readers' convenience only. Model parameters are given in the footnote of Table 3. RbSr isotope data are shown in Table 5.

3. Sm/Nd ratios and significance of Nd model ages as a tool for isotopic mapping The principles of calculation and use of Nd model ages as estimates of crustal residence times have been reviewed by Arndt and Goldstein ( 1987 ) and DePaolo ( 1988 ). The underlying principle of "mapping" isotopic provinces using Nd model ages is the observation that many upper-crustal lithologies (e.g. sediments, felsic volcanics and granitoids) display remarkably constant 147Sm/144Nd ratios

( ~ 0.11 _+0.02), which differ significantly and consistently from their (ultimate) mantle source (/> 0.2 ). This constancy of crustal S m / Nd ratios allows the estimation of crustal residence ages for many crustal rock types by assuming that the Nd isotope evolution can be approximated by a single-stage system evolving with upper-crustal-like S m / N d ratios. The resulting age provides the integrated average time since the S m - N d components of a crustal sample were fractionated from the mantle and became part of the continental crust. Sediments and metasediments derived from several sources of different primary ages can therefore provide only a weighted average of the actual crustal residence ages involved. The age significance of Nd model ages for mafic rocks is ambiguous since many of them show mantle-like S m / N d ratios and an iso-

f .(. MII,lSENl)-\ E! %i

100 T

ABLE 4

S m - N d data for rocks of the Wanni Complex (Nos. 37-56 ) and " A r e n a s " (No. 57-59) No. in Fig. 3

Field

Rock

No.

type

37 38 39 40

SL 18 SL 29 SL 30 SL 56 SL 414 SL 68 SL 71 SL 277-4a SL 277.4c SL 542 SL 544 SL 325 SL 330 SL 332 SL 333 SL 336 SL 339 SL 348 SL 417 K 442

41 42 43

44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

K 460 SL 622 SL 648 SL 706 SL 708 SL 789 SL 1 SL 2.1 SL 5.1

ga-cor-bio gneiss chamockite chamockite ga-bio gneiss ga-bio gneiss hbl-bio gneiss ga-bio gneiss hbl-bio gneiss

chamockite hbl-bio gneiss chamockite hbl-bio gneiss pink granite pink granite pink granite hbl-bio gneiss

pink granite chamockite

charnockite ga-hbl-bio gneiss gn-bio gneiss bio gneiss ga-bio gneiss

charnockite chamockite ga-bio gneiss ga-sil-bio gneiss pink granite pink granite

Assum~ Age (Ga)

Nd (ppm)

Sm (ppm)

147Sm/144Nd

143Nd/144Nda

~Ndb

T1)Mc

1.3 0.77 0.77 1.3 1.3 1,1 1.3 0.77 0.77 0.77 0.77 1.1 0.55 0.55 0.55 1. I 0.55 1.1 1.1 1.3 1.3 1.1 1.3 1.1 1.1 1.3 1.3 0.55 0.55

42.22 20.10 38.08 19.76 32.48 63.94 20.78 55.31 31.69 47.94 17.88 32.88 42.76 31.54 55.81 11.91 117.2 20.34 46.71 23.26 34.24 14.91 24.27 25.55 11.96 36.51 32.07 64.72 90.98

8.55 2.96 5.46 3.85 5.71 9.39 3,91 8.66 4.3(I 8.75 3.21 6.64 9.36 6.21 9.54 2.32 20.07 4.87 10.75 5.05 5.08 2.88 4.39 5.80 3.23 6.92 6.59 13.43 17.73

0.1224 0.0891 0.0867 0.1177 0.1063 0.0888 0.1139 0.0946 0.0820 0.1104 0.1087 0.1221 0.1323 0.1190 0.1034 0.1176 0.1030 0.1448 0.1391 0.1310 0.0897 0.1166 0.1093 0.1372 0.1632 0.1146 0.1243 0.1254 0.1178

0.512150L'_13 0.511795+25 0.511738+_23 0.511954+13 0.511948+15 0.511622-+ 11 0.512138-+17 0.5118405:12 0.511776-+ 8 0.511794-+19 0.511795+19 0.512137-+21 0.512303-+15 0.512099-+13 0.512156+13 0.511828+16 0.511937+10 0.512395+16 0.512333-+13 0.512264-+ 9 0.511839-+10 0.512175_+12 0.512139--+18 0.512315-+12 0.512610i--10 0.512185-+11 0.512166-+22 0.512273+28 0.512207+_13

