A geochemical investigation into the provenance of the Neoproterozoic Port Askaig Tillite, Dalradian Supergroup, western Scotland

Precambrian Research 85 (1997) 8!-96

ELSEVIER

A geochemical investigation into the provenance of the Neoproterozoic Port Askaig Tillite, Dalradian Supergroup, western Scotland Alireza Panahi *, Grant M. Young Department of Earth Sciences, University of Western Ontario, London, ON, Canada N6A 5B7 Received 24 February 1997; received in revised form 25 June 1997; accepted 25 June 1997

Abstract

The major and trace element geochemistry of matrix materials from glaciogenic diamictites (unsorted polymictic conglomerates consisting of clasts scattered throughout a finer grained matrix) of the Neoproterozoic Port Askaig Tillite of western Scotland shows evidence of an upward change in provenance. Values for a carbonate-corrected Chemical Index of Alteration (CIA) are fairly high ( ~ 70-80) in the lower diamictites, suggesting that they were derived mainly from an older sedimentary cover (older Dalradian Formations) that had undergone a previous weathering cycle. Low CIA values, more typical of unweathered material, were obtained from diamictites in the upper part of the formation. Thus, deposition of the Port Askaig diamictites is thought to involve a change in provenance, with unroofing of a cover of older sedimentary rocks and exposure of basement rocks. In spite of these inferred differences in provenance, diamictite materials throughout the formation have very uniform chondrite-normalized rare earth element (REE) distributions. REE distribution patterns from the diamictites closely resemble that of average post-Archean shales (PAAS), with a significant negative Eu anomaly and typical enrichment in LREE. These results, together with the presence of uniform (La/Yb)N ratios throughout the formation, suggest that the sedimentary cover rocks from which the lower diamictites were derived, were themselves derived from a similar post-Archean basement. A Th/Sc versus Sc plot suggests that siliciclastic matrix material in the lowest diamictite was derived essentially from provenances similar to PAAS. The higher diamictites appear to have been derived from a mixture of granitic material and PAAS, in a ratio of up to ca 4: 1. Sparse sedimentary structures suggest derivation of the Port Askaig Tillite from the southeast but the exact source remains unknown. © 1997 Elsevier Science B.V.

Keywords." Diamictites; Neoproterozoic; Port Askaig Formation; Provenance; Sedimentary geochemistry

1. Introduction

Establishment of provenance from the chemical and mineralogical characteristics o f sedimentary rocks is complicated by a number of factors,

* Corresponding author. 0301-9268/97/$17.00 © 1997 ElsevierScience B.V. All rights reserved. PII S0301-9268 (97) 00033-8

foremost a m o n g which are the effects of weathering and sorting (Nesbitt et al., 1996). Because the materials making up glaciogenic deposits have generally undergone virtually no chemical weathering (Nesbitt and Young, 1996), they should provide a particularly powerful tool for establishing the nature of their source materials. Perhaps the most c o m m o n rock type associated with ancient

82

A. PanahL G.M. Young / Preeambrian Research 85 (1997) 81-96

glacial deposits is diamictite. This is a non-genetic term referring to poorly sorted siliciclastic sedimentary rocks containing a wide range of clast sizes in an abundant fine-grained matrix in which the clasts are dispersed so that most of them are not in contact. Diamictites should be particularly useful in this regard because they are made up of material that is virtually unsorted and has therefore undergone little mineralogical or geochemical differentiation related to variations in typical grain sizes of different minerals. It is therefore surprising that there are so few geochemical studies of glaciogenic diamictites. Most geochemical studies of glaciogenic deposits have been directed towards attempting to differentiate among depositional settings. For example Frakes (1985) carried out geochemical investigations of recent glacio-marine sediments from the Antarctic region and proposed that terrestrial tills could be differentiated from glaciomarine diamictons on the basis of their iron content, which was thought to be relatively high in sediments formed in a more pervasively oxidized terrestrial setting. Based on a study of both major and trace elements, Sumartojo and Gostin (1976) suggested that the Neoproterozoic Sturt Tillite of South Australia was deposited in a glaciomarine environment. Howarth (1971) carried out a detailed geochemical study of the Port Askaig Tillite in Ireland. Howarth's study was also directed towards establishing the environment of deposition but was inconclusive in regard to depositional setting. Pettijohn and Bastron (1959) analysed some laminated argillite samples from the glaciogenic rocks of the Paleoproterozoic Gowganda Formation and, noting their high NazO content, concluded that they had undergone Na20 metasomatism (albitization). Pettijohn (1975) reported chemical analyses from six tills and tillites from around the world and noted that they were comparable to 'average greywacke' and similar chemically immature rocks. Young (1969) also published some analyses of samples from the Gowganda Formation. When Nesbitt and Young (1984) introduced the Chemical Index of Alteration (CIA: see the following for the explanation), they used glaciogenic rocks of the Gowganda Formation as an example of relatively unweathered material.

