Trace-element and Nd isotopes in shales as indexes of provenance and crustal growth: The early Paleozoic from the Brabant Massif (Belgium)

Chemical Geology, 57 (1986) 101-115 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

101

TRACE-ELEMENT AND Nd ISOTOPES IN SHALES AS INDEXES OF PROVENANCE AND CRUSTAL GROWTH" THE EARLY PALEOZOIC FROM THE BRABANT MASSIF (BELGIUM) L. A N D R I ~ 1, S. D E U T S C H

1 a n d J. H E R T O G E N

2

~Laboratoires Associds de Gdologie-Pdtrologie-G~ochronologie, Universit~ Libre de BruxeUes, B1050 Bruxelles (Belgium) 2Afdeling Fysico-chemische Geologie, University of Leuven, B-3030 Leuven (Belgium) (Accepted for publication July 4, 1986)

Abstract Andrd, L., Deutsch, S. and Hertogen, J., 1986. Trace-element and Nd isotopes in shales as indexes of provenance and crustal growth: The early Paleozoic from the Brabant Massif {Belgium). In: S. Deutsch and A.W. Hofmann (Editors), Isotopes in G e o l o g y - Picciotto Volume. Chem. Geol., 57: 101-115. The Cambrian- Ordovician shales from the Brabant Massif show an increase in Th (6.7-17,7 ppm) and La (16.6- 71.2 ppm) contents in LREE/HREE ratio (LaN/YbN = 3.8-13.9) and in e~d-Values ( - 3.3 to -- 8.9) with younger age of the sedimentary rocks. This evolution is related to a change in the dominant source rock from basic metavolcanics in the early Cambrian to granite-gneiss-schist rocks in the early Ordovician. The Ta content appears to be controlled by the provenance area as well. The modification in the origin of detritus is not reflected in the abundance of Sc, Cr and V, but it is slightly recorded in the Co and Ni concentrations. e~d responds more sensitively to source changes than Sm/Nd ratios and crustal residence ages and it is confirmed to be a more powerful parameter to detect new addition of mantle material to the crust. The evolution of e~d with stratigraphic age (e~d vs. t) for the Brabant shales parallels the trends for coeval clastic rocks from Brittany (France) and southern Britain ( Michard et al., 1985; Davies et al., 1985) and is considered to be a characteristic of the Armorica plate. The e~d vs. t path of sediments deriviug from crystalline rocks ("first-cycle" sediments) turns out to be a convenient function to delineate the boundaries of fossil lithospheric plates. On the contrary, such first-cycle sediments are not very useful to investigate the long-term continental growth, because their e~-d VS. t path represents a second-order pattern {episodic evolution) that disturbs the first-order trend of the sedimentary Nd-isotope record (secular evolution ).

1. I n t r o d u c t i o n During the past several years geochemical investigations of detrital sedimentary rocks have greatly added to our understanding of crustal evolution. These studies focused either on trace elements, especially Th and rare-earth e l e m e n t s ( R E E ) (e.g., M c L e n n a n et al., 1980;

0009-2541/86/$03.50

B h a t i a a n d T a y l o r , 1981; M c L e n n a n a n d T a y lor, 1982) o r o n N d i s o t o p e s (e.g., O ' N i o n s et al., 1983; All~gre a n d R o u s s e a u , 1984; M i c h a r d et al., 1985; M i l l e r a n d O ' N i o n s , 1985) b e c a u s e t h e s e t r a c e r s are t h o u g h t t o r e f l e c t t h e c o m p o sition of exposed continental crust. Indeed, a l t h o u g h f r a c t i o n a t i o n s o f R E E do o c c u r d u r i n g a l t e r a t i o n ( N e s b i t t , 1979; D u d d y , 1980) in

© 1986 Elsevier Science Publishers B.V.

102

response to a mineralogical control or during sedimentation in function of deposition environment ( Ronov et al., 1974; Dypvik and Brunfelt, 1976), the T h - R E E content of the finegrained fraction of clastic sediments has generally been found to mirror their parental source material (e.g., Cullers et al., 1979; Goldstein et al., 1984; Bhatia, 1985 ). To test this sensitivity of T h - R E E to source rocks, we have measured the Th and REE contents and Nd isotopic composition of polygenic detrital sediments within a very-restricted area: the Caledonian Brabant Massif (Belgium). We have especially studied shales of different ages and mineralogies which have been deposited into various sedimentary environments. A fundamental question in the evolution of the Earth is to define whether the continental crust has been created continuously from the mantle (e.g., Hurley et al., 1962), has been approximately constant in mass since the Archean (e.g., Patterson and Tatsumoto, 1964) or has grown by episodic additions from the mantle (e.g., Moorbath, 1977). Because the sedimentary system is known to respond to changes in Nd isotopic composition of exposed continental crust (McCulloch and Wasserburg, 1978), it should record new additions of mantle-derived material to the continent (O'Nions et al., 1983). This tenet is based o11 three assumptions: (1) The growth of the continental sedimentary mass approximated that of the continental crust itself (Veizer and Jansen, 1979, 1985). (2) The sedimentary mass is rapidly recycled at the surface of the Earth in less than 600 Ma ( ~ 2 0 0 - 5 0 0 Ma; e.g., Veizer and Jansen, 1979) and can continuously reflect the evolution of the average composition of the continental crust. (3) The Nd isotopic composition of continental rocks is primarily a function of the time when they or their crustal precursors were extracted from the upper mantle, because metamorphism, intra-crustal melting and sedimentation appear to have little effect on S m / N d