+2.9 -5.8 -6.7 -0.2 +1.6 -3.8 +4.0 -5.5 -5.5 -8.0 -7.8 +0.7 -2.0 -5.1 -2.8 -4.7 -7.1 +2.6 +2.2 +3.6 +2.2 +.2.2 +4.8 +2.1 +4.2 +4.8 +2.9 -2.1 -2~9

1.b 1.6 i Y~ 1.~ 1,6 1.,~ t.5 1~6 1,5 1.9 1,9 1.6 1.5 1.6 1.3 2.0 1.6 1.3" 1~6 1.5 1.5 1~5 1.4 1.6 1.|* t.4 1.6 l4 I ,,2

For notes see Table 3.

topic evolution path that is roughly parallel to their source. On the other hand, if such rocks show more crustal-like S m / N d ratios, this could reflect crustal assimilation and, again, the model age is doubtful. Some high-grade crustal rocks have obviously suffered severe chemical and mineralogical fractionation, especially when sampled in relatively small hand specimens. An extreme example of this is a highgrade metapelite analyzed by Black and McCuUoch (1987), which contained 30% garnet and yielded a 1 4 7 5 m / 1 4 4 N d ratio of 0.39, more than three times greater than the ordinary crustal value of0.11. Therefore, material of this type is completely unsuitable for determining crustal residence ages. Consequently, the present discussion will be simplified by presenting crustal residence ages only for representative samples of typical crustal rocks, such as rectasediments and felsic orthogneisses, the vast

majority of which has 1475m/144Ndratios between 0.08 and 0.14. The 1478m/'44Ndratios for the studied samples of the various lithotectonic units are summarized in a histogram in Fig. I. The mean 1478m/144Ndratio is 0.12_+0.02 ( n = 7 9 , la), consistent with data presented for other highgrade supracrustals and orthogneisses (e.g. Ben Othman et al., 1984; Rudnick and Presper, 1990). This finding indicates that "upper crustal-type" S m / N d ratios are retained by such rocks, implying that, contrary to the assertions of Burton and O'Nions (1990), highgrade metamorphism has not resulted in a gross fractionation of the S m / N d ratios on a wholerock scale. In addition, although the lithotectonic units experienced contrasting P - T conditions (e.g. Schumacher et al., 1990; Faulhaber and Raith, 1991 ) the mean S m / N d ratios of all three units are essentially the same.

Nd ISOTOPIC MAPPINGOF THE SRI LANKABASEMENT

101

TABLE 5 Rb-Sr data Samples

Sr ( p p m )

Rb ( p p m )

aTRb/*6Sr

875r/86Sra

246 320 90.6 146 376 87.2 287 371 294 383 146 78.6

214 121 252 237 55.4 263 134 131 178 146 264 323

2.529 1.097 8.149 4.787 0.426 8.979 1.358 1.024 1.762 1.108 5.308 12.243

0.776664_+ 25 0.722337 _+14 0.839522_+ 16 0.864275 _+25 0.718858 _+5 0.998184_+20 0.747056_+ 16 0.735332_+9 0.766476_+ 25 0.743715_+ 15 0.867143_+ 12 1.023870_+ 27

297 633 409 274 254 184 50.1 273 627 251

158 15.9 14.9 186 85.5 105 255 232 55.2 180

1.547 0.073 0.105 1.969 0.975 1.650 15.051 2.459 0.254 2.083

0.731191 _+25 0.706615 _+9 0.707212_+23 0.731713_+ 29 0.717570_+ 11 0.721359 _+25 0.921743_+ 15 0.736938 -+ 16 0.705870_+ 17 0.734266 -+20

75.5 153 224 104 182 133 75.8 209 73.4 104 113 103 86.3 76.6 144 86.0 170 166 87.8 59.7

0.920 5.411 11.944 0.786 1.844 1.380 1.330 1.054 0.695 0.287 3.093 4.145 1.597 0.298 20.230 1.501 1.090 1.869 0.651 0.799

0.716828+ 11 0.794866 + 33 0.829502_+ 39 0.715711 _+22 0.728588 _+ 17 0.722562 _+8 0.724986 + 11 0.718657 + 20 0.714816 _+20 0.707807 -+ 18 0.747523 _+29 0.754218-+ 11 0.724206_+ 11 0.713301 _+29 0.977916_+ 17 0.727627_+ 14 0.718021 _+34 0.728339_+ 24 0.711404_+ 14 0.721139_+ 12

Highland Complex and tectonic klippen SL 8.3 SL 45 SL 57 SL 82 SL 98 SL 110 SL 125 SL 137 SL 355 SL 357 SL 397 SL 402

Vijayan Complex SL 38 SL 66 SL 164 SL 362 SL 403 SL 584 SL 587 SL 611 SL 613 SL 616

Wanni Complex and "Arenas" SL 1 SL 2.1 SL 5.1 SL 18 SL 29 SL 30 SL 56 SL 68 SL 71 SL 325 SL 330 SL 332 SL 333 SL 336 SL 348 SL 417 SL 542 SL 544 SL 622 SL 648

238 82.4 54.9 385 286 279 165 573 306 1050 106 72.1 157 743 21.1 166 451 257 390 216

aErrors on 87Sr/86Sr are 2a=.