In this study, both major and trace elements are used in an attempt to document the geochemical characteristics of the Port Askaig Formation, a diamictite-dominated succession, which forms part of the Neoproterozoic to lower Cambrian Dalradian Supergroup in Scotland and Ireland. The Dalradian succession is estimated to be cumulatively ca 20 km thick. It forms a discontinuous outcrop belt ca 700 km by up to 100 km, extending from northern Scotland to Connemara in western Ireland. This thick, dominantly sedimentary succession, has been considered to have been deposited between ca 700 and 520 Ma in a fault-bounded rift basin that possibly evolved into a continental margin at the time of opening of the Iapetus Ocean (Anderton, 1982). The precise age and tectonic setting of these rocks remain contentious. Noting the absence of unconformities within the Dalradian succession, Soper (1994a,b) suggested that the Dalradian Supergroup, and also the thick Torridonian succession (Stewart, 1991), accumulated as the result of a complex series of long-lived rift episodes that culminated with extrusion of the Tayvallich volcanics and the Cambrian opening of the Iapetus ocean (Anderton, 1985). Others (Bluck and Dempster, 1991; Fitches et al., 1996) supported the idea of orogenic activity in this region. The Grampian Group considered by Harris et al. (1978) and Glover et al. (1995) to be the basal part of the Dalradian succession, is overlain by the Appin Group, the youngest formation of which is the Islay Limestone. The Appin Group is succeeded by the Argyll Group, of which the Port Askaig Formation forms the base. The diamictitebearing unit is succeeded by the Bonahaven Dolomite (Fairchild, 1980) and overlain by tidal shelf sandstones of the Jura Quartzite. Thus the diamictites of the Port Askaig Formation are sandwiched between the Islay Limestone and the Bonahaven Dolomite, as noted by Spencer (1971 ) and Spencer and Spencer (1972).

2. Overview of stratigraphy and paleoenvironments Diamictites of the Port Askaig Formation are exposed in a roughly linear series of outcrops with a northeast-southwest extent of >700 km. The

A. Panahi, G.M. Young/ PrecarnbrianResearch 85 (1997) 81-96

term 'Port Askaig' is used in both Ireland and Scotland but the formation name is taken from a small settlement on the island of Islay and the Islay-Garvellachs area is considered as the type area (Fig. 1). The degree of deformation and the metamorphic grade of the Dalradian rocks are exceptionally low on the island of Islay and the small Garvellachs islands where there is excellent exposure on raised beaches, particularly on the northeast coast of Garbh Eileach, where most of the samples were collected for this study. Details of the stratigraphy and sedimentology have been reported by Spencer (1971), Howarth (1971 ) and Eyles (1988). The sequence exposed in the Islay-Garvellachs area is made up of ca 40-50% diamictites. The succession also includes siltstones, sandstones, orthoconglomerates, dolomitic sandstones and dolostones. The diamictites show a very limited range of clast types; the lower part is dominated by orange- and yellow-weathering buff and grey dolostones, whereas the upper diamictites are characterized by the presence of grey and pink weathering granitic clasts and subordinate foliated granites. A recent geochemical study (Fitches et al., 1996) showed that these