ratios (e.g., McCulloch and Wasserburg, 1978; Muecke et al., 1979; Rollinson and Windley, 1980; Allen et al., 1985). The second assumption is not fulfilled when the sediment composition is mainly controlled by crystalline parent rocks whilst recycling of older sediments is minimal ("first-cycle" sediments). A gradual denudation could then result in fluctuations of sediment 143Nd/~44Nd ratios according to the nature of eroded rocks. Hence, these Nd-isotope data might not be valid to monitor the crustal growth, but should be useful to determine the sediment provenance. Since the Cambrian-Ordovician detrital sediments from the Brabant Massif derived from various crystalline rocks (Vander Auwera and Andrg, 1985; this paper, see p. 103), they provide an opportunity to examine the effects of progressive erosion on the geochemistry and Nd isotopic patterns of such first-cycle sediments. Taken together with published data concerning sediments from Britanny ( Michard et al., 1985) and southern Britain (Davies et al., 1985), our Nd isotopic measurements yield a detailed picture of Nd isotopic evolution in clastic sediments belonging to the same early Paleozoic lithospheric domain: the Armorica plate.

2. Geology and paleogeography The Brabant Massif is a Caledonian structural unit which extends from the London area to the Meuse Valley. Although a calc-alkaline volcanic activity is documented during the late Ordovician (Andrg and Deutsch, 1984), this massif is almost entirely composed of Cambrian to Silurian detrital sedimentary rocks. They consist of shelf and deep-water sediments of an estimated total thickness of ~ 10 km. We shall limit the geological description to the Cambrian-Ordovician strata considered in the present paper. The stratigraphy of the Ordovician column is based on micropaleontological data (Martin, 1976; Vanguestaine, 1977), but stratigraphic ages of older clastic rocks are mainly con-

103

strained by geometric evidence. The preOrdovician sediments are from the base upwards divided into three lithological subgroups: Blanmont, Tubize and Oisq u e r c q - M o u s t y Subgroups. The B l a n m o n t Subgroup is made up of quartzites; the age is unknown but could be late Precambrian ( Mortelmans, 1977). The Tubize Subgroup and its O l d h a m i a - b e a r i n g sediments must be attributed to the early Cambrian (Mortelmans, 1977 ). It has recently been subdivided into three major lithostratigraphic units: the Rogissart, Fabelta and Forges units (Vander Auwera and Andr6, 1985). The Oisquercq-Mousty Subgroup conformably underlies the Tremadocian strata (Michot, 1977; Vanguestaine, 1977) and is thought to represent the late Cambrian. The petrography of granules and pebbles from the early Cambrian strata shows that the clastics derived from various Precambrian rock types: granites, mica schists, low-grade schists and metavolcanics (Vander Auwera and Andr6, 1985). In the early Cambrian, greywackes are frequent. T h e y are sub-feldspathic to feldspathic with a b u n d a n t epidote, pointing to a derivation mainly from metavolcanics altered in the greenschist facies. Most coarse-grained specimens carry around 10-30% of volcanic fragments, but some of them carry up to 60%. Greywackes are rarer in the early Ordovician; they are sub-quartzose to quartzose in composition with polycrystalline quartz grains and ubiquitous detrital micas. T h e y are associated with a b u n d a n t micaceous shales (e.g., Michot, 1977), which suggests a provenance from crystalline rocks composed of granite, gneiss and schists. Possible sources of the B r a b a n t sedimentary mass must be investigated within the framework of paleogeographic models for Northwest Europe throughout the time span considered. Faunal and paleomagnetic data obtained since the 1970's (e.g., Cocks and Fortey, 1982; Perroud et al., 1984; and references cited therein) suggest that during the early Ordovician, the Precambrian cratonic blocks enclosed within

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Fig. 1. Paleogeographical reconstructions for the interval ~ 500 to ~ 400 Ma ago (after Perroud et al., 1984; Andr6 et al., 1986). During the early Ordovician, Scotland was part of the Laurentia land mass and England was located with southern Ireland, Brittany and Belgium close to Gondwana (rn = Midland craton, b = Brabant craton). Closure of the Iapetus and Medio-European oceans during the Ordovieian and Silurian brought Laurentia, Baltica and Armorica into juxtaposition and gave rise to the Appalachian-British-Norwegian Caledonides and the NorthGerman-Polish Caledonides. The shaded zones represent the Caledonian orogenic belts. the Caledonian fold belts of southern Britain (Midland craton; Watson and Dunning, 1979) and Belgium (Brabant craton; Andr6 and Deutsch, 1984) could be part of the same landmass: the Armorica microplate which included Brittany, Belgium and southern Britain. Recent reconstructions (Fig. 1) indicate that during Ordovician times Armorica detached from Gondwana to join Laurentia and Baltica in response to closure of the Iapetus and MedioEuropean oceans. The setting of the B r a b a n t