However, in each subdivision, a few samples (five samples in all) show S m / N d values that are significantly higher than those observed for

normal upper crust. The effect of S m / N d fractionation on the calculation of Nd model ages is illustrated in Fig. 2. Sample SL 110, a gar-

102

~i.~. M I L 1 S E N D A [ I

24-

•

Highland Complex

[]

Wanni Complex

[]

Vijayan Complex

1

18-

~

,\~

14-

1

12-

Z 8

Colot~

.08

.10

.12 .14 .16 147Sm/144Nd

.18

Fig. 1. Histogram of 147Sin/144Nd ratios for crustal rocks of the lithotectonic units in Sri Lanka with a mean 147Sm/ 144Nd ratio of 0.12+0.02 (HC: 0.11 +0.02, n=31; WC: 0.11 +0.02, n=29; VC: 0.12+0.02, n = 19). Note, that the Sm/Nd ratios are not biased by a different metamorphic grade.

Fig. 3. Sample location map showing the distribution of Nd model ages in Sri Lanka and separation of the three distinct crustal provinces Wanni Complex (including Arenas: AN), Highland Complex and Vijayan Complex. The approximate isotopic boundary between HC and WC is represented by a dashed line punctuated by question marks, In the eastern part the isotopic boundary coincides with a major thrust zone separating the Vijayan from the Highland Complex.

10 5

~o -5 -I0 -15 -20

J i ) I '11"2 0.5

147Sm/14aNd~ 0.17

1,0

1.5

J J i ' T1 2.0

2.5

3.0

3,5

I"3 ' 4,0

A G E (Ga)

Fig. 2. Forward model for the end evolution of a sample with an anomalously high Sm/Nd ratio. At T1 = 2.2 Ga, the rock is derived from the depleted mantle and evolves with a normal 147Sm/~Nd=0.12 until it is subjected to high-grade metamorphism a t / 2 = 600 Ma ago, which increases its 147Sm/t'~Nd ratio to the anomalously high value of 0.17. A single-stage calculation for this sample would yield an anomalously high model age of/3---4 Ga.

netiferous charnockite from the Highland Complex with TDM = 2.2 Ga (T1 in Fig. 2) is assumed to evolve with an average crustal 147Sm/144Nd ratio of 0.12 until the time of metamorphism ( / 2 ) which, on the basis of recent geochronological data (Baur et al., 1991; HSlzl et al., 1994), was in the Neoproterozoic --, 600 Ma ago. In this particular case, thehighgrade event caused a significant increase in the S m / N d ratio, which may have been controlled by the formation and enrichment of garnet during metamorphic differentiation. If the measured 147Sm/144Ndratio of ~ 0.17 is used, the resulting model age ( / 3 = 4 Ga) is unrealistieally high. Similarly, a significant decrease

103

Nd ISOTOPIC MAPPING OF THE SRI LANKA BASEMENT

3.1. The Highland Complex

12 10 =" 8 6 _E 4 Z 2 1.0

1.2

1.4

1.6

1.8 2.0

2.2

2.4

2.6

2.8

3.4

apparent crustal residence age (Ga)

Fig. 4. H i s t o g r a m o f ToM m o d e l a g e s s h o w i n g t h e r a n g e of crustal residence ages for Highland Complex, Vijayan

Complex and Wanni Complex, where the boundary between Wanni and HighlandComplexis definedby the data as shown in Fig. 3.

in the S m / N d ratio would yield a model age that is too young. These effects on the Nd model ages are very large when the S m / N d ratios fractionated very late in the evolution of a crustal rock. Therefore, we corrected the Nd model ages for five samples with 147Sm/144Nd ratios > 0.14 by using measured ~47Sm/144Nd ratios for the age interval 0 to 600 Ma ago, and then calculating model ages by using an average crustal ratio of 0.12. The depleted mantle model age (TDM) is then defined as:

1 E

Trot = ~ In 1+{0.513151 -[(

141

Nd/

144

Nd)meas.-

( e 2 > 600Ma

-1)

