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Pb isotopic discrimination of crustal domains within the highgrade basement of Sri Lanka
Precambrian Research, 66 (1994) 111-121
111
Elsevier Science B.V., Amsterdam
Pb isotopic discrimination of crustal domains within the highgrade basement of Sri Lanka T.C. Liew*, C.C. Milisenda and A.W. Hofmann Max-Planck Institut J~r Chemie, Postfach 3060, 6500 Mainz, Germany Received February 17, 1992; revised version accepted January 5, 1993
ABSTRACT Pb isotope data confirm the recognition of two isotopically distinct crustal domains, originally proposed on the basis of Nd model age data, within the central belt of granulite-grade rocks in Sri Lanka. One domain, the Highland Complex, comprise rocks that have late Archaean-Palaeoproterozoic Nd model ages and unusually high 2°7pb/2°4Pbcompositions. They plot well above the 2°Tpb/2°apb-2°rpb/2°"Pbplumbotectonic growth curves for average upper crust and orogeny of Zanman and Doe ( 1981 ), and have been isolated from plumbotectonic exchange more than 1 Ga prior to metamorphism ~ 600 Ma ago. Two Highland samples suggest that, in some cases, this isolation occurred some 2 Ga ago or earlier. This isolation age is considered to date the time the pre-granulite Highland terrain was "cratonised", i.e. incorporated into a stable shield environment. Pb isotopic compositions of rocks from the second domain, the Wanni Complex, straddle the upper crust and orogeny plumbotectonic curves. In contrast to the Highland granulites, there is no evidence for an extended plumbotectonic isolation period prior to metamorphism. The available U - P b zircon, Nd and Pb isotope data indicate that the Wanni Complex represents a mid-Neoproterozoic crustal package (0.7 Ga < T< 1.3 Ga ) metamorphosed some 600 Ma ago. A time gap exceeding 0.5 Ga separates the age of "cratonisation" ( ~ 1.8-2.0 Ga) of the pre-granulite Highland terrain from the times of formation of the earliest Wanni protoliths ( < 1.3 Ga). The distinctive isotopic characteristics and the unrelated geological evolution reconstructed from these data indicate that the two now-adjacent Highland and Wanni complexes were geologically unrelated crustal packages prior to high-grade metamorphism and, as such, represent "suspect terranes" brought together during a collision event.
1. Introduction
Milisenda et al. (1988) demonstrated that the distribution of Nd model ages of rocks from the central belt of granulite-grade rocks (Fig. 1 ) in Sri Lanka forms a relatively simple pattern consisting of an eastern part having old model ages (2-3 Ga) and a western part having younger model ages (1-2 Ga). The isotopic differences are large and there is minimal overlap of the two groups on a histogram plot (Milisenda et al., 1994). The exact geological significance of these isotopic differences is still *Corresponding author. Present address: Dept. of Nuclear Physics, University of Oxford, Oxford OX1 3RH, U.K. (Reprint requests to Mainz address. )
SSDIO301-9268(93)EO029-C
being debated, but Milisenda et al. (1988) suggested that the two distinct isotopic domains may be separated by a major structural/ tectonic break. These results, together with those of subsequent U - P b zircon dating work (Baur et al., 1991; H/51zl et al., 1994; KriSner et al., 1994) and petrological studies (Schumacher et al., 1990; Faulhaber and Raith, 1991 ) led Kr/Sner et al. (1991) to propose a two-fold division of the central granulite belt in Sri Lanka. The Wanni Complex comprises rocks that have young Nd model ages and young zircon ages (0.7 G a < T< 1.3 Ga) and was metamorphosed at low to medium pressures ~ 550-600 Ma ago. It can be differentiated from the Highland Complex which contains rocks having the older Nd model ages and
]C FLI~Lkl~ S, c h a l n c l t kig~l
9
.
7°.
Color
50
6 °--
Fig. 1. Simplified geologicalmap of Sri Lanka (modified from Geological Map ofSri Lanka, 1982) showing distribution of the central granulite belt and the inferred division into the Highland Complexand Wanni Complexafter Milisendaet al. ( 1988). Sample numbers followthat listed in Tables 1 and 2. The location of the "boundary" is only a crude approximation inferred from the previous Nd "model age mapping" work. There have since been some indications from petrological studies that this "boundary" may coincide with a break in metamorphic pressure gradient in the central portion of the island (M. Raith, pers. commun, based on examination of data of Schureacher et al., 1990, and Schenk et al., 1991 ), and with a zone of anomalously strong strain gradient (Kriegsman, 1994). The exact nature and regional distribution of a possible tectonic boundary between the two isotopically distinct crustal domains remain to be worked out. old zircon ages ( > 1.8 G a ) and which was m e t a m o r p h o s e d at m e d i u m to high pressures ~ 550-600 M a ago. This nomenclature is followed in this paper. The two-fold division m a y not prove to be adequate in future, and additional ( m o r e restrictive) lithotectonic subdivisions o f the central granulite belt m a d e on the basis o f rock associations m a y become desira-
ble. Such additional lithological subdivisions of the two isotopic domains will not alter the main conclusions reached in this paper in which we use Pb isotope data (together with earlier Nd model age information ) to argue that the nowadjacent Wanni and Highland complexes tor future subdivisions thereof) have had unrelated p r e m e t a m o r p h i c histories. The methodology adopted illustrates how isotope data facilitate the recognition of distinct crustal packages juxtaposed in complex terrains and m a y have general application in the field of "terrane analysis". An earlier publication (Liew et al., 1991 ) used then unpublished data for the Highland Complex in a comparative study of the Highland, Lewisian (Scotland) and N u k (SW Greenland) high-grade terrains, and discussed the relevance of their Pb isotope compositions for understanding the global Pb mass balance. This paper will draw on some of the conclusions arrived at in that earlier study without repeating the arguments. 2. Pb isotopes as tracers of crustal evolution The U - P b system, in contrast to the S m - N d system, is often severely fractionated during high-grade metamorphism. Severe depletions o f Th and U relative to Pb in high-grade felsic rocks have been well d o c u m e n t e d by m a n y early studies (e.g. Heier, 1973) and the timeintegrated effects o f these depletions can be studied using Pb isotopes (e.g. Moorbath et al., 1969 ). Studies involving a combination of N d and Pb isotope work are especially interesting since Pb isotopes provide information related to the chronology of intracrustal differentiation associated with high-grade metamorphism, whereas the S m - N d system yields important information concerning times of derivation o f juvenile crust from the mantle. Two classes o f Pb isotope evolution systematics are o f particular relevance to the discussion in this paper: ( 1 ) multi-stage, closed-system Pb evolution and (2) complex open-system be-
Pb ISOTOPIC
DISCRIMINATION
OF CRUSTAL
113
DOMAINS
haviour as described by the "plumbotectonic model" of Zartman and Doe ( 1981 ) and Zartman and Haines (1988 ). The systematics are not as straightforward as those of Sr and Nd isotopes and, because of this, a brief two-part explanation follows.
Ibll ('
E
('
{I 155
I
'
X
Ga
2Ga e-I 150
14.5 3 0 Ga A
2.1. Multi-stage Pb isotope evolution 140
Liew et al. ( 1991 ) argued that the late Archaean protoliths of the Highland granulites had resided for extended intervals of time in a high U / P b upper crustal-like environment prior to high-grade metamorphism ~ 600 Ma ago because their Pb isotope ratios record a long history of high p ( U / P b ratio expressed as 238U/E°4pb) even though the granulites now have low measured U / P b concentration ratios. Fig. 2 and its accompanying extended caption analyses the 2°7pb/z°4pb-E°6pb/2°4pb systematics of a hypothetical multi-stage system (closed with respect to Pb) derived from primitive mantle 3 Ga ago (stage 1 ), attaining crustal U / P b ratios at this time and evolving with such ratios till 600 Ma ago (stage 2). Pb isotopic growth from then on was either "frozen" or severely retarded (stage 3 ). Measured Pb isotopic ratios of the Highland granulites will be examined relative to this simple model in Sect. 4. Fig. 2 also illustrates another aspect of Pb isotope evolution especially relevant for an appreciation of Pb isotopic differences, and this is the retarded evolution of 2°7pb/2°4pb relative to 2°6pb/2°4pb during the last 2 Ga compared to its rapid evolution during the Archaean. Point X in Fig. 2 represents the present-day Pb isotopic composition of a system that "took off" from the primitive mantle (/1= 8.5 ) growth curve 2 Ga ago and evolved with a high crustal ~ of 13 for the last 2 Ga. Note that there is only a small increase of 0.2 in the 2°Tpb/2°4pb ratio compared to the present-day primitive mantle composition despite the large increase in U / P b and long age interval. Differences of such magnitude in post-Ar-
12
13
14
15
/6
17
I~
Itt
2(I
206Pb/204Pb
Fig. 2. Illustration of systematics of Pb isotopic evolution of multi-stage systems (closed with respect to Pb) on a 2°7pb/2°4pb-E°6pb/2°4pb plot. The reference Pb growth curve used in the figure is that representing a primitive mantle evolving with p = 8.5 and To = 4.51 Ga (Liew and Hofmann, in prep.). Ticks mark 250 Ma intervals along the primitive mantle curve. The terms "high" and "low" values, whether time-averaged values calculated from Pb isotopes or present-day measured values, are compared relative to this primitive mantle value of 8.5. The line A B C represents a "3000 Ma-600 Ma isochron" along which all systems evolving from point A with a variety of /~ values until 600 Ma ago will fall, if their p values are effectively "frozen" by a 600 Ma event. Systems that evolved with 1~> 8.5 will lie on the segment B C and those with/~ < 8.5 will lie on the segment AB. There has been very little growth of 2°TpbF°4Pb in the last l Ga because 235U is almost extinct, so any post-600 Ma isotopic growth (i.e. non-zero p values) will merely shift points from their 600 Ma initials sub-horizontally (along a line like C - C ' ) to the right of the line ABC. Points that are located left of this isochron must have experienced an earlier lowering of# values, sometime between 3000 Ma and 600 Ma. The line A D E is a "3000 Ma-2000 Ma isochron". Systems affected by two high-grade events, at 2000 Ma and at 600 Ma, will plot on or close to the line A D E if low ~ values are produced 2000 Ma ago. The effects of further decreases of ~ at T= 600 Ma are practically indiscernible. Note that the slope of line A B C is steeper than that for a 3000 Ma system (i.e. 3000 Ma-0 Ma isochron) and, as a rule, rock systems that have an extended period of crustal residence prior to high-grade metamorphism will always yield Pb-Pb "isochron ages" older than both their true protolith ages and the age of metamorphism. It is for this reason we have not given any weight to linear arrays on a 2°Tpb/E°4pb vs. 2°6pb/E°4pb plot in this study. Point X represents the present-day composition of a system starting from the primitive mantle curve 2 Ga ago that then evolved with a crustal-like p of 13. Despite this extended time-separation of two systems with contrasting/~, an absolute 2°7pb/2°4pb difference of only 0.2 is produced and this demonstrates the importance of small 2°Tpb#°4pb differences in post-2 Ga rocks.