104

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turbidites, while VDA115 and VDA120 are siltstones formed in littoral conditions under slow deposition rate (Vander Auwera and Andre, 1985). CL1 is a micaceous shale essentially composed of coarse-grained (50-250 /~m) intergrowths of chlorite-illite. The chlorite-rich shales (VDA188 and VDA211) contain up to 75% chlorite. For the late Cambrian and the early Ordovician, we have concentrated our sampling towards "normal" shales in order to study the stratigraphic variations alone.

4. Analytical techniques I

L_.

([ate Cambrian } Tublze Subgroup ~ { earty Cambrian} B[anmont Subgroup

Terf,ary Ordovician

Fig. 2. Geological sketch map of the central part of the B rabant Massif showing the location of the samples: 1 = VDA6, VDA72; 2 = V D A l l 5 , VDA120, V D A 1 8 8 , VDA21I; 3 = CL1; 4 = CL38; 5 = CL55; 6 = CL29; 7 = SED23; 8 =: SED2I; 9 = S E D I l O ; 10 = SEDI9; 11 = SED13.

Massif at a destructive plate margin along the northern edge of the Armorica plate is suggested by the spatial distribution of the calcalkaline and tholeiitic Caledonian volcanisms from Belgium (Andr~ et al., 1986).

3. Sample selection Fresh samples were collected from quarries or recent outcrops. Sampling has been restricted to the early Cambrian to early Ordovician time span wherein no magmatic activity has been recorded in the area. With the exception of rocks CLI and SED19, care was exercised in selecting sedimentary rocks with grain-size less than 10 /lm. Approximate sample locations are indicated in Fig. 2. Exact locations can be obtained from the first author on request. For the early Cambrian, we have deliberately sampled sediments with different mineralogies and from various depositional environments. VDA6 and VDA72 consist of terms Td (Bouma, 1962) of intermediate to proximal

Trace elements were determined by standard instrumental neutron activation analysis ( INAA ) ( Sc, Co, Cr, Th, U, Ta, Hf, REE) and by X-ray spectrometry techniques (Ni, V, Zr). Nd isotope analytical methods are described in detail in Weis and Deutsch (1984) and only an outline is given here. Samples of ~ 100 mg were decomposed by a mixture of HF, HC1Q and HNO3 in a Teflon ® vessel. The search for undissolved minerals after sample digestion was negative in the case of Cambrian shales. For the Ordovician shales, a black residue of fine organic matter (estimated visually at a few volume percent) remained. Separation of Nd and Sm was performed in two steps: a first elution with 4 N HC1 on a cation-exchange column (Dowex ® 50WX8) concentrates the LREE; a second elution with 0.3 N HC1 on a column prepared with diethylhexylorthophosphoricacid on Teflon ® powder provides a Sm-free Nd fraction. The blank is < 2 ng Nd and is negligible. Isotopic compositions were determined with a Finnigan ® MAT 260 mass spectrometer. About 1 zg Nd was loaded on the side filament of a double Re filament. The Nd ÷ species was run in the sequence: 146, 143, 144 and 145. For this peak sequence, the values obtained for the CaD tech ® n N d fl (22) standard solution are: 143Nd/i44Nd = 0.51200 + 0.00004 (unweighted m e a n _+2(7M), 145Nd/M4Nd : 0.34834 (normalized t o 146Nd/144Nd -- 0.7219), to be compared with 0.51193 and 0.34842 ( Wasserburg et

105 al., 1981). Results for BCR-1 are 143Nd/144Nd = 0.512705 + 0.000026 (_+20"M) to be compared to 0.51264 recommended in the literature. Consequently, all measured 143Nd/144Nd ratios were lowered by an absolute value of 7.10 -5 . The reproducibility tested by duplicates of 8 samples varies between 0.5.10 -5 and 8.10 ~ (average: 3.10 5). Hence, the precision of ~4:~Nd/144Nd is quoted as 5"10 -5, even if the within-run precision is better than 2' 10 -s. Sm and Nd concentrations were determined by INAA with a precision of around 3% on Sm/Nd ratios. Duplicate analyses by INAA and isotope dilution (ID) have previously demonstrated that the difference in Sm/Nd ratios between these methods is generally lower than _+2% ( Andre, 1983 ). 5. R e s u l t s