Samples from the central granulite belt of the Highland Complex yield model ages from 2.0 to 3.4 Ga and confirm the Palaeoproterozoic to Archaean origin for this crustal province. Rock types analyzed include metasediments such as sillimanite-garnet and cordierite-garnet gneisses, garnetiferous hornblende orthogneisses, charnockites and garnet-biotite gneisses of as yet uncertain origin. Thus, the old model ages appear to be independent of the rock type. Garnetiferous gneisses collected from the high-grade Kudu Oya and Kataragama inliers (Nos. 21-24 in Fig. 3 ) within the Vijayan Complex yield comparable Palaeoproterozoic to Archaean model ages, which supports the suggestion that these inliers represent tectonic klippen of Highland Complex rocks (e.g. Vitanage, 1985; Kr/Sner et al., 1991). The actual depositional ages of the supracrustal sequence of the Highland Complex is still uncertain, but are estimated to be around Palaeoproterozoic based on the absence of detrital zircons younger than ~ 2 Ga as well as the ~ 1.9 Ga intrusive ages of Highland orthogneisses (Baur et al., 1991; H/Slzl et al., 1994). However, for most of the samples presented in this study, the formation and emplacement ages are unknown which makes the interpretation of the isotope data base difficult. It was therefore found useful to calculate

)< [ (147Sm/144Sm)meas - 0 . 1 2 ] ]}

[] 8_-

__

sillimanite and cordiefite gneisses

[ ] chamockites

/(0.2188--0.12) 1 "~ 4 -

We emphasize that this procedure was applied to only five samples, mainly in order to show that their seemingly aberrant compositions are still consistent with the model ages of the more typical samples. The distribution of the Nd model ages in Sri Lanka is shown in Fig. 3 and summarized in the histogram of Fig. 4.

.~

_

-~2-24

-22

-20

-18 -16 -14 ENd (600 Ma)

-12

-10

-8

Fig. 5. H i s t o g r a m o f end v a l u e s at t h e t i m e o f h i g h - g r a d e m e t a m o r p h i s m ( 6 0 0 M a ) for H i g h l a n d C o m p l e x g r a n u lites. T h e end v a l u e s s h o w n o d e p e n d e n c e o n rock type.

i04

: ( ' . MII.ISENIAA E l AI

initial ~Nd and 875r/86Sr at 600 Ma, the inferred age of granulite-metamorphism. The large negative eNd values ( --7 to --25; see Fig. 5 ) and the high initial S7Sr/86Sr ratios between 0.713 and 0.921 indicate that these rocks had an extensive pre-600 Ma crustal history. In addition, the large range in initial Nd and Sr isotope compositions provides strong evidence that the lower crustal rocks of the Highland Complex represent extensive reworking of old continental crust and that no new crustal material was generated during the granulite event. It should be noted, however, that mafic rocks were systematically excluded from this study, for the reasons given above. Therefore, mafic additions to the crust during the high-grade event may or may not have occurred.

3.2. The Vijayan Complex Most of the analyzed samples from the Vijayan Complex are amphibolite-grade hornblende +_biotite gneisses of granitoid composition, biotite migrnatites, garnet-hornblende gneisses and calc-silicates. These rocks yield Nd model ages varying between 1.0 and 1.8 Ga. The majority of these gneisses are of tonalitic to leucogranitic composition and, based on geochemical criteria, are classified as I-type granitoids (Milisenda et al., 1991; Pohl and Emmermann, 1991 ). Three representative Vijayan orthogneisses collected from widely separated localities yielded U - P b zircon ages of •

hornblende+biotitegneisses, biotite gneisses [ ] garnet-hornblendegneisses D calc-silicates

,,O =

1022+_ 10 Ma, 1021+_ 18 Ma and 1032_~::100 Ma, indicating that the majority of the Vijayan gneisses were emplaced ~ 1 Ga ago (H61zl et al., 1994). If measured Nd isotope data are corrected using this age, they give end initials ranging from -- 2 to + 7 (Fig. 6 ), with most of the samples displaying strongly positive values. The most positive end initials are actually found in rocks collected from a quarry at Mahiyangana (No. 28 in Fig. 3 ). The observation of relict granulite facies mineral assemblages in these rocks has led Kleinschrodt and VoU (1991 ) to suggest that these rocks represent retrogressed equivalents of the Highland Complex and that the boundary separating the Highland Complex from the Vijayan Complex is located to the East of that locality. The strongly positive end values, however, cause us to question this interpretation. Instead, we attribute these rocks to the Vijayan Complex, a point of view that appears to be supported by geophysical data (Biichel, 1994). The calculation of precise initial 87Sr/86Sr ratios is complicated by the fact that some samples show high Rb/Sr ratios and, therefore, the correction factor is very large. In addition, De Maesschalck et al. ( 1990 ) reported Rb-Sr data for Vijayan hornblende-biotite gneisses, which plot close to a 800 Ma reference isochron and thus require ages younger than 1 Ga. However, the samples with low Rb/ Sr ratios show, independent of the age, initial 87Sr/S6Sr ratios in the range ~0.704-0.707. The positive end values and the low to intermediate Sr initials confirm the suggestion of Milisenda et al. ( 1988, 1991 ) that the gneisses of the Vijayan Complex represent, at least in part, juvenile crustal additions in the Neoproterozoic. In addition, their compositional characteristics closely resemble those of I-type granitoids found in continent-margin plutonic arcs.