chaean crustal rocks therefore imply long periods of residence in systems with very different U / P b ratios.
2.2. Open-system Pb evolution and plumbotectonics Zartman and Doe ( 1981 ) and Zartman and Haines ( 1988 ) argue that the Pb isotopic evolution of the upper mantle and continental crust cannot be viewed as the product of longterm closed-system evolution. They devised a theoretical model to describe the Pb isotopic evolution of the crust and upper mantle which assumes that semi-continuous, bi-directional exchange takes place between the various terrestrial silicate reservoirs. The basis for this "plumbotectonic" model for Pb evolution is the mechanism of plate tectonics. It involves the constant subduction of oceanic crust ( + incorporated sediments) into the convecting mantle that is continuously tapped to produce juvenile crust which is then eroded to restart the cycle. In fact, workers like Armstrong ( 1981 ) have, for a long time, independently championed the case for crustal recycling in order to explain the combined isotopic and chemical record of the Earth. Theoretical growth curves have been constructed for average upper continental crust, lower continental crust, oceanic mantle and orogeny (where rapid mixing and homogenisation of components from contrasting reservoirs are envisaged during pulses of "orogenies"). The orogenic curve is actually quite similar to the conformable ore Pb curve (e.g. C u m m i n g and Richards, 1975 ). It is considered to represent the average erosional record of large segments of the exposed continental crust. The shape of the curve is influenced by the signatures o f old upper conlinental crust to a larger degree than by those of juvenile components of mantle origin and lower crust. It should be borne in mind that these curves are well constrained by data for young rocks (especially in the age range 0 300 Ma) and, to a much lesser degree, by early
Archaean galenas which indicate that isotopic differences between crust and mantle were small. The other parts of the curves have to be inferred on the basis of small data sets and an assumption of reasonable compositional parameters. As a result, the curves are of limited use as precise dating tools. Their shapes are; nevertheless, quite well constrained and they are very useful reference curves that allow an evaluation of the average Pb isotopic evolution of large crustal segments relative to those of model averages.
3. Experimental techniques Approximately 150 mg of sample powder are dissolved with HF-HNO3. Firstly an open beaker attack followed by pressure dissolution in Teflon ® vessels at 200°C for 3-4 days. This is followed by a HC104 attack and then 6N HCI pressure dissolution. A small aliquot of the sample solution in 0.5N HBr is taken for Pb separation using 0.5N HBr and 6N HCI passes through BioRad AG 1 × 8 anion exchange resin. An additional purification step uses 2N HC1 and 6N HC1 passes through a second anion exchange resin column. For determinations of U and Pb concentrations and Pb isotopic compositions from single dissolutions, aliquots of final solutions are homogenised with a mixed 2°2pb-233Utracer. Separation of Pb is as described previously but now includes an additional 2.5N HC1 clean-up pass through a small cation exchange resin colu m n which we found useful in greatly lowering the a m o u n t of Ba manifested as BaPO2 at mass 201 during mass spectrometer runs and which may have a small interference at mass 202 (Roddick et al., 1987). U is separated and purified on anion exchange resin columns using 7N HNO3 and a 1N H F - 2 N HC1 mix. Pb is run using the SiO2-gel + H3PO4 method at 1250-1300°C and measured on a Finnigan MAT 261 multi-collector machine in the static mode. All measurements are corrected for a 0.0014 a m u - 1 fractionation factor (2tr error
Pb ISOTOPICDISCRIMINATIONOF CRUSTALDOMAINS
115
TABLE 1 Pb isotopic compositions and U and Pb abundances of samples from the Highland Complex, central granulite belt of Sri Lanka Field No.
Rocktype
2°6pb/2°4pb
2°7pb/2°4pb 2°8pb/2°4pb
l
SL8.3
ga-hbl-bio gneiss
2 3 4 5 6 7
SL8.6 SL45 SL57 SL62 SL82 SL98
ga-granulite sil-ga-bio gneiss sil-ga-bio gneiss charnockite ga-bio gneiss charnockite
8 9 10
SL102 SLI08 SL l 10
ga-charnockite ga-granulite ga-charnockite
ll 12 13 14 15 16 17
SL125 SLI37 SL145 SL146 SLI84 SL355 SL366
ga-charnockite charnockite ga-bio gneiss charnockite ga-charnockite ga-granulite bio-opx-ga granulite
18 19 20
SL402 PR22 PR41
ga-charnockite sil-cord-ga-bio gneiss sil-cord-ga-bio gneiss
18.1 l0 18.097 20.574 18.103 20.223 17.856 19.347 17.732 17.763 18.629 18.679 20.159 20.161 17.834 18.163 19.013 18.192 19.766 18.669 25.516 25.488 19.826 18.754 25.689
15.714 15.708 16.098 15.720 15.957 16.127 15.910 15.689 15.716 15.817 15.785 15.969 15.980 15.809 15.807 15.949 16.329 16.075 15.914 16.385 16.379 15.943 15.763 16.672
No. in Fig. 1
38.26 38.10 37.44 38.30 40.68 37.26 39.68 38.94 39.00 39.71 39.40 39.34 39.47 38.07 38.70 38.30 40.08 39.45 38.18 41.63 42.05 39.41 39.38 54.79
U (ppm)
Pb (ppm)
238U/2°4pb
4.01 0.72
39.06 7.01
6.47 6.67
5.87 0.45
33.18 20.54
11.92 1.37
0.14 2.05 2.34
9.55 34.29 28.28
0.93 3.88 5.35
1.48 0.36
27.33 19.64
3.59 1.15
1.11 0.18 0.47
39.82 14.45 ll.53
1.79 0.81 2.69
3.44
3.45
73.2
TABLE 2 Pb isotopic compositions and U and Pb abundances of samples from the Wanni Complex, central granulite belt of Sri Lanka No. in Fig. 1
Field No.
Rocktype
2°6pb/2°4pb
2°7pb/2°4pb
/°SPb/2°4pb
21 22 23 24 25 26 27 28 29 30 31 32
SL1 SL2.1 SL5.1 SL17 SL18 SL29 SL30 SL33.1 SL56 SL68 SL71 SL346
sil-ga-bio gneiss pink feldspar gneiss granite gneiss cord-bio gneiss sil-cord-ga gneiss bio-hbl gneiss charnockite granite gneiss ga-bio gneiss hbl-bio gneiss sil-cord-ga gneiss hbl-bio gneiss
33 34 35 36
SL348 SL417 SL622 SL648
charnockite charnockite bio-migmatite cord-ga gneiss
17.59 18.067 17.875 17.689 17.534 17.827 17.677 16.727 18.603 17.651 17.575 17.435 17.403 18.061 17.490 17.639 17.838
15.562 15.628 15.629 15.607 15.569 15.602 15.618 15.396 15.706 15.609 15.520 15.548 15.534 15.542 15.519 15.560 15.604
38.593 39.718 38.174 39.115 37.61 38.938 39.421 36.533 39.275 38.889 38.756 37.861 37.758 38.687 37.533 38.032 38.691
U (ppm)
Pb (ppm)
238U/2°4pb
1.28 0.43
14.99 22.43
5.5 1.22
1.62 1.31
21.58 13.09
4.65 6.34
!16
is_ 0.00016 amu-1 ) estimated from measurements of the NBS 982 standard. Each dissolution is run twice and the results averaged. The error associated with the estimate of the fractionation factor is the main contributor to the uncertainty of the measurements reported for each dissolution. Separate dissolutions, however, indicate variations beyond this error estimate (duplicate results reported in Tables 1 and 2 are all separate dissolutions) and indicate some degree of sample powder heterogeneity but these variations are still small and not significant for the purposes of the interpretation. U is run using the SiO2-gel+H3PO4 method and measured as U O f using an electron multiplier. Laboratory procedures were relaxed during the processing of these samples because of their high Pb contents. The total processing blank for Pb is typically 300-400 pg and is negligible. 4. Results and discussion
General introductions to the geology of Sri Lanka (e.g. Cooray, 1978; Geological Map of Sri Lanka, 1982; KrSner et al., 1991 ) and discussion of aspects of the metamorphic petrology (e.g. Schumacher et al., 1990; Faulhaber and Raith, 1991; Schenk et al., 1991 ) of the Highland and Wanni granulites relevant for an understanding of the isotope data are found elsewhere and are not repeated here. Most of the rock powders analysed for Pb isotopic compositions in this study have also been analysed for Nd (Milisenda et al., 1988, 1994) and this allows an uncomplicated discussion of the combined P b - N d isotopic characteristics of the rocks within the central granulite belt. Some of the samples analysed can be identified as metapelites and metapsammopelites (e.g. sillimanite-garnet-biotite gneisses and garnet-cordierite-biotite gneisses with varying amounts of quartz and feldspar) although an occasional sample may be an S-type granite. Some samples of biotite _ amphibole + garnet gneisses still preserve deformed phenoerysts
[ < ;it?,~ E i \~.