The trace-element data are listed in Table I. The most remarkable feature is the increase in T h - R E E contents with decreasing stratigraphic age of shales (Fig. 3). All early Cambrian samples (except VDA120, lowermost curve in Fig. 3 ) show similar trace-element patterns in the extended Coryell-Masuda diagram: a moderate depletion of Ta relative to Th and La ( mean Ta/Ta ~"= 0.45 ), moderate light REE (LREE: La-Sm) enrichment coupled to a limited fractionation of heavy REE (HREE: Tb-Lu) with LaN/YbN ratios in the range of 3.8-7.5. All early Ordovician sedimentary rocks exhibit a more pronounced negative Ta anomaly (mean T a / T a ~ ' = 0.32), higher LREE concentrations and higher LaN/YbN ratios (8.9-13.9). The sandstone SED19 contains similar but lower Th, Ta and REE contents than related shales SEDI3 and SEDI10, in accordance with dilution of fine-grained clastics by quartz with low Th, Ta and REE concentrations (Cullers et al., 1979). The trace-element patterns of late Cambrian sediments are generally intermediate between those of the two previous groups: LaN/YbN = 7.2-10.8; mean T a / T a * = 0.38. All shales have the small neg-

ative Eu anomaly which is a worldwide characteristic of most post-Archean sediments (McLennan and Taylor, 1981). U, Hf, Zr, Sc, Cr and V contents vary from sample to sample, but there is no systematic correlation with the age of sediments. The Co and Ni concentrations are lower in Ordovician shales than in their early Cambrian counterparts. The Nd isotope data are listed in Table II. Epsilon values are expressed relative to the present-day chondritic values: 143Nd//144Nd _ 0.51262 and 1478m//144Nd = 0.1966. e~d-Values ( i.e. the conventional epsilon value at the time of sediment deposition) are calculated using absolute age estimates of Odin (1982). The crustal residence ages relative to the depleted mantle (tDM) or bulk Earth (t~E) represent the time at which the sediment (or its precursors ) was extracted from depleted or chondritic parts of the upper mantle, respectively. These model ages are calculated assuming a linear evolution of the mantle and present-day values of 0.51315 (DM) or 0.51262 (BE) for 14:~Nd/144Nd and values of 0.2136 (DM) or 0.1966 (BE) for 1478m/144Nd. The tDM model ages used in this paper are comparable to the estimates of Miller and O'Nions (1984), but are older by some 0.25 Ga in comparison with those of Davies et al. (1985) and Michard et al. (1985). In Fig. 4, e~d-values and tDM-crustal residence ages are plotted against La content. The most striking feature is the decrease of ~d-values with La content of the sediments and their stratigraphic age. tDMages are typically in excess of ~ 1.1-1.4 Ga over stratigraphic ages; this difference is larger in the younger units, but tDMmodel ages are poorly correlated with stratigraphic ages and La content of sedimentary rocks. There is no significant linear relationship between 143Nd/144Nd and 147Sm/144Nd (Fig. 5) in the Brabant shales. The points are rather scattered between the mantle sources and the data from the Rushton Schists and the Rosslare Gneisses, which may represent the metamorphic Proterozoic basement at the south of the Iapetus suture (Thorpe et al., 1984).

13 n.d. 59 22 n.d. n.d. 21.3 46.3 21.1 4.42 0.99 0.72 2.49 0.38 4.27 1.11 1.4 6.7 0.56

21.1 123 91 23.6 59 164 34.0 64.7 30.4 6.3 1.31 0.87 3.01 0.5 4.42 1.24 2.0 12.1 0.36

22.7 128 121 27.7 68 163 24.1 47.3 23.9 4.95 1.09 0.73 2.73 0.44 4.74 1.39 1.9 I0.3 0.50

VDA72 S 16.9 125 93 36.3 95 284 35.8 74.6 35.2 7.00 1.55 1.05 3.63 0.60 7.5 1.66 1.27 12.5 0.48

14.2 119 76 23.6 84 297 31.8 64.5 30.3 5.98 1.30 0.89 3.09 0.51 7.3 1.36 1.17 11.2 0.41

VDA211 Ch 21.1 120 93 18.6 47 154 26.2 61.7 26.4 5.24 1.08 0.78 2.93 0.48 4.25 1.32 1.03 12.3 0.41

VDA115 Si 22.9 129 92 18.2 43 167 16.6 37.9 16.5 3.44 0.70 0.59 2.90 0.48 4.28 1.30 1.12 I1.9 0.45

VDA120 Si 17.6 n.d. 82 4 n.d. n.d. 40.1 84 33 6.7 1.26 0.81 2.8 0.44 4.5 1.5 2.1 13.0 0.39

20.5 n.d. 90 29.6 n.d. n.d. 40.8 72 31.4 5.76 1.14 0.63 2.59 0.42 4.0 1.5 1.6 12.9 0.39

CL29 S 20 n.d. 92 26.7 n.d. n.d. 27.6 57.4 24.6 4.69 0.93 0.59 2.55 0.41 4.04 1.33 1.3 1:].2 0.38