Z -2

0

+2

+4

+6

3.3. The Wanni Complex

ENd (1000 Ma) Fig. 6. Histogram of end values at 1 Ga for Vijayan gneisses.

Samples from the Wanni Complex include a large diversity of amphibolite- to granulite-fa-

Nd ISOTOPIC

MAPPING

OF THE SRI LANKA BASEMENT

cies rocks such as metasediments of predominantly pelitic to semipelitic composition, hornblende and biotite gneisses, charnockitic gneisses and late- to post-tectonic pink granites of alkaline affinity. Nd model ages for both para- and orthogneisses range between 1.1 and 2.0 Ga. The Mesoproterozoic ages of 1.4 to 1.6 Ga for rocks collected from the large synformal "Arena" structures (Nos. 57-59 in Fig. 3 ) allow us to include provisionally these gneisses with the Wanni Complex, as proposed already by Kehelpannala ( 1991 ). Radiometric ages on rocks of the western model age province are sparse and insufficient to portray its depositional and igneous evolution, but available data indicate that the various granitoid suites were emplaced ~<1.1 Ga ago. The largest and probably most distinct time interval of magmatic activity ranges between ~ 1000 Ma and ,-, 1100 Ma. These ages are provided by the massive charnockite bodies (No. 54 and 55 ) N and NE of Anuradhapura in the Vavuniya area ( U - P b zircon ages between 1050 and 1100 Ma, Kr/Sner et al., 1994) and granodioritic gneisses (No. 52) some 25 km NW ofKurunegala ( U - P b zircon, A. Krtiner, pers. commun., 1993). Additional intrusion ages have been reported on a hornblende-biotite gneiss (No. 43, SL 542 ) and an associated in-situ charnockite (No. 43, SL 544 ) at Udadigana in the Kurunegala District. Zircon populations of both specimens yielded an upper Concordia intercept age of 771 +~7 --14 Ma, which was interpreted as the intrusion age of the original granite (Baur et al., 1991 ). The last episode of magmatic activity is represented by the late- to post-tectonic pink granites with U - P b zircon ages of ~ 550 Ma (Htilzl et al., 1994 ). The depositional age of the Wanni metasediments remains unknown, but detrital zircons from metapelites are not older than ~ 1.3 Ga (Htilzl et al., 1994; KriSner et al., 1994) which supports the young age of the Wanni Complex relative to the Highland Complex as already indicated by the model age data.

105

Ages in Table 4 represent assumed emplacement ages for magmatic rocks and maximum stratigraphic ages for metasedimentary rocks. Note that most of the ages have been inferred from broad geological and geographical correlations. However, the purpose of this section is to confirm the "young" Nd isotope signature of the Wanni Complex relative to the Highland Complex and therefore, errors in the absolute values of the ages have no effect on the interpretation, because the calculated model ages are not dependent on the age of intrusion. Fig. 7 shows initial end versus geological ages for the Wanni samples. Also shown for reference are the depleted-mantle (DM) and the bulk Earth ( C H U R ) evolution lines, the field of initial ~Nd for the Vijayan gneisses (VC) and the evolution line of typical average Highland Complex crust separated from the depleted mantle 2.5 Ga ago. The massive charnockites from the Anuradhapura and Vavunyia areas (Nos. 48, 49, 54, 55 ) form an isotopically distinct group with ~Nd initials ranging from + 2.1 to + 4.2 (MC in Fig.

0

CHUR

~

~

~

i - n- -

-

s

i

t

u

-

~

"-

20

.~

~

0.5

1.0

9 0 _ % , charnockites, Kumn~gala a hbl-bio grmfitoids o ga-bio, sil-ga, cor-ga gneisses + pird~ granites 1.5

Z0

AGE (Ga)

Fig. 7. Initial end versus age (Ga) for rocks of the Wanni Complex: M C (massive charnockites); VC (Vijayan gneisses). The reference2 Ga averagecontinentalcrust is calculated assuming a crustal 147Sm/144Ndratio of 0.12 and a linear evolution of depleted mantle (DM) from 6Nd3"5Ga=0 to eNOd=+ 10. The evolution line of 2.5 Ga Highland crust is calculatedfrom averagesillimanite-and cordierite-bearinggneissesof clearlymetasedimentaryorigin. The 1.5 Ga evolution line is calculatedfrom sillimanite- and cordierite-bearing gneisses of the Wanni Complex.