83

granitic clasts have geochemical features of withinplate granites. There are few unequivocal directional structures but the Port Askaig Formation includes some irregularly distributed folds that have been attributed by Spencer (1971) to "ice push". These structures, and folding of a huge (> 40 m long) dolostone fragment, suggest a northwesterly transport direction (Spencer, 1971). The base of the formation is best exposed on the northeast side of Garbh Eileach, the largest island in the Garvellach Islands, which are ca 50 km northeast of Islay (Fig. 1). At this location there is a continuous stratigraphic section of the basal 500 m of the formation (Fig. 2). The lower diamictites are dominated by dolomite clasts, which were probably derived from the underlying Islay Limestone (Spencer, 1971; Anderton, 1985). The upper diamictites differ in containing abundant extra-basinal granitic clasts. The provenance of the exotic clasts in the Port Askaig Formation has long been debated. Fitches et al. (1996), noted that although the granitic clasts within the diamictites are petrographically similar to those of the Rhinns Complex syenites, they are geochemically different. They suggested that these clasts were

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Fig. 1. Area of study in western Scotland.

A. Panahi, G.M. Young / Precambrian Research 85 (1997) 81-96

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A. Panahi, G.M. Young / Precambrian Research 85 (1997) 81-96

possibly derived from other plutonic rocks closely associated in space and time with the Rhinns Complex. Striking similarities between the crystalline clasts in the Port Askaig diamictites and Rapakivi-type granites in Finland and Sweden led Spencer (1971) to suggest that their source might be a southern extension of the 1000 Ma Gothide complex. Fitches et al. (1996) have also noted that the granitic clasts may have been derived from Ketilidian and Svecofennian provinces that contain plutonic rocks of diverse ages. The mechanism of deposition of the diamictites was thought by Spencer (1971) to have been from grounded ice sheets. Evidence for in situ basal melt-out is provided by the sharp lower contacts of the diamictite beds and by the nature of the internal bedding in the diamictites. The interbedded sediments were mostly deposited in a subaqueous environment. The sandstone wedges which occur in several horizons in the formation are considered as subaerial (permafrost) features (Spencer, 1975, 1985). By contrast, Eyles and Eyles (1983), Eyles et al. (1985) and Eyles (1988) suggested an origin for the diamictites by glaciomarine deposition below floating ice. They based their interpretation on the absence of glaciotectonic deformation structures which may indicate either relatively deeper water during diamict accumulation or insubstantial floating ice masses. Lithological variation in the form of conglomerate, sandstone and siltstone interbeds may have resulted from changes in relative sea level within a continually subsiding basin. A periglacial origin for sandstone wedge structures developed on the surface of several diamictites was reassessed and a subaqueous deformation model was presented (Eyles and Clark, 1985). A glaciomarine origin by ice rafting, however, was rejected by Spencer (1985) and he argued that deposition from floating ice would have produced gradational lower contacts as ice rafted sedimentation gradually became dominant over normal sedimentation. Recent discovery of a Grenville-age terrain in western South America (Wasteneys and Clark, 1995) and the proposal that eastern Laurentia (including Scotland) may have been juxtaposed against South America (Dalziel, 1991) in the

85

Neoproterozoic, together with the suggested northwesterly transport directions for the Port Askaig diamictites (Spencer, 1971) open up the possibility that the exotic granitic clasts in the Port Askaig Formation may have been derived from western South America (Astini and Benedetto, 1995; Dalla Salda et al., 1992; Dalziel et al., 1994; Tosdal, 1996; Wasteneys and Clark, 1995).