CL38 S

CL55 S

VDA6 S

VDA188 Ch

CLI M

Oisquercq- Mousty Subgroup

Fabelta U n i t

Rogissart U n i t

20.8 n.d. 91 28.3 n.d. n.d. 40.1 84 33 6.7 1.26 0.81 2.8 0.44 4.5 1.5 2.1 13.0 0.31

CL68 S

21.3 145 103 5.50 30 164 48.0 104 47.4 8.66 1.84 1.17 3.51 0.54 4.78 1.68 2.5 15.7 0.37

SED23 S

Ch = chlorite-rich shale; M - micaceous shale; S = shale; S a - sands~ne; Si - siltstone; ( I ) = Villers-la-Ville formation; (2) - Tribotte formation; (3) T a / T a * has been calculated with T a * interpolated between T h and La in the extended Coryell Masuda diagram.

Sc V Cr Co Ni Zr La Ce Nd Sm Eu Tb Yb Lu Hf Ta U Th Ta/Ta*

Sample Rock type

Late Cambrian

Early Cambrian

Trace-element contents ( in ppm) for the sedimentary rocks from the Brabant Massif

TABLE I

21.7 135 117 10.7 34 179 62.6 133 56.8 10.6 2.23 1.37 3.93 0.59 5.53 1.71 2.4 17.6 0.32

SED110 S

Rigen6e formation; n.d.

20.7 138 101 4.56 27 183 50.0 79 46.1 8.75 1.75 1.19 3.58 0.56 5.25 1.82 3.9 15.7 0.39

SED21 S

(1)

19.3 124 106 10.1 36 145 71.5 152 58.2 10.9 2.17 1.33 3.43 0.52 4.26 1.65 2.6 17.7 0.29

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14.4 n.d. 80 6.5 n.d. 238 41.5 91 38.8 7.59 1.73 1.02 3.11 0.47 6.7 1.31 1.8 12.0 0.36

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6. Discussion 6.1. Geochemical trends and composition of source rocks 6.1.1. Th-REE. Since we are not dealing with pure mudstones but with coarser fractions (2-10 #m) which may contain mineral fragments of various origin, we have first to determine whether the stratigraphic evolution in the REE content has sedimentogenetic significance or is merely caused by a modification in the proportions of quartz, feldspars and heavy minerals. The time-related gradual increase in LaN/YbN ratios (Fig. 3 ) precludes that the variation of absolute trace-element abundances is due to dilution of fine-grained clastics with variable amounts of quartz or feldspar. Petro-

graphic investigation of the siltstones-sandstones associated with the shales established that only two REE-bearing heavy minerals might be present in unusual concentrations in the 2-10-#m fraction: zircon and epidote. The absence of HREE fractionation and the rather constant Yb and Hf concentrations in shales (Table I) are at variance with large variations in their zircon content. The similarity of Eu anomalies (Fig. 3) is not consistent with a geochemical control by epidote since REE-rich epidote has a strong negative Eu anomaly (Mahood and Hildreth, 1983; Reed, 1985). The most important factors in determining the REE content of a shale are: REE contents of the source rocks, intensity of weathering in the provenance area, and depositional environment (e.g, Cullers et al., 1979). Our data show that the major controlling factor of the Th-REE patterns of Brabant shales is the nature of the source rocks because: (1) the early Cambrian shales differ greatly in mineralogy and depositional environments, but have similar T h - R E E patterns; and (2) the correlation between the stratigraphic age of shales, their REE content and their e~a-values (Table II; Fig. 4) is difficult to explain by modifications of weathering conditions. The time-related geochemical and isotopic variations can readily be interpreted in terms of mixing between two major types of sources. The sources which have been first eroded during the early Cambrian must have had low Th ( < 6 ppm) and La ( < 1 6 ppm) contents, LaN/YbN < 4 and a rather radiogenic Nd isotopic composition ( ~ d > -- 3 ). The other crustal component which became dominant during early Ordovician must have had high Th ( > 18 ppm) and La ( > 7 2 ppm) contents, LaN/YbN> 14 and a rather unradiogenic Nd isotopic composition (e~4d<-8). The two "end-members" have thus geochemical features which fit into the probable lithology of sources as inferred from petrographical studies. Indeed, the low abundance of T h - L R E E of the

108 T A B L E II Nd isotopic data Sample/rock type* ~

Stratigraphical age .2

CL1/M

Absolute age (Ma)

t47Sm/144Nd

530 530 530 500 500 500 460 460

0.1266 0.1252 0.1260 0.1233 0.1109 0.1104 0.1128 0.1132

E.C. E.C. E.C. L.C. L.C. L.C. Ar-L1 Ar-LI

VDA72/S VDA120/Si CL68/S CL29/S SED23/S SED110/S SED13/S

14SNd/144Nd( *sl x

ftNd

tDM (Ga)

tB~: (Ga)

- 3.3 -6.1 -4.2 -7.6 -7.8 -7.4 8.9 -8.8

1.64 1.88 1.72 1.95 1.79 1.78 1.87 1.86

0.89 1.19 0.99 1.31 1.21 1.16 1.29 1.28

_+2(7M

0.51221 +- 0.00003 0.51206__+0.00002 0.51216+__0.00003 0.51199+_0.00002 0.51194+_0.00003 0.51196_+0.00009 0.51191 +_0.00004 0.51192_+0.00004

* t M = mieaceous shale; S = shale; Si = siltstone. **E.C. = early Cambrian; L.C. = late Cambrian; Ar-L1 = A r e n i g i a n - L l a n v i r n i a n . *:~Normalized to 146Nd/144Nd = 0.7219.