10(3

7), indicating an isotopically little evolved, mantle-derived precursor as already postulated for most of the Vijayan orthogneisses. Their 1475m/144Nd ratios (0.1372-0.1632) are moderately to significantly higher than a typical crustal ratio of 0.12. If the Nd isotope composition is corrected for post-metamorphic decay using the method outlined above, and end initials are calculated using a t47Sm/144Nd ratio of0.12, the resulting end initials increase to more positive values, thus further reducing the calculated model ages and confirming the juvenile character of these rocks. Consequently, the large charnockite province in northern Sri Lanka is not part of the Highland Complex but should be grouped with the Wanni Complex. The range of initial end for the hornblendebiotite granitoids is from - 4 . 7 to +2.2. The four samples analyzed indicate two distinct groups. SL 325 and SL 622 have positive values ( + 0.7 and + 2.2, respectively) whereas SL 336 and SL 68 show negative values of - 3 . 8 and - 4 . 7 , respectively. This indicates that older crustal material may have participated in the genesis of these rocks. The arrested in-situ charnockites and the hornblende-biotite gneisses collected in the Kurunegala District (Nos. 38, 39, 43) show similar initial Nd isotopic ratios with end values between - 5 . 5 and - 8 . 0 . The least negative values are predominantly found in the charnockites. This is due to the fact that in-situ charnockite formation is accompanied by a local decrease of the S m / N d ratio (e.g. Burton and O'Nions, 1990; Milisenda et al., 1991 ). Burton and O'Nions (1990) showed that along a small-scale gneiss-charnockite traverse, the decreasing S m / N d ratio causes a reduction of the Nd model ages from maximum 1.9 Ga in the hornblende-biotite gneiss to minimum 1.5 Ga in the charnockite and, therefore, they questioned the significance of these ages. However, independently of the S m / N d ratios, the resulting model ages still coincide with those observed in other rocks of the Wanni Complex. In addition, significant S m / N d frac-

,~ C . M I L I S E N ~ } ~ f f

'.,i

tionation is only observed on the centimetre scale. Sample SL 277-4a, a hornblende-biotite gneiss yields a model age of 1.6 Ga and SL 2774c, the corresponding charnockite, yields a model age of 1.5 Ga. For samples SL 542 (hornblende-biotite gneiss) and Sir 544 (charnockite) the resulting model ages are identical ( 1.9 Ga). Thus, if model ages are estimated on whole rocks, the effect of local Sm/ Nd changes is largely averaged out. In summary, the arguments and evidence presented by Burton and O'Nions (1990) are not relevant to the present distinction of model age provinces, because: (a) the specific local changes in S m / N d ratios, on which their arguments are based, are so small that the resulting model ages remain within the scatter of the other model ages of the region, and (b) because the observed local changes in S m / N d have clearly not affected the vast majority of the samples irrespective of their metamorphic grade (see Fig. 1 ). The Sr initial ratios for the Kurunegala samples range from 0.706 to 0.708, and are compatible with the finding of Baur et al. ( ! 991 ) that these gneisses have major and trace element characteristics similar of I-type granitoids. However, their comparatively tow end values contrast with the less evolved isotopic signatures of the Vijayan Complex (see Fig. 7 ), and they therefore do not represent juvenile crustal additions. The garnet-biotite gneisses which, based on stable isotope data (Fiorentini et al., 1990; Hoernes et al., 1994), are at least in part of sedimentary origin, and the cordierite-garnet and sillimanite-garnet gneisses have initial end values ranging between - 0 . 2 and +4.8. It is important to point out that the sillimanitegarnet gneiss (No. 57) from the Arena structures has an initial end value of + 2.9, which is identical to the value found in the cordieritegarnet gneiss (No. 37) from the Wanni Complex. This indicates derivation from a similar crustal provenance, because metasediments commonly inherit the integrated isotopic com-