that allow them to be identified as orthogneisses, but a metasedimentary or meta-igneous parentage is unclear in many cases because of the high strain suffered by these rocks. Samples identified as charnockites are gneisses of granite-granodiorite-tonalite compositions that possess a very distinctive grey-green lustre in hand specimen, but which may or may not contain orthopyroxene +, clinopyroxene + garnet ___ amphibole + biotite. Their parentage is also uncertain, although where these charnockites have been bleached or in some localities where a network of grey-green charnockites are observed to infiltrate grey gneisses (these units are sometimes described as "arrested in-situ charnockites" ), the protoliths can be identified as orthogneisses. Seventeen samples from both the Highland Complex and Wanni Complex were analysed for U and Pb concentrations (Tables 1 and 2 ). Of these, only two show/t values higher than the estimate for primitive mantle ( ~ 8.5); all the rest (irrespective of rock type ) have the low /t values generally assumed to be characteristic of granulites. Some samples display very low/~ values ( ~ 1 or lower). Of the two high # sampies, one (SL366, an unusual quartz- and feldspar-poor biotite-orthopyroxene-garnet granulite) is clearly anomalous (/z=73! and correspondingly highly radiogenic Pb isotope ratios), but this appears to be a result of very low Pb content and perhaps non-removal of U rather than U enrichment. Sample SL57 (a garnetiferous charnockite) has a It value ( = 12 ) typical of upper crustal rocks and this may be a relict protolith signature rather than a result of preferential U enrichment. 2°7pb/2°apb-2°6pb/2°4pb compositions of the Sri Lankan high-grade rocks are compared with those of modern pelagic sediments (their Pb isotopic compositions are used here to approximate that of average present-day upper crust) and galenas from Phanerozoic stratabound ore deposits (these provide crude estimates of average upper crustal signatures during the Mesozoic and Palaeozoic), as well as
Pb ISOTOPIC
DISCRIMINATION
OF
CRUSTAL
1 17
DOMAINS
the orogeny and upper crust plumbotectonic curves in Fig. 3. Samples from the Highland Complex can be clearly distinguished from those of the Wanni Complex by their unusually high 2°Tpb/2°4pb for a particular 2°6pb/ 2°4pb value on this plot. Irrespective of any interpretation of the Pb isotopic systematics, this finding confirms the inference of the earlier Nd study that these two complexes are isotopically distinct entities. Only one sample (SL56, number 29 in Fig. 1 ) can be regarded as slightly ambiguous. SL56 is located very near the "isotopic break" and has an intermediate 16.4 [
n
Ifi 2 o o
~
160
~'~
15 8 ~" 600 M a reference line
o
0
15.6
8
• +
0
OR "
18.5
+ m o d e m sediments • Phanerozoic galenas O Highland C o m p l e x
19.5
'
'
'
"
'
'
"
'
" E '
"
'
("
D Wanni Complex eO 16.0
O Highland Complex e Highland initials
~-~ 15.5
~
I Ga
~
a
B
Oa
o~
m0 15
O O
PM
2 Ga 15.0 A5 Ga
145 t~
14
15
~6
17
/8
it~
21)
21
206Pb/204Pb
Fig. 4. Plot of measured 2°7pb/2°4pb vs. 2°6pb/z°4pb compositions of metamorphic rocks of the Highland Complex and Wanni Complex, and initial compositions (corrected to 600 Ma ago) for the Highland Complex. The growth curve plotted is that of the primitive mantle (PM) assumi n g # = 8.5 and To=4.51 Ga. Ticks correspond to 250 Ma intervals. The lines marked ABC and ADE represent the 3000 Ma-600 Ma and 3000 Ma-2000 Ma isochrons, respectively, discussed in Fig. 2 and in the text.
o
....................
PM
17.5
0
Oo
.......................
15.4
152 16.5
O
o
o"
16.5
205
206Pb/204Pb
Fig. 3. Plot of 2°Tpb/2°4pb vs. 2°6pb/2°4pb compositions of metamorphic rocks of the Highland Complex and Wanni Complex, modern pelagic sediments and galenas from Phanerozoic sedimentary- or felsic volcanic-hosted massive sulphide deposits. Also included in the Wanni population are the data for arrested "in-situ" charnockites and orthogneisses of the Kurunegala area of Burton and O'Nions (1990). Two Highland samples with 2°6pb/ 2°4pb > 25 have not been included in the plot. Note clear separation of the Highland samples from those of the Wanni Complex as a result of high 2°7pb/z°4Pb ratios of the former. The plumbotectonic curves for average upper crust (UC) and orogeny (OR) (see text for discussion) used in this figure are those of model II of Zartman and Doe ( 1981 ). Ticks along these curves denote 400 Ma time intervals. PM denotes the reference primitive mantle curve as used in Fig. 2. Data for modern pelagic sediments are taken from Ben Othman et al. (1989). Data for Phanerozoic galenas represent a selection from various sources, but primarily Wedepohl et al. ( 1978 ), Doe and Zartman ( 1979 ), Godwin and Sinclair (1982), Sundblad and Stephens ( 1983 ), MacFarlane et al. (1990) and Gunnesch et al. (1990). Uncertainties associated with the measurements for individual sample dissolutions are smaller than the symbols used to plot Highland and Wanni samples in the figure.
Nd model age of 1.8 Ga. Although the point for this sample plots close to the field for Highland samples in Fig. 3, it actually lies on an extension of the Pb isotopic growth band for the Wanni population. This is what would be expected for a Wanni sample that evolved with high U / P b (relative to other Wanni samples) prior to metamorphism. We tentatively interpret SL56 to be a Wanni unit but acknowledge that this interpretation is not unambiguous and should be resolved by future U - P b zircon work. We have so far not discussed initial ratios. This is so for two reasons: the shallow slope of a 600 Ma reference isochron as shown in Fig. 3 and the limited magnitude of the age corrections (with the exception of SL366) for samples that were analysed for U and Pb abundances means that the spatial separation of the two populations remains when initial ratios (at the time of metamorphism) are considered (see Fig. 4).