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025 1&7Sm//l&4 Nd

Fig. 5. Plot of 14SNd/144Nd vs. 1478m/144Nd. T h e data for the different rock types are from the following sources: midocean ridge basalts ( O ' N i o n s et al., 1977; Cohen et al., 1980; Cohen a n d O'Nions, 1982; W h i t e a n d H o f m a n n , 1982); ocean island basalts (O'Nions et al., 1977; W h i t e a n d Hofmann, 1982); continental basalts (Carlson et al., 1981; Hart, 1985); R u s h t o n Schists and Rosslare Gneisses (Davies et al., 1985). Symbols as in Fig. 4.

109 former indicates rather mafic sources while the high T h - L R E E content of the latter points to felsic materials. Since a mixture between mafic rocks and crustal components is expected to impose an upward concave curve in the e~d-La diagram (cf. calculated curve in Fig. 4), the hyperbolic relationship between e~d and La in the Brabant shales is consistent with the assumed mixing process. However, the fact that the data points do not clearly define a mixing line in the isochron diagram (Fig. 5) suggests that the Cambrian-Ordovician shales do not represent the different stages of a mixture between two constant end-members. It favours the existence of two sedimentary reservoirs composed of various rock types.

6.1.2. Sc, V, Cr, Co and Ni. McLennan and Taylor (1984) have recently suggested that Co and Sc contents of sediments could mirror the composition of their source. This behaviour is confirmed for Co, but is not verified for Sc. Indeed, although the concentrations of the divalent ferromagnesian trace elements (Co, Ni) from Brabant shales slightly reflect their detrital source, the distributions of Sc, V and Cr are not readily explained by provenance considerations. 6.1.3. Tantalum. Ta (or Nb) has proved to be an exceptionally valuable indicator of magma provenance (Wood et al., 1979) and of crustal contributions to mantle-derived magmas (Dupuy and Dostal, 1984). Considerably less attention has been given to its distribution in sedimentary rocks. In a pioneering study, Pachadzhanov (1963) estimated that half of the Ta found in shales is contained in Ti minerals of the silt fraction, the other half being in clay minerals. Pushkina (1974) confirmed that the Ta content of pelagic sediments is related to its inflow into sediments as clay material. The comparison of T h - T a - R E E patterns of early Ordovician shales and sandstones may provide an independent criterion by which we can identify that Ta was transferred in clay and

not in Ti-oxides. Compared to the concentrations of Th, Ta and REE in shales (SED110 and SED13), those found in the sandstone (SEDI9) are lower by a similar factor of around 1.4. Since the TiO2 content of these rocks (SED13: 1.05%, SED110: 1.02%, SED19: 0.95% ) is not significantly affected by change in granulometry, we deduce that Ta follows Th and REE in the clay fraction of sediments. The progressive transition from a metavolcanic source to a granite-gneiss-schist component is reflected in the geochemistry of shales by the modification of Ta depletion relative to Th and La (Fig. 3 ). Hence, we believe that the Ta anomaly of shales reflects that of the provenance area. The deep negative Ta anomaly observed for the early Ordovician shales is in agreement with their derivation from granite-gneiss-schists lithologies since crustal rocks are known to have such an important negative anomaly (Weaver, 1985). The moderate Ta depletion of the early Cambrian shales suggests that their mafic metavolcanic source rock was not part of a calc-alkaline sequence wherein the Ta depletion is much more pronounced (e.g., Briqueu et al., 1984; Holm, 1985). Because in Fig. 5 the Brabant shales fall between the data for the Rushton Schists and the Rosslare Gneisses and the values for the basalts characterized by high 147Sm//144Ndratios, the best candidate for the basic metavolcanic source should probably be found among these basalts.