Nd ISOTOPIC

MAPPING

OF THE

SRI LANKA

BASEMENT

| 07

position of the continental material from which they were derived. Although the geographic position of this provenance remains unknown, it is likely from overall Nd isotopic compositions that these sediments were derived from sources similar to the exposed magmatic rocks of the Wanni Complex. They cannot be derived from sources similar to the Highland Complex. The syn- to post-tectonic alkaline pink granites (PG in Fig. 7) show a spread of initial eNd values from -- 2.0 to -- 7.1. They show high and variable Sr initials ranging from 0.712 to 0.750, i.e., typically crustal ratios, which indicate that these rocks represent anatectic derivatives of pre-existing crustal material as typified by rocks of the Wanni Complex. 4. Comparison of the model age provinces with regional Sr and Pb isotope data The Rb-Sr data for all samples studied are compared in a conventional Rb-Sr isochron diagram (Fig. 8 ). Both the Wanni and the Vijayan gneisses plot close to a 1 Ga reference isochron, consistent with the assumed intrusion and stratigraphic ages for the majority of the ortho- and paragneisses of these two crustal provinces. In contrast, the vast majority of the Highland granulites are far off that trend and are approximately aligned with a 2 Ga refer• .i.*a.. C;°p,ex

/

o Wanni Complex + Viiavan C o m p l e x

/ ~

/ •

~

.xx~e~ .

0.78

o

0.76 0,74

\0~¢ 0.8

o •

.

~ ¢ ~c~~ "

o

0.72 [

0.~

5

l0

__ 15

,

,

•

.

,

° •

o

o 1

2_3 20

4 25

87Rb/86Sr

Fig. 8. Rb-Sr isochron diagram for crustal rocks of the lithotectonic units in Sri Lanka. Note the high 87Sr characteristics of Highland Complex rocks.

ence isochron. Thus, for a given 87Rb/86Sr ratio, these rocks possess much higher 875r/86Sr ratios than the Wanni and Vijayan gneisses do. Although the Rb-Sr system can be strongly disturbed by weathering, sedimentation processes and metamorphism, the ancient Highland Complex and the younger Vijayan and Wanni Complexes still exhibit contrasting Sr isotope signatures that are comparable to the differences in the crustal residence ages. Previously reported Rb-Sr data for Highland Complex rocks also show alignments along ~ 1.9 to 2.3 Ga reference isochrons (Crawford and Oliver, 1969; Wickremasinghe, 1969; De Maesschalck et al., 1990), whereas Wanni and Vijayan gneisses cluster around reference isochrons between 800 Ma and 1100 Ma (Cordani and Cooray, 1989; De Maesschalck et al., 1990). These authors interpreted the Rb-Sr arrays as reflecting the ages of high-grade metamorphism. We disagree with this interpretation, because recent U - P b geochronology on zircons and monazites clearly indicates that the granulite event occurred in the latest Proterozoic to early Phanerozoic time (Baur et al., 1991; H/51zl et al., 1994; Kr/Sner et al., 1994). Instead, the consistency of the apparent Rb-Sr ages with the Nd isotopic characteristics and with recent estimates of depositional and intrusive ages strongly suggests that the Rb-Sr system inherited the isotopic characteristics of the protoliths and thus provides a rough estimate of the precursor age. Liew et al. ( 1991, 1994) analyzed samples collected from the various lithotectonic units for Pb isotope compositions. They found that samples from the Highland Complex plot consistently above the Vijayan and Wanni gneisses on a 2°Tpb/2°4pb versus 2°6pb/2°4pb diagram. The high 2°7pb/2°4pb ratios for Highland Complex granulites reflect an extended period in a high-U/Pb environment relatively early in Earth history when 235U, the parent nuclide of 2°7pb, was still abundant. In contrast, the 2°6pb/2°4pb ratios require residence in a lowU / P b environment which developed much

]{)8

later, presumably at the time of granulite-grade metamorphism. Thus, the argument for a large time interval between crust formation (/> 2 Ga) and metamorphism (600 Ma) is also supported by Pb isotope data. The high 2°7pb signature of the Highland Complex rocks relative to the Wanni and Vijayan gneisses strongly supports the identification of distinct isotopic provinces in Sri Lanka based on the Nd model ages. 5. Discussion

The updated Nd data base confirms the existence of distinct crustal provinces within the high-grade basement of Sri Lanka and highlights the use of Nd model ages to map crustal provinces in high-grade terrains which cannot be readily differentiated by conventional mapping. Nd model ages between 2.0 to 3.4 Ga show that both ortho- and paragneisses of the Highland Complex had an extended crustal history and were derived from an Archaean to Palaeoproterozoic source terrain. This finding is consistent with the documentation of ~ 1.9 Ga intrusive ages for Highland orthogneisses and the presence of detrital zircons that preserved ages in the range ~ 2 Ga to 3.2 Ga suggesting that the Highland supracrustals were deposited in the Palaeoproterozoic (Kr~iner et al., 1987; Baur et al., 1991; H/51zl et al., 1994). Furthermore, the highly radiogenic Pb and Sr isotope characteristics suggest that this complex has preserved a record.of an extended residence in an upper crustal environment characterized by high U / P b and Rb/Sr ratios. In contrast to the Highland Complex, the Vijayan gneisses show significantly younger Nd model ages ranging from 1.0 to 1.8 Ga. Most of the Vijayan Complex consists of a variety of gneisses, hornblende___biotite bearing granitoids, and the original protoliths of tonalitic to leucogranitic composition were emplaced some 1 Ga ago. The positive initial ~Na values and the low initial 875r/a6Sr ratios indicate that the