3. Sampling and analytical procedures Typical samples of diamictite matrix were collected from most of the diamictite beds in the Garbh Eileach section of the Port Askaig Formation (Panahi, 1996). Ideally, an extremely large sample would best represent the average composition of poorly sorted rocks like diamictites, but for practical reasons, fist-sized fragments of diamictite matrix material were taken as representative samples. Because of the lack of chemical weathering in a glacial regime, and the general absence of sorting in diamictites, such hand samples are probably more representative of source rock compositions than either sandstones or mudstones, which are generally used for such studies. Some preferential breakdown of some minerals may, however, take place (Young, 1969). The diamictites matrix samples were split into three groups: the first group includes samples from the lower part of the formation, where clasts are predominately carbonate. This group includes diamictites 1-13. The second group includes diamictite matrix materials from the middle part of the formation (diamictite 14-32), where granite clasts first appear and tend to increase upwards. The third group (diamictites 33-38) includes samples from the arenaceous part of the section, where clasts are mainly granitic. Twenty-one samples were chemically analysed for major and trace elements (Table 1). Only the matrix material of the diamictites was separated for analysis. Si, A1, total Fe reported as Fe203, Mn, Mg, Ca, K, Ti, P, Rb, Sr, Ba, Zr, Y, Nb, V, Cr, Ni, Cu, Zn and Pb were analysed by X-ray fluorescence spectrometry at the University of Western Ontario. Na was determined by atomic absorption spectrophotometry (AAS). Rare earth

A. Panahi, G.M. Young / Precambrian Research 85 (1997) 81-96

86

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A. Panahi, G.M. Young / Precambrian Research 85 (1997) 81-96

elements (REE) abundances were determined by instrumental neutron activation analysis (INAA) at the University of Western Ontario. The procedure followed that outlined by Jolly et al. (1992). The analysed elements include: La; Ce; Nd; Sm; Eu; Yb; Lu; Th; Sc; Hf; and U. Individual aliquots, consisting of 250-300 g of powder, were irradiated for 3 h in the reactor at McMaster University, Hamilton, Ontario. Counting of ~'-ray spectra occurred in two stages: La; Ce; Nd; and Sm were counted 1 week after irradiation for 1000 s, the remaining REEs were counted four weeks after irradiation over a counting time of 100000s. International standards MRG-1 and BHVO-1 [from reference values of Govindaraju (1989)] were analysed together with the samples to check the accuracy of the method. Most of the analysed rocks contain carbonate minerals (dolomite and calcite). To minimize the effects of different amounts of carbonates present in samples and in order to calculate the dilution factor, between 1.700 and 0.850g of the rock powders were analysed for carbonates (calcite and dolomite) using the technique suggested by Dreimanis (1962). The chemical compositions of the samples were then re-calculated on a carbonate-free basis.

4. Geochemical results

4.1. The chemical index of alteration." implication for the provenance of the Port Askaig Tillites The weathering history of sedimentary rocks may be evaluated by examining the relationships among alkali and alkaline earth elements (Nesbitt and Young, 1982, 1984). Chemical weathering largely involves the degradation of feldspars, which are estimated to be the most abundant labile minerals in the upper crust. During weathering, calcium, sodium and potassium are largely removed from feldspars; the amount surviving in soil profiles (and sediments derived therefrom) is a sensitive index of the intensity of weathering. The secondary products of such feldspar alteration are mainly aluminous clay minerals, so that the degree of alteration can be calculated as a CIA [of

87

Nesbitt and Young (1989)] as follows: CIA = [AlzOa/(AlzO 3 + CaO* + Na20 + K20)] x 100 where the oxides are expressed as molar proportions and CaO* refers to CaO 0in silicates only. Corrections can be made for carbonates and phosphates [see: Fedo et al. (1995) for methodology of calculations]. This value provides an estimate of the relative proportions of aluminous secondary clay minerals and 'primary' minerals such as feldspars. CIA values vary from ca 50 for unweathered 'primary' igneous rocks to 100 for severely weathered residual clays (Nesbitt and Young, 1984). Estimates of the average composition of shales typically yield intermediate values of ca 70-75, indicating that weathering has not proceeded to the stage where alkalis and alkaline earth elements are totally removed. Before calculation of CIA values for sedimentary rocks it is essential to ensure that the chemical analyses represent only the silicate fraction of the rock. Variable amounts of carbonates in the Port Askaig diamictite samples, particularly in the lower part of the formation, complicate use of CIA in evaluating paleoweathering history. In order to make appropriate corrections for CaO in calcite and dolomite, an estimate of the amounts of different carbonates in each sample was made, using the gasometric Chittick apparatus, as outlined by Dreimanis (1962). The major element proportions were then recalculated on a carbonatefree basis and the data were plotted on a ternary diagram showing the molecular proportions of A1203, (CaO* +Na20) and K20 (Fig. 3). An estimate of the composition of the average postArchean shale [PAAS of Taylor and McLennan (1985)] and an estimate of the average composition of the Proterozoic upper continental crust (Condie, 1993) are also plotted. By drawing a best-fit line through the linear array of points representing the analysed diamictites, an estimate of the composition of the source material can be obtained (Fedo et al., 1995). The heavy arrow in Fig. 3 (labelled "predicted weathering trend") represents the theoretical path that should be expected from weathering of material of this initial composition (Nesbitt