6.2. Nd isotopes in shales and plate delimitation A notable feature of the Brabant sediments is the rather large extent to which their geochemical characteristics and Nd isotopic compositions were homogenized at the time of' deposition. This is consistent with the presence of numerous rock types among the lithic detritus of early Cambrian turbidites (Vander Auwera and Andrd, 1985). Indeed, both observations underscore the fact that the drainage

110

system from which these sediments have been supplied was relatively wide. Hampton and Taylor (1983) and Thorpe et al. (1984) have put upper limits of 1.2 and 0.9 Ga to the age of the basement in southern Britain. Miller and O'Nions (1984) have obtained a m e a n tDM of 1.55 Ga for sediments of southern Britain, but they consider that this model age is unlikely to represent a crust-forming event. They suggest that the source of this "old" detritus came from somewhere in Gondwana through a series of sedimentary cycles. The evidence for a "granite-gneiss-schist" clastic source which has a crustal residence age higher than 1.9 Ga is not necessarily at variance with these conclusions. It points out that the mean age of' the protoliths from which these granites and gneisses derived were older than 1.9 Ga, but it does not give any information concerning the age of the Midland-Brabant craton itself. Indeed, a 1.9-Ga tDM age fits the average age estimate for the entire preserved sedimentary mass (2.0_+0.2 Ga; Miller et al., 1986 in this special issue) and these granites and gneisses could themselves represent a mixture between recent (t<2.0 Ga) and ancient (t>2.0 Ga) components. The contrast in Nd isotopic composition of clastic sediments between the two sides of the Iapetus suture led Miller and O'Nions (1984) to conclude that ~d-values of shales could be a tool in paleogeographical reconstructions. In Fig. 6, the e~d-values of sediments are plotted against their age of deposition. This diagram shows the trend of progressively lower ~ d in the younger sediments of Brabant Massif. The data for sediments from Brittany (Michard et al., 1985) and southern Britain (Davies et al., 1985) are also reported. The evolution of e~d with age of deposition ( e~a vs. t) is remarkably similar for the shales from the three regions. This observation raises the question as to whether these trends trace the progressive erosion in the Armorica plate or are a mere coincidence without any geotectonic significance. The Precambrian basement in the three areas

0

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A" °

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00

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i

•

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600 550 500 ~50 ~0 sfrafigraphic age (Ma)

Fig. 6. e~d-values plotted against the stratigraphic age of shales from Brabant (open circles), southern Britain (solid circles) and Brittany (triangles). Data from this work, Davies et al. (1985), and Michard et al. (1985).

turns out to be very similar. Both Brittany and southern Britain exhibit stratified basements with late Proterozoic (Cadomian, t> 560 Ma) metavolcanics outpoured upon older gneissic rocks (e.g., Cogn~ and Wright, 1980). Each volcanic suite is composed of continental flood basalts and subduction-related calc-alkaline basalts (Cabanis et al., 1986; Pharaoh et al., 1986; Roach et al., 1986). In Belgium, we may confidently assume that the slightly metamorphosed volcanic source which is first eroded during the Caledonian sedimentary cycle is younger than the "granite-gneiss-schist" source. This suggests that the unexposed Brabant craton is probably made up of medium- or high-grade metamorphic rocks overlain by basic lavas. Erosion of the Precambrian basement from southern Britain, Belgium and Brittany was thus likely able to produce equivalent e~a vs. t paths in the subsequent shales: slightly negative e~a-values characteristic of a large contribution from the volcanic rocks at the onset, followed by gradually more negative e~avalues when the crustal component became predominant. The three parallel e~d vs. t trends are thus interpreted as the Nd isotopic signa-

111 ture of the early Paleozoic sediments from the Armorica plate, and the fine structure of the e~d vS. t sedimentary record is expected to be very much useful to the identification of fossil lithospheric plates.

6.3. Nd isotopes in shales and crustal growth One of the striking results of the most recent studies on shales is the constancy of their 147Sm/144Ndratios which range from 0.10 to 0.13 (e.g., O'Nions et al., 1983; Patchett et al., 1984), in an interval similar to that measured in river and eolian particulates ( Goldstein et al., 1984 ). Such constancy has generally been used to check whether the collected fine-grained sediments are representative mixtures of the continental crust. The temporal modification in the provenance of Brabant shales only caused small changes of their 1478m/144Nd: from 0.127 to 0.110. Therefore, although their REE patterns are not representative of the average uppercrust composition (Taylor, 1979), this fact is not clearly reflected in their 147Sm/144Nd. This ratio may thus be inadequate to discriminate shales which represent mere recycled sediments from those deriving from mantle-derived protoliths (andesites, continental tholeiites, etc. ). Since an efficient mixing of lithologically complex upper-crust is assumed only when uniform EPSC*-like REE patterns are recorded in shales (e.g., Nance and Taylor, 1976; Taylor, 1979; M c L e n n a n and Taylor, 1982), selection of shales for Nd isotopic studies should take complete T h - T a - R E E profiles into account and not merely the 147Sm//144Ndratio. The tDM crustal residence age of shales is a parameter which is widely used to infer new mantle additions to continental crust (e.g., O'Nions et al., 1983). Indeed, any inflow of mantle material within the continent should add more radiogenic Nd to the crust and impose a decrease of crustal residence ages of subsequent sediments if their 147Smf144Nd ratio is *EPSC: European Paleozoicshale composite.