4.1. Highland Complex The Highland samples plot subparallel to and above the field of m o d e m pelagic sediments and Phanerozoic stratabound galenas. Two
samples (SL62 and SL146 ), however, plot well above and to the left of the "main population" in Fig. 3 and a discussion of these two samples is deferred till the end of this section. For any given 2°6pb/2°apb value, the Highland samples have 2°7pb/2°apb values that are 0.2 or higher relative to the orogenic plumbotectonic curve. The high 2°Tpb/2°4pb signatures of the "main population" (i.e. excluding SL62 and SL 146 ) relative to the two plumbotectonic curves indicate that the protoliths of the Highland granulites were isolated from plumbotectonic mixing and homogenisation early in their evolution history, since a long isolation interval is required to produce these anomalous and elevated present-day Z°Tpb/2°4pb ratios. This time interval is difficult to estimate precisely since it is sensitive to both the # history and the choice of the orogenic or upper crustal curve, but it exceeds 1 Ga (prior to the 600 Ma metam o r p h i s m ) even for high p values in the range 12-15. Strictly speaking, this conclusion applies only to the protoliths of metasediments and S-type orthogneisses, but there is no systematic isotopic difference between the Highland metasediments and orthogneisses, and many Phanerozoic felsic igneous rocks do lie on the plumbotectonic curves at approximately the appropriate positions, indicating that the characteristics of these curves have wider applicability than just to sediments. The isotopic characteristics of the Highland granulites are assessed with respect to a 3-stage Pb evolution model (as explained in Fig. 2 ) in Fig. 4. Measured ratios of the samples plot on or to the left of the 3 Ga-0.6 Ga isochron, and almost all samples plot to the left of a 2.5 G a 0.6 Ga isochron (not shown). This means that the crustal residence history prior to the 600 Ma m e t a m o r p h i s m is quite complicated. It requires both a lowering of p to produce the retardation shift to the left of the reference isochron, as well as a large increase of/t to produce high 2°Tpb/z°4Pb values. The relative timing of the/~ increase and duration of this high # history that affected all rock types would be use-
ful pieces of information, but they are not sufficiently well constrained for our purposes by the data for the "main population" samples and we are forced to search for alternative solutions. An examination of the two anomalous samples (SL62 and SL 146 ) lying above the "main population" indicate that they do allow surprisingly tight constraints. These two samples are rather ordinary-looking charnockites but their protolith lithologies are unknown. They have very low measured # values (1.37 and 0.81, respectively). Their 2°6pb/2°4pb ratios are exceptionally unradiogenic relative to their high 2°7pb/2°4pb values and such combinations of values can only be explained if the isotopic evolution of these samples were "frozen" very early in their history, but this was preceded by an even earlier, very high # stage. This sequence of stages is amenable to simple modelling as it allows us to make the assumption that isotopic evolution after crust formation can be approximated by just a single, early high-/z stage. A single-stage evolution must intersect a closed-system (in contrast to the opensystem plumbotectonic curves) growth curve at two positions--in the specific case of our approximation, one corresponds to the time when juvenile crust was formed, the other when Pb isotopic evolution was "frozen" as the result of a drastic reduction of/t. Even if we use Nd model age data as the crudest approximations of the time of juvenile crust formation (the Nd model age of SL62 is 2.7 Ga), the slopes of the Pb growth curves in the 2.5-3.2 Ga interval, as well as the convergence of crust and mantle growth curves in Archaean times actually constrain the starting isotopic compositions to a very small region of the 2°7pb/z°4pb-2°6pb/ 2°4pb diagram. An isochron originating from compositions in the ~ 3 Ga portion and passing through the points for SL62 and SL 146 always produce a second intersection of the closed-system growth curve at the ~ 2 Ga position (see Fig. 4 ). This means that 2 Ga is an approximate m i n i m u m estimate of the time
Pb ISOTOPIC DISCRIMINATION OF CRUSTAL DOMAINS
the areas around Dodangaslanda and Kandy. In: A. KrSner (Editor), The Crystalline Crust of Sri Lanka. Geol. Surv. Sri Lanka, Prof. Pap., 5: 69-88. Kriegsman, L.M., 1994. Evidence for a fold nappe in the high-grade basement of central Sri Lanka: terrane assembly in the Pan-African lower crust? In: M. Raith and S. Hoernes (Editors), Tectonic, Metamorphic and Isotopic Evolution of Deep Crustal Rocks, With Special Emphasis on Sri Lanka. Precambrian Res., 66: 5976 (this volume). Kr6ner, A., Cooray, P.G. and Vitanage, P.W., 1991. Lithotectonic subdivision of the Precambrian basement in Sri Lanka. In: A. Kr6ner (Editor), The Crystalline Crust of Sri Lanka. Geol. Surv. Sri Lanka, Prof. Pap., 5: 5-21. KriSner, A., Jaeckel, P. and Williams, I.S., 1994. Pb-loss patterns in zircons from a high-grade metamorphic terrain as revealed by different dating methods: U - P b and P b - P b ages for igneous and metamorphic zircons from northern Sri Lanka. In: M. Raith and S. Hoernes (Editors), Tectonic, Metamorphic and Isotopic Evolution of Deep Crustal Rocks, With Special Emphasis on Sri Lanka. Precambrian Res., 66:151-181 (this volume). Liew, T.C. and Hofmann, A.W., in prep. Bulk Earth T h U-Pb, the early Archean Pb isotopic record, and implications for evolution of the mantle sources of MORBs. Liew, T.C., Milisenda, C.C. and Hofmann, A.W., 1991. Isotopic contrasts, chronology of elemental transfers and high-grade metamorphism: the central Sri Lankan granulites, and the Lewisian (Scotland) and Nuk (SW Greenland) gneisses. Geol. Rundsch., 80: 279-288. MacFarlane, A.W., Marcet, P., LeHuray, A.P. and Petersen, U., 1990. Lead isotope provinces of the central Andes inferred from ores and crustal rocks. Econ. Geol., 85: 1857-1880. Milisenda, C.C., Liew, T.C., Hofmann, A.W. and Kr6ner, A., 1988. Isotopic mapping of age provinces in Precambrian high-grade terrains: Sri Lanka. Jour. Geol., 96:608-615. Milisenda, C.C., Liew, T.C., Hofmann, A.W. and Ktihler, H., 1994. Nd isotopic mapping of the Sri Lanka basement: update, and additional constraints from Sr iso-
121 topes. In: M. Raith and S. Hoernes (Editors), Tectonic, Metamorphic and Isotopic Evolution of Deep Crustal Rocks, With Special Emphasis on Sri Lanka. Precambrian Res., 66: 95-110 (this volume). Moorbath, S., Welke, H. and Gale, N.H., 1969. The significance of lead isotope studies in ancient, high-grade metamorphic basement complexes, as exemplified by the Lewisian rocks of northwest Scotland. Earth Planet. Sci. Lett., 6: 245-256. Roddick, J.C., Loveridge, W.D. and Parrish, R.R., 1987. Precise U / P b dating of zircon at the sub-nanogram Pb level. Chem. Geol. (Isotope Geosci. Sect.), 6 6 : 1 1 1 121. Schenk, V., Raase, P. and Schumacher, R., 1991. Metamorphic zonation and P - T history of the Highland Complex in Sri Lanka. In: A. Kr6ner (Editor), The Crystalline Crust of Sri Lanka, Part 1. Geol. Surv. Sri Lanka, Prof. Pap., 5:150-163. Schumacher, R., Schenk, V., Raase, P. and Vitanage, P.W., 1990. Granulite facies metamorphism ofmetabasic and intermediate rocks in the Highland Series of Sri Lanka. In: M. Brown and J.R. Ashworth (Editors), HighGrade Metamorphism and Crustal Anatexis. Allen and Unwin, London, pp. 235-271. Sundblad, K. and Stephens, M.B., 1983. Lead isotope systematics of strata-bound sulfide deposits in the higher nappe complexes of the Swedish Caledonides. Econ. Geol., 78:1090-1107. Voll, G. and Kleinschrodt, R., 1991. Sri Lanka: Structural, magmatic and metamorphic development of a Gondwana fragment. In: A. KrOner (Editor), The Crystalline Crust of Sri Lanka. Geol. Surv. Sri Lanka, Prof. Pap., 5: 22-51. Wedepohl, K.H., Delevaux, M.H. and Doe, B.R., 1978. The potential source of lead in the Permian Kupferschiefer bed of Europe and some selected Paleozoic mineral deposits in the Federal Republic of Germany. Contrib. Mineral. Petrol., 65:273-281. Zartman, R.E. and Doe, B.R., 1981. Plumbotectonics-the model. Tectonophysics, 75:135-162. Zartman, R.E. and Haines, S.M., 1988. The plumbotectonic model for Pb isotopic systematics among major terrestrial reservoirs--a case for bi-directional transport. Geochim. Cosmochim. Acta, 52: 1327-1339.
1 19
Pb ISOTOPIC DISCRIMINATION OF CRUSTAL DOMAINS
when /~ values were drastically reduced because we have ignored post-2 Ga growth. We currently do not know what process caused the extreme reduction of the # values of SL62 and SL146 some 2 Ga ago, but feel confident this is not related to regional granulite-grade metamorphism, which must be young. The extreme measured # and Pb isotopic compositions of sample SL366 actually set a useful upper limit for the age of m e t a m o r p h i s m - - i t s 2°6pb/2°apb and 2°7pb/2°apb initials are similar to those of other Highland samples if 600 Ma is used for age correction, but these initials shift very rapidly and yield highly implausible compositions (2°6pb/2°apb= 14.5; 2°7pb/2°apb= 15.62, see Fig. 4) if, for example, 900 Ma is used for age correction. The inference that the extreme high-/t history of SL62 and SL146 is pre-2 Ga is important. If this is a result of upper crustal processing, as is likely, then we can take the inference a step further to argue that the high-/t history required to produce the elevated 2°Vpb/ 2°4pb ratios of other Highland lithologies is pre2 Ga, and that the time when the signatures of samples SL62 and SL146 were "frozen" corresponds directly to the time when the pregranulite Highland terrain was isolated from plumbotectonic mixing and exchange. We consider the most plausible interpretation of this isolation age to be the time when the crustal package was "cratonised" (taken to loosely imply that it was shielded from younger orogenic activity, erosion, mixing and homogenisation) without being metamorphosed to granulite-grade until ~ 600 Ma ago.