88

A, PanahL G.M. Young / Precambrian Research 85 (1997) 81-96

10090-

AI "corrected"CIA range of the diamicites

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Fig. 3. Ternary plot of A-CN-K for the Port Askaig diamictite samples. The correction for CaO* was made using the technique discussed in the text. The compositions for major minerals and typical rock types are also plotted. The CIA scale is given on the left side of the diagram. Note that the samples from lower part have relatively higher CIA values than those from the upper and middle part. The weathering trend follows a path towards the K20 apex, suggesting a possible addition of K20. CIA values for average diamictites (lower and upper part) is between 60-80%. ka, Kaolinite; gi, gibbsite; chl, Chlorite; ill, lllite; plag, Plagiocalse; k-sp, K-Feldspar.

and Young, 1984, 1989). This path should be subparallel to the A-CN boundary (Nesbitt et al., 1996). Fedo et al. (1995) have shown that a distribution such as that of the Port Askaig samples, indicates that the samples have undergone K-metasomatism. When lines are drawn from the K20 apex, through the individual diamictite samples, the points of intersection with the predicted weathering trend (heavy arrow) should show the original (pre-metasomatism) CIA values of the samples. This procedure cannot be used for samples plotting on the A1203-K20 boundary of the triangle because their original position could have been anywhere between the point of intersection of the heavy arrow with the A 1 2 0 3 - K 2 0 boundary and the A1203 apex. Apart from such samples, which must have had original CIA values of at least 80 (the point of intersection between

the predicted trend and the A1203-KzO join), the matrix materials of the Port Askaig diamictites yield values from ca 60 to 80. Many of these values are significantly higher than those reported by Nesbitt and Young (1984) for diamictite matrix materials of the much older (Paleoproterozoic) glaciogenic Gowganda Formation in Ontario. Samples from the lower part of the Port Askaig Formation generally yield high CIA values, whereas those from the middle and upper parts have lower values, more in keeping with the glacial interpretation of these diamictites. The high CIA values obtained from the lower diamictite samples probably reflect incorporation of a significant proportion of siliciclastic material from the underlying Islay Limestone, and possibly other Dalradian sedimentary units which have been through a previous cycle of weathering. The low values for

A. Panahi, G.M. Young / Precambrian Research 85 (1997) 81-96

the upper part of the formation could reflect unroofing of a crystalline basement (Spencer, 1971), as suggested by the appearance of a high proportion of felsic plutonic clasts in the upper diamictites.

4.2. Stratigraphic variations Howarth (1971) selected and analysed a group of samples from the Port Askaig Formation, mainly from the Glencolumbkille and Kiltyfanned regions of northwestern Ireland. He noted that the suite of specimens from the Kiltyfanned section shows a pattem of geochemical variation with stratigraphic height. He reported a gradual upwards increase in SiO2 and decrease in TiO2 and A1203. In contrast, there is lack of well defined trends for Fe203, MnOz, K20 and P205. Above the dolomitic lower Port Askaig Tillite in the Kiltyfanned area, MgO, CaO, S and C1 fall off rapidly. Abundant Na20, Cr, Ni, Sr, La and Ce were also reported in the semipelitic Middle Port Askaig Tillite (Howarth, 1971, p. 22). The most significant geochemical variations with stratigraphic height in the Garbh Eileach section of the Port Askaig Formation involve CaO, MgO and SiO2. The CaO and MgO contents of the diamictite matrices progressively decrease upwards, whereas the SiO2 content increases. SiO 2 content in the Port Askaig Formation diamictites ranges from 37.7 to 79% with an average of 54.0%. A1203 ranges between ,-~7 and 11.7% with an average of 8.85%. Na20 content of the diamictite matrices changes progressively from 0% in the lower part to 1.7% in the upper part. K20 remains relatively constant. The marked increase in PzOs with stratigraphic height from ca 0.06% near the base, to ca 0.37% in the upper part of the formation may be due to the increasing abundance of apatite group minerals which are common accessory minerals in almost all igneous rocks and particularly in granite and granite pegrnatites. In such rocks apatite may amount to as much as 5% by volume, although 0.1-1% is the more normal range (Deer et al., 1966). Covariation of TiO2 and A1203 with stratigraphic height is also characteristic. Titanium in the form of insoluble heavy minerals, may be concentrated in certain residual sediments. On the