t-t

MILISF~',[):~}!I a1

larger part of the Vijayan Complex is isotopically "unevolved". It represents a juvenile crustal fragment derived from melting of a mantle-derived precursor and not from reworking of significantly older crustal material such as the Highland gneisses. The contrasting isotopic systematics and chemical characteristics clearly imply that the Highland Complex and the Vijayan Complex represent two distinct crustal provinces of different origin. The voluminous granitoids of the Vijayan Complex are similar to those found in the root zones of Andean-type magmatic arcs, and this complex appears to fit best a detached continental margin plutonic terrane dominated by subduction-related, calc-alkaline, Itype granitoid intrusions. The predominantly Mesoproterozoic Nd model ages between 1.0 and 2.0 Ga for orthoand paragneisses of the Wanni Complex indicate that this lithotectonic unit constitutes a third distinct crustal domain within the Sri Lankan basement. Although the exact geographical extent of this complex has not yet been delineated, an origin distinct from that of the Highland Complex is substantiated by its relatively low 2°7pb/2°apb ratios and by the absence of zircon ages older than ~ 1.3 Ga. The isotopic characteristics of the metasediments are particularly noteworthy since their Nd, Pb and Sr characteristics clearly indicate that the former sedimentary protoliths must have had a different source from those of the Highland metasediments farther East. In addition, recent estimates of depositional ages suggest that the supracrustal sequence of the Wanni Complex is some 1 Ga younger than the Highland Complex assemblage, which is consistent with the Nd model age data. The consistent isotopic contrasts between the three basement units suggest that these units may represent tectonically accreted ("suspect") terranes, similar to those documented in North America (e.g. Saleeby, 1983; Hoffman, 1988) and elsewhere, This concept is comparatively easy to accept for the relation-

Nd ISOTOPIC MAPPING OF THE SRI LANKA BASEMENT

ship b e t w e e n t h e V i j a y a n a n d H i g h l a n d C o m plexes, b e c a u s e t h e i r b o u n d a r y is w i d e l y acc e p t e d to b e a t h r u s t c o n t a c t (e.g. H a t h e r t o n et al., 1975; V i t a n a g e , 1985; K r r n e r et al., 1987; Kleinschrodt, 1994; K r i e g s m a n n , 1994). W h e t h e r t h e t e r r a n e c o n c e p t is also a p p l i c a b l e to the W a n n i - H i g h l a n d b o u n d a r y is likely to r e m a i n c o n t r o v e r s i a l u n t i l this c o n t a c t c a n b e delineated with much greater precision than w a s p o s s i b l e in t h e p r e s e n t study. W e s u b m i t , h o w e v e r , t h a t f i n d i n g t h e c o n t a c t in t h e field m a y u l t i m a t e l y b e e a s i e r t h a n e x p l a i n i n g the ~ 1 G a age c o n t r a s t e n t i r e l y w i t h o u t t h e terr a n e c o n c e p t . It is p r i m a r i l y for this r e a s o n t h a t we f a v o u r t h e " U n i t e d P l a t e s o f Sri L a n k a " ( w i t h a p o l o g i e s to P a u l H o f f m a n , 1988 ).

Acknowledgements T h i s s t u d y f o r m s p a r t o f the Special R e search Project "Composition, structure, and e v o l u t i o n o f the l o w e r c o n t i n e n t a l c r u s t " , supp o r t e d b y the D e u t s c h e F o r s c h u n g s g e m e i n schaft ( D F G ) . The following colleagues are t h a n k e d f o r p r o v i d i n g s a m p l e s , scientific c o m m e n t s , h o s p i t a l i t y a n d g u i d a n c e d u r i n g field w o r k : S. F a u l h a b e r , S. H/51zl, K . V . W . K e h e l p a n n a l a , A. K r t i n e r , K. M e z g e r , W . K . B . N . P r a m e , J. Pohl, V. S c h e n k a n d P.W. V i t a n a g e . W e a c k n o w l e d g e N . T . A r n d t a n d S. M o o r b a t h for reviews.

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