4.2. Wanni Complex Like the case for the Highland Complex, results for the Wanni Complex identify a "main population" and two deviants. The "main population" shows very limited dispersion of isotopic ratios in spite of the wide range of rock types analysed. This group of samples straddle the orogenic and upper crustal plumbotectonic curves, and occupy a compositional space just
slightly to the left (i.e. less radiogenic) side of the field for modern sediments and Phanerozoic stratabound galenas. Interpretation of the Wanni signatures appears to be relatively straightforward. The Pb data, when analysed jointly with previous Nd model age and a handful of U - P b zircon ages, indicate that the Wanni Complex was a former mid-Neoproterozoic crustal package that evolved with normal (according to plumbotectonic models) orogeny-upper crustal characteristics until ~ 600 Ma ago when Pb isotopic evolution was retarded as a consequence of high-grade metamorphism. Sample SL56 plots slightly above the plumbotectonic curve for the upper crust and its Pb isotopic composition can be tentatively interpreted as that of a Wanni sample that evolved with unusually high U / P b until the ~ 600 Ma m e t a m o r p h i s m caused a moderate retardation. In contrast, sample SL33.1 has unradiogenic 2°7pb/2°4pb and 2°6pb/2°apb, and it lies well below the orogenic plumbotectonic curve. It must have evolved with low/z well before 600 Ma. We interpret it to represent juvenile crust formed in Mesoproterozoic times that has evolved since then with low/t but we do not consider this low/t signature as indicating an older, regional high-grade metamorphic event. 5. Conclusions
Pb isotope data confirm the earlier recognition of two isotopically distinct crustal domains within the central granulite belt of Sri Lanka. The Pb isotope data for granulites of the Highland Complex indicate that their premetamorphic protoliths were removed from plumbotectonic exchange more than 1 Ga prior to the ~ 600 Ma metamorphism. Two samples that have preserved unusually radiogenic 2°Tpb/2°apb ratios indirectly suggest that this isolation age is as old as 2 Ga in some cases. A plausible geological interpretation of this isolation age is that it corresponds to the time of "cratonisation" or incorporation of the pre-
111
Elsevier Science B.V., Amsterdam
Pb isotopic discrimination of crustal domains within the highgrade basement of Sri Lanka T.C. Liew*, C.C. Milisenda and A.W. Hofmann Max-Planck Institut J~r Chemie, Postfach 3060, 6500 Mainz, Germany Received February 17, 1992; revised version accepted January 5, 1993
ABSTRACT Pb isotope data confirm the recognition of two isotopically distinct crustal domains, originally proposed on the basis of Nd model age data, within the central belt of granulite-grade rocks in Sri Lanka. One domain, the Highland Complex, comprise rocks that have late Archaean-Palaeoproterozoic Nd model ages and unusually high 2°7pb/2°4Pbcompositions. They plot well above the 2°Tpb/2°apb-2°rpb/2°"Pbplumbotectonic growth curves for average upper crust and orogeny of Zanman and Doe ( 1981 ), and have been isolated from plumbotectonic exchange more than 1 Ga prior to metamorphism ~ 600 Ma ago. Two Highland samples suggest that, in some cases, this isolation occurred some 2 Ga ago or earlier. This isolation age is considered to date the time the pre-granulite Highland terrain was "cratonised", i.e. incorporated into a stable shield environment. Pb isotopic compositions of rocks from the second domain, the Wanni Complex, straddle the upper crust and orogeny plumbotectonic curves. In contrast to the Highland granulites, there is no evidence for an extended plumbotectonic isolation period prior to metamorphism. The available U - P b zircon, Nd and Pb isotope data indicate that the Wanni Complex represents a mid-Neoproterozoic crustal package (0.7 Ga < T< 1.3 Ga ) metamorphosed some 600 Ma ago. A time gap exceeding 0.5 Ga separates the age of "cratonisation" ( ~ 1.8-2.0 Ga) of the pre-granulite Highland terrain from the times of formation of the earliest Wanni protoliths ( < 1.3 Ga). The distinctive isotopic characteristics and the unrelated geological evolution reconstructed from these data indicate that the two now-adjacent Highland and Wanni complexes were geologically unrelated crustal packages prior to high-grade metamorphism and, as such, represent "suspect terranes" brought together during a collision event.
1. Introduction
Milisenda et al. (1988) demonstrated that the distribution of Nd model ages of rocks from the central belt of granulite-grade rocks (Fig. 1 ) in Sri Lanka forms a relatively simple pattern consisting of an eastern part having old model ages (2-3 Ga) and a western part having younger model ages (1-2 Ga). The isotopic differences are large and there is minimal overlap of the two groups on a histogram plot (Milisenda et al., 1994). The exact geological significance of these isotopic differences is still *Corresponding author. Present address: Dept. of Nuclear Physics, University of Oxford, Oxford OX1 3RH, U.K. (Reprint requests to Mainz address. )
SSDIO301-9268(93)EO029-C
being debated, but Milisenda et al. (1988) suggested that the two distinct isotopic domains may be separated by a major structural/ tectonic break. These results, together with those of subsequent U - P b zircon dating work (Baur et al., 1991; H/51zl et al., 1994; KriSner et al., 1994) and petrological studies (Schumacher et al., 1990; Faulhaber and Raith, 1991 ) led Kr/Sner et al. (1991) to propose a two-fold division of the central granulite belt in Sri Lanka. The Wanni Complex comprises rocks that have young Nd model ages and young zircon ages (0.7 G a < T< 1.3 Ga) and was metamorphosed at low to medium pressures ~ 550-600 Ma ago. It can be differentiated from the Highland Complex which contains rocks having the older Nd model ages and
]C FLI~Lkl~ S, c h a l n c l t kig~l
9
.
7°.
Color
50
6 °--
Fig. 1. Simplified geologicalmap of Sri Lanka (modified from Geological Map ofSri Lanka, 1982) showing distribution of the central granulite belt and the inferred division into the Highland Complexand Wanni Complexafter Milisendaet al. ( 1988). Sample numbers followthat listed in Tables 1 and 2. The location of the "boundary" is only a crude approximation inferred from the previous Nd "model age mapping" work. There have since been some indications from petrological studies that this "boundary" may coincide with a break in metamorphic pressure gradient in the central portion of the island (M. Raith, pers. commun, based on examination of data of Schureacher et al., 1990, and Schenk et al., 1991 ), and with a zone of anomalously strong strain gradient (Kriegsman, 1994). The exact nature and regional distribution of a possible tectonic boundary between the two isotopically distinct crustal domains remain to be worked out. old zircon ages ( > 1.8 G a ) and which was m e t a m o r p h o s e d at m e d i u m to high pressures ~ 550-600 M a ago. This nomenclature is followed in this paper. The two-fold division m a y not prove to be adequate in future, and additional ( m o r e restrictive) lithotectonic subdivisions o f the central granulite belt m a d e on the basis o f rock associations m a y become desira-
ble. Such additional lithological subdivisions of the two isotopic domains will not alter the main conclusions reached in this paper in which we use Pb isotope data (together with earlier Nd model age information ) to argue that the nowadjacent Wanni and Highland complexes tor future subdivisions thereof) have had unrelated p r e m e t a m o r p h i c histories. The methodology adopted illustrates how isotope data facilitate the recognition of distinct crustal packages juxtaposed in complex terrains and m a y have general application in the field of "terrane analysis". An earlier publication (Liew et al., 1991 ) used then unpublished data for the Highland Complex in a comparative study of the Highland, Lewisian (Scotland) and N u k (SW Greenland) high-grade terrains, and discussed the relevance of their Pb isotope compositions for understanding the global Pb mass balance. This paper will draw on some of the conclusions arrived at in that earlier study without repeating the arguments. 2. Pb isotopes as tracers of crustal evolution The U - P b system, in contrast to the S m - N d system, is often severely fractionated during high-grade metamorphism. Severe depletions o f Th and U relative to Pb in high-grade felsic rocks have been well d o c u m e n t e d by m a n y early studies (e.g. Heier, 1973) and the timeintegrated effects o f these depletions can be studied using Pb isotopes (e.g. Moorbath et al., 1969 ). Studies involving a combination of N d and Pb isotope work are especially interesting since Pb isotopes provide information related to the chronology of intracrustal differentiation associated with high-grade metamorphism, whereas the S m - N d system yields important information concerning times of derivation o f juvenile crust from the mantle. Two classes o f Pb isotope evolution systematics are o f particular relevance to the discussion in this paper: ( 1 ) multi-stage, closed-system Pb evolution and (2) complex open-system be-
Pb ISOTOPIC
DISCRIMINATION
OF CRUSTAL
113
DOMAINS
haviour as described by the "plumbotectonic model" of Zartman and Doe ( 1981 ) and Zartman and Haines (1988 ). The systematics are not as straightforward as those of Sr and Nd isotopes and, because of this, a brief two-part explanation follows.
Ibll ('
E
('
{I 155
I
'
X
Ga
2Ga e-I 150
14.5 3 0 Ga A
2.1. Multi-stage Pb isotope evolution 140
Liew et al. ( 1991 ) argued that the late Archaean protoliths of the Highland granulites had resided for extended intervals of time in a high U / P b upper crustal-like environment prior to high-grade metamorphism ~ 600 Ma ago because their Pb isotope ratios record a long history of high p ( U / P b ratio expressed as 238U/E°4pb) even though the granulites now have low measured U / P b concentration ratios. Fig. 2 and its accompanying extended caption analyses the 2°7pb/z°4pb-E°6pb/2°4pb systematics of a hypothetical multi-stage system (closed with respect to Pb) derived from primitive mantle 3 Ga ago (stage 1 ), attaining crustal U / P b ratios at this time and evolving with such ratios till 600 Ma ago (stage 2). Pb isotopic growth from then on was either "frozen" or severely retarded (stage 3 ). Measured Pb isotopic ratios of the Highland granulites will be examined relative to this simple model in Sect. 4. Fig. 2 also illustrates another aspect of Pb isotope evolution especially relevant for an appreciation of Pb isotopic differences, and this is the retarded evolution of 2°7pb/2°4pb relative to 2°6pb/2°4pb during the last 2 Ga compared to its rapid evolution during the Archaean. Point X in Fig. 2 represents the present-day Pb isotopic composition of a system that "took off" from the primitive mantle (/1= 8.5 ) growth curve 2 Ga ago and evolved with a high crustal ~ of 13 for the last 2 Ga. Note that there is only a small increase of 0.2 in the 2°Tpb/2°4pb ratio compared to the present-day primitive mantle composition despite the large increase in U / P b and long age interval. Differences of such magnitude in post-Ar-
12
13
14
15
/6
17
I~
Itt
2(I
206Pb/204Pb
Fig. 2. Illustration of systematics of Pb isotopic evolution of multi-stage systems (closed with respect to Pb) on a 2°7pb/2°4pb-E°6pb/2°4pb plot. The reference Pb growth curve used in the figure is that representing a primitive mantle evolving with p = 8.5 and To = 4.51 Ga (Liew and Hofmann, in prep.). Ticks mark 250 Ma intervals along the primitive mantle curve. The terms "high" and "low" values, whether time-averaged values calculated from Pb isotopes or present-day measured values, are compared relative to this primitive mantle value of 8.5. The line A B C represents a "3000 Ma-600 Ma isochron" along which all systems evolving from point A with a variety of /~ values until 600 Ma ago will fall, if their p values are effectively "frozen" by a 600 Ma event. Systems that evolved with 1~> 8.5 will lie on the segment B C and those with/~ < 8.5 will lie on the segment AB. There has been very little growth of 2°TpbF°4Pb in the last l Ga because 235U is almost extinct, so any post-600 Ma isotopic growth (i.e. non-zero p values) will merely shift points from their 600 Ma initials sub-horizontally (along a line like C - C ' ) to the right of the line ABC. Points that are located left of this isochron must have experienced an earlier lowering of# values, sometime between 3000 Ma and 600 Ma. The line A D E is a "3000 Ma-2000 Ma isochron". Systems affected by two high-grade events, at 2000 Ma and at 600 Ma, will plot on or close to the line A D E if low ~ values are produced 2000 Ma ago. The effects of further decreases of ~ at T= 600 Ma are practically indiscernible. Note that the slope of line A B C is steeper than that for a 3000 Ma system (i.e. 3000 Ma-0 Ma isochron) and, as a rule, rock systems that have an extended period of crustal residence prior to high-grade metamorphism will always yield Pb-Pb "isochron ages" older than both their true protolith ages and the age of metamorphism. It is for this reason we have not given any weight to linear arrays on a 2°Tpb/E°4pb vs. 2°6pb/E°4pb plot in this study. Point X represents the present-day composition of a system starting from the primitive mantle curve 2 Ga ago that then evolved with a crustal-like p of 13. Despite this extended time-separation of two systems with contrasting/~, an absolute 2°7pb/2°4pb difference of only 0.2 is produced and this demonstrates the importance of small 2°Tpb#°4pb differences in post-2 Ga rocks.