significantly lower than the depleted mantle values (0.20-0.23). This scenario must be dominant in the destructive plate margins wherein the continental crust is supposed to grow through the input of calc-alkaline andesites with low ( < 0.14) 147Sm/144Nd(e.g., Taylor, 1967). However, the tDM residence age is certainly much less sensitive to juvenile additions in the form of mid-ocean ridge basalts and back-arc or continental tholeiitic basalts, all of which have 1478m//144Ndratios higher t h a n 0.13 (Fig. 5). Such a relative intensitivity is well documented in the case of Brabant shales. Indeed, despite the strong time-related modification in the sources of detritus, their tDM crustal residence ages fall in the range of Phanerozoic shales so far analysed (1.8 + 0.2). The important mafic component of the early Cambrian shales is thus poorly detected. In contrast, the e~d -values vary significantly more t h a n tDM crustal residence ages in response to the change in detrital sources ( Fig. 4 ). Following Michard et al. (1985), we thus conclude that the e~d of shales is more powerful t h a n their tDM model ages to detect mantle additions to the crust. Previously published S m - N d isotopic analyses of fine-grained clastic sediments suggest that two categories of trends can be resolved in the Nd isotope record of the sedimentary mass: first-order ones ( secular evolution) which represent the global evolution of the sedimentary mass (e.g., O'Nions et al., 1983; All~gre and Rousseau, 1984) and second-order ones (episodic evolution) which trace regional differences in the response of the sedimentary system to juvenile additions (Davies et al., 1985; Michard et al., 1985 ). A major characteristic of the secular evolution is that the tDM crustal residence age of shales is rather uniform over the last 2 Ga. This is interpreted as an evidence for a decreasing rate of mantle influx to the continental crust over Proterozoic-Phanerozoic times (e.g., Goldstein et al., 1984; O'Nions, 1984). Using the more "mantle" sensitive ~ d parameter in the case of French Phanerozoic

112

shales, Michard et al. (1985) refine this model by pointing out the episodic evolution in a restricted area. These authors describe periodic variations of e~d which yield a serrate pattern in the e~d vs. t plot. This periodicity is ascribed to the combination of two independent geological processes: "internally driven" discrete addition of juvenile material to the crust during orogenic events; "externally driven" rapid mixup of these materials with pre-existing sediments during the anorogenic sedimentary cycles. Our data confirm the portion of the serrate e~d vs. t path which concerns the Cadomian-Caledonian interorogenic time span and they lead to an interpretation of its origin. The steep decrease in e~d which occurred during this interval of time (Fig. 6) is related to a progressive unroofing of a heterogeneous basement rather than to a rapid turnover of the sedimentary mass in between the two orogenic events. For lack of accurate volume estimates concerning the Cambrian and Ordovician sediments from the Armorica plate, the relative importance of the various mafic and felsic Precambrian parent rocks remains undetermined. Consequently, whereas the e~d signatures of shales record an actual juvenile inflow to the crust during the Cadomian orogenic cycle ( 800? Ma < t < 560 Ma ), they cannot provide an eval.uation of the net Cadomian youthful addition to the mass of upper crust. Care must thus be exerciced in using the fine structure of a sedimentary sequence, particularly in the case of first-cycle sediments, to evaluate the input of mantle-derived materials to the continents and thereby to quantify crustal growth. Indeed, their REE-e~d signature highlights the episodic evolution of the sedimentary mass, but fails to give any information about its secular evolution even when the drainage systems sample very large regions.

7. Conclusions The significant differences in the T h - R E E patterns of Brabant shales indicate that var-

ious profiles are possible in post-Archean firstcycle sediments and that they are controlled by their source rocks. Hence, the uniform pattern observed in many post-Archean lithologies (Haskin and Haskin, 1966; Nance and Taylor, 1976) is attributed to the rapid turnover of the sedimentary mass. Recycled sedimentary rocks must therefore be chosen in order to determine crustal abundance and long-term evolution of T h - R E E budgets. Since Ta appears to be almost quantitatively transferred into the clay fraction of fine-grained sediments, investigation of its depletion relative to Th and La in shales might provide a useful index to trace continental growth and to evaluate the role of calc-alkaline magmatism in its development. The e~d signature of first-cycle sediments is highly biased by the lithology of their provenance area and it offers a good means to improve plate-tectonic reconstructions. In contrast, it does not reflect the average Nd-isotope record of the sedimentary mass and it cannot yield accurate information concerning the development of continental crust. The procedure of shale selection in the Nd isotopic studies of crustal growth should therefore be improved by collecting well-stirred sediments deriving from more than one sedimentary cycle and comparing their REE contents to the post-Archean shale composites. The extent to which the REE-e~d patterns of metamorphic clastic sediments are homogenized could in turn be exploited to deduce information about the maturity of ancient sediments and to infer the degree of sedimentary recycling.

Acknowledgements The present study was carried out as a part of a programme of the Belgian Centre of Geochronology. It was supported by grants from the National Fund for Scientific Research (Belgium). We are grateful to Mrs. N. Cromps for drawing the figures. The authors wish to express their appreciation to Drs. F. Albar~de and R.K. O'Nions for helpful reviews.

113

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