chaean crustal rocks therefore imply long periods of residence in systems with very different U / P b ratios.
2.2. Open-system Pb evolution and plumbotectonics Zartman and Doe ( 1981 ) and Zartman and Haines ( 1988 ) argue that the Pb isotopic evolution of the upper mantle and continental crust cannot be viewed as the product of longterm closed-system evolution. They devised a theoretical model to describe the Pb isotopic evolution of the crust and upper mantle which assumes that semi-continuous, bi-directional exchange takes place between the various terrestrial silicate reservoirs. The basis for this "plumbotectonic" model for Pb evolution is the mechanism of plate tectonics. It involves the constant subduction of oceanic crust ( + incorporated sediments) into the convecting mantle that is continuously tapped to produce juvenile crust which is then eroded to restart the cycle. In fact, workers like Armstrong ( 1981 ) have, for a long time, independently championed the case for crustal recycling in order to explain the combined isotopic and chemical record of the Earth. Theoretical growth curves have been constructed for average upper continental crust, lower continental crust, oceanic mantle and orogeny (where rapid mixing and homogenisation of components from contrasting reservoirs are envisaged during pulses of "orogenies"). The orogenic curve is actually quite similar to the conformable ore Pb curve (e.g. C u m m i n g and Richards, 1975 ). It is considered to represent the average erosional record of large segments of the exposed continental crust. The shape of the curve is influenced by the signatures o f old upper conlinental crust to a larger degree than by those of juvenile components of mantle origin and lower crust. It should be borne in mind that these curves are well constrained by data for young rocks (especially in the age range 0 300 Ma) and, to a much lesser degree, by early
Archaean galenas which indicate that isotopic differences between crust and mantle were small. The other parts of the curves have to be inferred on the basis of small data sets and an assumption of reasonable compositional parameters. As a result, the curves are of limited use as precise dating tools. Their shapes are; nevertheless, quite well constrained and they are very useful reference curves that allow an evaluation of the average Pb isotopic evolution of large crustal segments relative to those of model averages.
3. Experimental techniques Approximately 150 mg of sample powder are dissolved with HF-HNO3. Firstly an open beaker attack followed by pressure dissolution in Teflon ® vessels at 200°C for 3-4 days. This is followed by a HC104 attack and then 6N HCI pressure dissolution. A small aliquot of the sample solution in 0.5N HBr is taken for Pb separation using 0.5N HBr and 6N HCI passes through BioRad AG 1 × 8 anion exchange resin. An additional purification step uses 2N HC1 and 6N HC1 passes through a second anion exchange resin column. For determinations of U and Pb concentrations and Pb isotopic compositions from single dissolutions, aliquots of final solutions are homogenised with a mixed 2°2pb-233Utracer. Separation of Pb is as described previously but now includes an additional 2.5N HC1 clean-up pass through a small cation exchange resin colu m n which we found useful in greatly lowering the a m o u n t of Ba manifested as BaPO2 at mass 201 during mass spectrometer runs and which may have a small interference at mass 202 (Roddick et al., 1987). U is separated and purified on anion exchange resin columns using 7N HNO3 and a 1N H F - 2 N HC1 mix. Pb is run using the SiO2-gel + H3PO4 method at 1250-1300°C and measured on a Finnigan MAT 261 multi-collector machine in the static mode. All measurements are corrected for a 0.0014 a m u - 1 fractionation factor (2tr error
Pb ISOTOPICDISCRIMINATIONOF CRUSTALDOMAINS
115
TABLE 1 Pb isotopic compositions and U and Pb abundances of samples from the Highland Complex, central granulite belt of Sri Lanka Field No.
Rocktype
2°6pb/2°4pb
2°7pb/2°4pb 2°8pb/2°4pb
l
SL8.3
ga-hbl-bio gneiss
2 3 4 5 6 7
SL8.6 SL45 SL57 SL62 SL82 SL98
ga-granulite sil-ga-bio gneiss sil-ga-bio gneiss charnockite ga-bio gneiss charnockite
8 9 10
SL102 SLI08 SL l 10
ga-charnockite ga-granulite ga-charnockite
ll 12 13 14 15 16 17
SL125 SLI37 SL145 SL146 SLI84 SL355 SL366
ga-charnockite charnockite ga-bio gneiss charnockite ga-charnockite ga-granulite bio-opx-ga granulite
18 19 20
SL402 PR22 PR41
ga-charnockite sil-cord-ga-bio gneiss sil-cord-ga-bio gneiss
18.1 l0 18.097 20.574 18.103 20.223 17.856 19.347 17.732 17.763 18.629 18.679 20.159 20.161 17.834 18.163 19.013 18.192 19.766 18.669 25.516 25.488 19.826 18.754 25.689
15.714 15.708 16.098 15.720 15.957 16.127 15.910 15.689 15.716 15.817 15.785 15.969 15.980 15.809 15.807 15.949 16.329 16.075 15.914 16.385 16.379 15.943 15.763 16.672
No. in Fig. 1
38.26 38.10 37.44 38.30 40.68 37.26 39.68 38.94 39.00 39.71 39.40 39.34 39.47 38.07 38.70 38.30 40.08 39.45 38.18 41.63 42.05 39.41 39.38 54.79
U (ppm)
Pb (ppm)
238U/2°4pb
4.01 0.72
39.06 7.01
6.47 6.67
5.87 0.45
33.18 20.54
11.92 1.37
0.14 2.05 2.34
9.55 34.29 28.28
0.93 3.88 5.35
1.48 0.36
27.33 19.64
3.59 1.15
1.11 0.18 0.47
39.82 14.45 ll.53
1.79 0.81 2.69
3.44
3.45
73.2
TABLE 2 Pb isotopic compositions and U and Pb abundances of samples from the Wanni Complex, central granulite belt of Sri Lanka No. in Fig. 1
Field No.
Rocktype
2°6pb/2°4pb
2°7pb/2°4pb
/°SPb/2°4pb
21 22 23 24 25 26 27 28 29 30 31 32
SL1 SL2.1 SL5.1 SL17 SL18 SL29 SL30 SL33.1 SL56 SL68 SL71 SL346
sil-ga-bio gneiss pink feldspar gneiss granite gneiss cord-bio gneiss sil-cord-ga gneiss bio-hbl gneiss charnockite granite gneiss ga-bio gneiss hbl-bio gneiss sil-cord-ga gneiss hbl-bio gneiss
33 34 35 36
SL348 SL417 SL622 SL648
charnockite charnockite bio-migmatite cord-ga gneiss
17.59 18.067 17.875 17.689 17.534 17.827 17.677 16.727 18.603 17.651 17.575 17.435 17.403 18.061 17.490 17.639 17.838
15.562 15.628 15.629 15.607 15.569 15.602 15.618 15.396 15.706 15.609 15.520 15.548 15.534 15.542 15.519 15.560 15.604
38.593 39.718 38.174 39.115 37.61 38.938 39.421 36.533 39.275 38.889 38.756 37.861 37.758 38.687 37.533 38.032 38.691
U (ppm)
Pb (ppm)
238U/2°4pb
1.28 0.43
14.99 22.43
5.5 1.22
1.62 1.31
21.58 13.09
4.65 6.34
!16
is_ 0.00016 amu-1 ) estimated from measurements of the NBS 982 standard. Each dissolution is run twice and the results averaged. The error associated with the estimate of the fractionation factor is the main contributor to the uncertainty of the measurements reported for each dissolution. Separate dissolutions, however, indicate variations beyond this error estimate (duplicate results reported in Tables 1 and 2 are all separate dissolutions) and indicate some degree of sample powder heterogeneity but these variations are still small and not significant for the purposes of the interpretation. U is run using the SiO2-gel+H3PO4 method and measured as U O f using an electron multiplier. Laboratory procedures were relaxed during the processing of these samples because of their high Pb contents. The total processing blank for Pb is typically 300-400 pg and is negligible. 4. Results and discussion
General introductions to the geology of Sri Lanka (e.g. Cooray, 1978; Geological Map of Sri Lanka, 1982; KrSner et al., 1991 ) and discussion of aspects of the metamorphic petrology (e.g. Schumacher et al., 1990; Faulhaber and Raith, 1991; Schenk et al., 1991 ) of the Highland and Wanni granulites relevant for an understanding of the isotope data are found elsewhere and are not repeated here. Most of the rock powders analysed for Pb isotopic compositions in this study have also been analysed for Nd (Milisenda et al., 1988, 1994) and this allows an uncomplicated discussion of the combined P b - N d isotopic characteristics of the rocks within the central granulite belt. Some of the samples analysed can be identified as metapelites and metapsammopelites (e.g. sillimanite-garnet-biotite gneisses and garnet-cordierite-biotite gneisses with varying amounts of quartz and feldspar) although an occasional sample may be an S-type granite. Some samples of biotite _ amphibole + garnet gneisses still preserve deformed phenoerysts
[ < ;it?,~ E i \~.
that allow them to be identified as orthogneisses, but a metasedimentary or meta-igneous parentage is unclear in many cases because of the high strain suffered by these rocks. Samples identified as charnockites are gneisses of granite-granodiorite-tonalite compositions that possess a very distinctive grey-green lustre in hand specimen, but which may or may not contain orthopyroxene +, clinopyroxene + garnet ___ amphibole + biotite. Their parentage is also uncertain, although where these charnockites have been bleached or in some localities where a network of grey-green charnockites are observed to infiltrate grey gneisses (these units are sometimes described as "arrested in-situ charnockites" ), the protoliths can be identified as orthogneisses. Seventeen samples from both the Highland Complex and Wanni Complex were analysed for U and Pb concentrations (Tables 1 and 2 ). Of these, only two show/t values higher than the estimate for primitive mantle ( ~ 8.5); all the rest (irrespective of rock type ) have the low /t values generally assumed to be characteristic of granulites. Some samples display very low/~ values ( ~ 1 or lower). Of the two high # sampies, one (SL366, an unusual quartz- and feldspar-poor biotite-orthopyroxene-garnet granulite) is clearly anomalous (/z=73! and correspondingly highly radiogenic Pb isotope ratios), but this appears to be a result of very low Pb content and perhaps non-removal of U rather than U enrichment. Sample SL57 (a garnetiferous charnockite) has a It value ( = 12 ) typical of upper crustal rocks and this may be a relict protolith signature rather than a result of preferential U enrichment. 2°7pb/2°apb-2°6pb/2°4pb compositions of the Sri Lankan high-grade rocks are compared with those of modern pelagic sediments (their Pb isotopic compositions are used here to approximate that of average present-day upper crust) and galenas from Phanerozoic stratabound ore deposits (these provide crude estimates of average upper crustal signatures during the Mesozoic and Palaeozoic), as well as
Pb ISOTOPIC
DISCRIMINATION
OF
CRUSTAL
1 17
DOMAINS
the orogeny and upper crust plumbotectonic curves in Fig. 3. Samples from the Highland Complex can be clearly distinguished from those of the Wanni Complex by their unusually high 2°Tpb/2°4pb for a particular 2°6pb/ 2°4pb value on this plot. Irrespective of any interpretation of the Pb isotopic systematics, this finding confirms the inference of the earlier Nd study that these two complexes are isotopically distinct entities. Only one sample (SL56, number 29 in Fig. 1 ) can be regarded as slightly ambiguous. SL56 is located very near the "isotopic break" and has an intermediate 16.4 [
n
Ifi 2 o o
~
160
~'~
15 8 ~" 600 M a reference line
o
0
15.6
8
• +
0
OR "
18.5
+ m o d e m sediments • Phanerozoic galenas O Highland C o m p l e x
19.5
'
'
'
"
'
'
"
'
" E '
"
'
("
D Wanni Complex eO 16.0
O Highland Complex e Highland initials
~-~ 15.5
~
I Ga
~
a
B
Oa
o~
m0 15
O O
PM
2 Ga 15.0 A5 Ga
145 t~
14
15
~6
17
/8
it~
21)
21
206Pb/204Pb
Fig. 4. Plot of measured 2°7pb/2°4pb vs. 2°6pb/z°4pb compositions of metamorphic rocks of the Highland Complex and Wanni Complex, and initial compositions (corrected to 600 Ma ago) for the Highland Complex. The growth curve plotted is that of the primitive mantle (PM) assumi n g # = 8.5 and To=4.51 Ga. Ticks correspond to 250 Ma intervals. The lines marked ABC and ADE represent the 3000 Ma-600 Ma and 3000 Ma-2000 Ma isochrons, respectively, discussed in Fig. 2 and in the text.
o
....................
PM
17.5
0
Oo
.......................
15.4
152 16.5
O
o
o"
16.5
205
206Pb/204Pb
Fig. 3. Plot of 2°Tpb/2°4pb vs. 2°6pb/2°4pb compositions of metamorphic rocks of the Highland Complex and Wanni Complex, modern pelagic sediments and galenas from Phanerozoic sedimentary- or felsic volcanic-hosted massive sulphide deposits. Also included in the Wanni population are the data for arrested "in-situ" charnockites and orthogneisses of the Kurunegala area of Burton and O'Nions (1990). Two Highland samples with 2°6pb/ 2°4pb > 25 have not been included in the plot. Note clear separation of the Highland samples from those of the Wanni Complex as a result of high 2°7pb/z°4Pb ratios of the former. The plumbotectonic curves for average upper crust (UC) and orogeny (OR) (see text for discussion) used in this figure are those of model II of Zartman and Doe ( 1981 ). Ticks along these curves denote 400 Ma time intervals. PM denotes the reference primitive mantle curve as used in Fig. 2. Data for modern pelagic sediments are taken from Ben Othman et al. (1989). Data for Phanerozoic galenas represent a selection from various sources, but primarily Wedepohl et al. ( 1978 ), Doe and Zartman ( 1979 ), Godwin and Sinclair (1982), Sundblad and Stephens ( 1983 ), MacFarlane et al. (1990) and Gunnesch et al. (1990). Uncertainties associated with the measurements for individual sample dissolutions are smaller than the symbols used to plot Highland and Wanni samples in the figure.
Nd model age of 1.8 Ga. Although the point for this sample plots close to the field for Highland samples in Fig. 3, it actually lies on an extension of the Pb isotopic growth band for the Wanni population. This is what would be expected for a Wanni sample that evolved with high U / P b (relative to other Wanni samples) prior to metamorphism. We tentatively interpret SL56 to be a Wanni unit but acknowledge that this interpretation is not unambiguous and should be resolved by future U - P b zircon work. We have so far not discussed initial ratios. This is so for two reasons: the shallow slope of a 600 Ma reference isochron as shown in Fig. 3 and the limited magnitude of the age corrections (with the exception of SL366) for samples that were analysed for U and Pb abundances means that the spatial separation of the two populations remains when initial ratios (at the time of metamorphism) are considered (see Fig. 4).
4.1. Highland Complex The Highland samples plot subparallel to and above the field of m o d e m pelagic sediments and Phanerozoic stratabound galenas. Two
samples (SL62 and SL146 ), however, plot well above and to the left of the "main population" in Fig. 3 and a discussion of these two samples is deferred till the end of this section. For any given 2°6pb/2°apb value, the Highland samples have 2°7pb/2°apb values that are 0.2 or higher relative to the orogenic plumbotectonic curve. The high 2°Tpb/2°4pb signatures of the "main population" (i.e. excluding SL62 and SL 146 ) relative to the two plumbotectonic curves indicate that the protoliths of the Highland granulites were isolated from plumbotectonic mixing and homogenisation early in their evolution history, since a long isolation interval is required to produce these anomalous and elevated present-day Z°Tpb/2°4pb ratios. This time interval is difficult to estimate precisely since it is sensitive to both the # history and the choice of the orogenic or upper crustal curve, but it exceeds 1 Ga (prior to the 600 Ma metam o r p h i s m ) even for high p values in the range 12-15. Strictly speaking, this conclusion applies only to the protoliths of metasediments and S-type orthogneisses, but there is no systematic isotopic difference between the Highland metasediments and orthogneisses, and many Phanerozoic felsic igneous rocks do lie on the plumbotectonic curves at approximately the appropriate positions, indicating that the characteristics of these curves have wider applicability than just to sediments. The isotopic characteristics of the Highland granulites are assessed with respect to a 3-stage Pb evolution model (as explained in Fig. 2 ) in Fig. 4. Measured ratios of the samples plot on or to the left of the 3 Ga-0.6 Ga isochron, and almost all samples plot to the left of a 2.5 G a 0.6 Ga isochron (not shown). This means that the crustal residence history prior to the 600 Ma m e t a m o r p h i s m is quite complicated. It requires both a lowering of p to produce the retardation shift to the left of the reference isochron, as well as a large increase of/t to produce high 2°Tpb/z°4Pb values. The relative timing of the/~ increase and duration of this high # history that affected all rock types would be use-
ful pieces of information, but they are not sufficiently well constrained for our purposes by the data for the "main population" samples and we are forced to search for alternative solutions. An examination of the two anomalous samples (SL62 and SL 146 ) lying above the "main population" indicate that they do allow surprisingly tight constraints. These two samples are rather ordinary-looking charnockites but their protolith lithologies are unknown. They have very low measured # values (1.37 and 0.81, respectively). Their 2°6pb/2°4pb ratios are exceptionally unradiogenic relative to their high 2°7pb/2°4pb values and such combinations of values can only be explained if the isotopic evolution of these samples were "frozen" very early in their history, but this was preceded by an even earlier, very high # stage. This sequence of stages is amenable to simple modelling as it allows us to make the assumption that isotopic evolution after crust formation can be approximated by just a single, early high-/z stage. A single-stage evolution must intersect a closed-system (in contrast to the opensystem plumbotectonic curves) growth curve at two positions--in the specific case of our approximation, one corresponds to the time when juvenile crust was formed, the other when Pb isotopic evolution was "frozen" as the result of a drastic reduction of/t. Even if we use Nd model age data as the crudest approximations of the time of juvenile crust formation (the Nd model age of SL62 is 2.7 Ga), the slopes of the Pb growth curves in the 2.5-3.2 Ga interval, as well as the convergence of crust and mantle growth curves in Archaean times actually constrain the starting isotopic compositions to a very small region of the 2°7pb/z°4pb-2°6pb/ 2°4pb diagram. An isochron originating from compositions in the ~ 3 Ga portion and passing through the points for SL62 and SL 146 always produce a second intersection of the closed-system growth curve at the ~ 2 Ga position (see Fig. 4 ). This means that 2 Ga is an approximate m i n i m u m estimate of the time
Pb ISOTOPIC DISCRIMINATION OF CRUSTAL DOMAINS
the areas around Dodangaslanda and Kandy. In: A. KrSner (Editor), The Crystalline Crust of Sri Lanka. Geol. Surv. Sri Lanka, Prof. Pap., 5: 69-88. Kriegsman, L.M., 1994. Evidence for a fold nappe in the high-grade basement of central Sri Lanka: terrane assembly in the Pan-African lower crust? In: M. Raith and S. Hoernes (Editors), Tectonic, Metamorphic and Isotopic Evolution of Deep Crustal Rocks, With Special Emphasis on Sri Lanka. Precambrian Res., 66: 5976 (this volume). Kr6ner, A., Cooray, P.G. and Vitanage, P.W., 1991. Lithotectonic subdivision of the Precambrian basement in Sri Lanka. In: A. Kr6ner (Editor), The Crystalline Crust of Sri Lanka. Geol. Surv. Sri Lanka, Prof. Pap., 5: 5-21. KriSner, A., Jaeckel, P. and Williams, I.S., 1994. Pb-loss patterns in zircons from a high-grade metamorphic terrain as revealed by different dating methods: U - P b and P b - P b ages for igneous and metamorphic zircons from northern Sri Lanka. In: M. Raith and S. Hoernes (Editors), Tectonic, Metamorphic and Isotopic Evolution of Deep Crustal Rocks, With Special Emphasis on Sri Lanka. Precambrian Res., 66:151-181 (this volume). Liew, T.C. and Hofmann, A.W., in prep. Bulk Earth T h U-Pb, the early Archean Pb isotopic record, and implications for evolution of the mantle sources of MORBs. Liew, T.C., Milisenda, C.C. and Hofmann, A.W., 1991. Isotopic contrasts, chronology of elemental transfers and high-grade metamorphism: the central Sri Lankan granulites, and the Lewisian (Scotland) and Nuk (SW Greenland) gneisses. Geol. Rundsch., 80: 279-288. MacFarlane, A.W., Marcet, P., LeHuray, A.P. and Petersen, U., 1990. Lead isotope provinces of the central Andes inferred from ores and crustal rocks. Econ. Geol., 85: 1857-1880. Milisenda, C.C., Liew, T.C., Hofmann, A.W. and Kr6ner, A., 1988. Isotopic mapping of age provinces in Precambrian high-grade terrains: Sri Lanka. Jour. Geol., 96:608-615. Milisenda, C.C., Liew, T.C., Hofmann, A.W. and Ktihler, H., 1994. Nd isotopic mapping of the Sri Lanka basement: update, and additional constraints from Sr iso-
121 topes. In: M. Raith and S. Hoernes (Editors), Tectonic, Metamorphic and Isotopic Evolution of Deep Crustal Rocks, With Special Emphasis on Sri Lanka. Precambrian Res., 66: 95-110 (this volume). Moorbath, S., Welke, H. and Gale, N.H., 1969. The significance of lead isotope studies in ancient, high-grade metamorphic basement complexes, as exemplified by the Lewisian rocks of northwest Scotland. Earth Planet. Sci. Lett., 6: 245-256. Roddick, J.C., Loveridge, W.D. and Parrish, R.R., 1987. Precise U / P b dating of zircon at the sub-nanogram Pb level. Chem. Geol. (Isotope Geosci. Sect.), 6 6 : 1 1 1 121. Schenk, V., Raase, P. and Schumacher, R., 1991. Metamorphic zonation and P - T history of the Highland Complex in Sri Lanka. In: A. Kr6ner (Editor), The Crystalline Crust of Sri Lanka, Part 1. Geol. Surv. Sri Lanka, Prof. Pap., 5:150-163. Schumacher, R., Schenk, V., Raase, P. and Vitanage, P.W., 1990. Granulite facies metamorphism ofmetabasic and intermediate rocks in the Highland Series of Sri Lanka. In: M. Brown and J.R. Ashworth (Editors), HighGrade Metamorphism and Crustal Anatexis. Allen and Unwin, London, pp. 235-271. Sundblad, K. and Stephens, M.B., 1983. Lead isotope systematics of strata-bound sulfide deposits in the higher nappe complexes of the Swedish Caledonides. Econ. Geol., 78:1090-1107. Voll, G. and Kleinschrodt, R., 1991. Sri Lanka: Structural, magmatic and metamorphic development of a Gondwana fragment. In: A. KrOner (Editor), The Crystalline Crust of Sri Lanka. Geol. Surv. Sri Lanka, Prof. Pap., 5: 22-51. Wedepohl, K.H., Delevaux, M.H. and Doe, B.R., 1978. The potential source of lead in the Permian Kupferschiefer bed of Europe and some selected Paleozoic mineral deposits in the Federal Republic of Germany. Contrib. Mineral. Petrol., 65:273-281. Zartman, R.E. and Doe, B.R., 1981. Plumbotectonics-the model. Tectonophysics, 75:135-162. Zartman, R.E. and Haines, S.M., 1988. The plumbotectonic model for Pb isotopic systematics among major terrestrial reservoirs--a case for bi-directional transport. Geochim. Cosmochim. Acta, 52: 1327-1339.
1 19
Pb ISOTOPIC DISCRIMINATION OF CRUSTAL DOMAINS
when /~ values were drastically reduced because we have ignored post-2 Ga growth. We currently do not know what process caused the extreme reduction of the # values of SL62 and SL146 some 2 Ga ago, but feel confident this is not related to regional granulite-grade metamorphism, which must be young. The extreme measured # and Pb isotopic compositions of sample SL366 actually set a useful upper limit for the age of m e t a m o r p h i s m - - i t s 2°6pb/2°apb and 2°7pb/2°apb initials are similar to those of other Highland samples if 600 Ma is used for age correction, but these initials shift very rapidly and yield highly implausible compositions (2°6pb/2°apb= 14.5; 2°7pb/2°apb= 15.62, see Fig. 4) if, for example, 900 Ma is used for age correction. The inference that the extreme high-/t history of SL62 and SL146 is pre-2 Ga is important. If this is a result of upper crustal processing, as is likely, then we can take the inference a step further to argue that the high-/t history required to produce the elevated 2°Vpb/ 2°4pb ratios of other Highland lithologies is pre2 Ga, and that the time when the signatures of samples SL62 and SL146 were "frozen" corresponds directly to the time when the pregranulite Highland terrain was isolated from plumbotectonic mixing and exchange. We consider the most plausible interpretation of this isolation age to be the time when the crustal package was "cratonised" (taken to loosely imply that it was shielded from younger orogenic activity, erosion, mixing and homogenisation) without being metamorphosed to granulite-grade until ~ 600 Ma ago.
4.2. Wanni Complex Like the case for the Highland Complex, results for the Wanni Complex identify a "main population" and two deviants. The "main population" shows very limited dispersion of isotopic ratios in spite of the wide range of rock types analysed. This group of samples straddle the orogenic and upper crustal plumbotectonic curves, and occupy a compositional space just
slightly to the left (i.e. less radiogenic) side of the field for modern sediments and Phanerozoic stratabound galenas. Interpretation of the Wanni signatures appears to be relatively straightforward. The Pb data, when analysed jointly with previous Nd model age and a handful of U - P b zircon ages, indicate that the Wanni Complex was a former mid-Neoproterozoic crustal package that evolved with normal (according to plumbotectonic models) orogeny-upper crustal characteristics until ~ 600 Ma ago when Pb isotopic evolution was retarded as a consequence of high-grade metamorphism. Sample SL56 plots slightly above the plumbotectonic curve for the upper crust and its Pb isotopic composition can be tentatively interpreted as that of a Wanni sample that evolved with unusually high U / P b until the ~ 600 Ma m e t a m o r p h i s m caused a moderate retardation. In contrast, sample SL33.1 has unradiogenic 2°7pb/2°4pb and 2°6pb/2°apb, and it lies well below the orogenic plumbotectonic curve. It must have evolved with low/z well before 600 Ma. We interpret it to represent juvenile crust formed in Mesoproterozoic times that has evolved since then with low/t but we do not consider this low/t signature as indicating an older, regional high-grade metamorphic event. 5. Conclusions
Pb isotope data confirm the earlier recognition of two isotopically distinct crustal domains within the central granulite belt of Sri Lanka. The Pb isotope data for granulites of the Highland Complex indicate that their premetamorphic protoliths were removed from plumbotectonic exchange more than 1 Ga prior to the ~ 600 Ma metamorphism. Two samples that have preserved unusually radiogenic 2°Tpb/2°apb ratios indirectly suggest that this isolation age is as old as 2 Ga in some cases. A plausible geological interpretation of this isolation age is that it corresponds to the time of "cratonisation" or incorporation of the pre-