89

other hand, TiO 2 tends to vary together with Fe203, particularly in the upper part of the formation. This can be explained by the presence of Ti in Fe-bearing minerals such as magnetites (Deer et al., 1966). Most samples form the lower part of the formation have K20/Na20 ratios higher than those of average shale (NASC) and average greywacks. In terms of SIO2/A1203 ratios, the samples closely resemble average greywackes (Fig. 4).

5. Provenance

5.1. Transition elements A plot of Cr versus Ni for the analysed samples is shown in Fig. 5. All samples fall within the Ni-Cr field designated as typical of post-Archean terrigenous sediments (Taylor and McLennan, 1985). Average Ni, Co and Cr values of the Port Askaig diamictites as well as Cr/Th and Co/Th ratios are, however, relatively lower than NASC (Tables 1 and 2). These elements are more abundant in the lower part of the Port Askaig

0 diamictitematrix, upperp a ~ + diamictitematrix,middlepartI ® diamictitematrix,lowerpart j

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90

A. Panahi, G.M. Young / Precambrian Research 85 (1997) 81-96 1000.0

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Formation. V and Sc chiefly reside with the mafic component of greywackes (Bhatia and Crook, 1986). Sc is particularly useful because its geochemical behaviour differs from that of the REE and Th, for Sc is much smaller (IR=0.87 A) and is typically much more compatible than the other REEs. High V and Sc are taken to signify a considerable proportion of basic material in source regions, whereas low values suggest incorporation of more felsic materials (Condie and Wronkiewicz, 1990; Prame and Pohl, 1994). Low values for Cr, V, Sc and Ni in the Port Askaig diamictites rule out a major contribution from a basic source. 5.2. Lanthanides-Th

The La-Th relationship for the samples from the Port Askaig Formation is shown in Fig. 6A. A significant positive correlation (r = 0.93) between La and Th in diamictite samples suggests that these elements behaved concordantly during sedimentary processes (Nance, 1976; McLennan et al., 1980). All samples have La/Th ratios between 2 and 4 which corresponds to a relatively felsic composition. La/Th and Th/Sc ratios (Fig. 6B) are essentially within the ranges suggested for

fine-grained post-Archean sedimentary rocks (McLennan, 1989). As pointed out by McLennan and Taylor (1991), the REE, Th and Sc are useful for inferring crustal composition, because their abundances in sediments are thought to be similar to those of the source rocks and also because their distribution is not seriously affected by secondary processes like diagenesis and metamorphism. REE abundances and the shape of the REE patterns can provide information about the bulk composition of the provenance as well as the nature of the dominant igneous processes that affected the provenance region. Igneous differentiation processes commonly result in LREE enrichment in more evolved felsic rocks. Eu-depletion in sedimentary rocks, has been interpreted to indicate that the ultimate igneous source rocks were strongly affected by intracrustal differentiation processes, resulting from plagioclase fractionation. Correlations between La or Yb and Zr in the Port Askaig diamictites are somewhat smaller than those with A1203 and K20, suggesting that zircon is less important than clay minerals in controlling the distribution of LREEs. The lack of correlation between La, Th and P 2 0 5 also indicates that neither apatite nor monazite controls the La-Th distribution in the diamictites. There is no correlation between either A1103 or Zr and Eu/Eu* (Eu* is the calculated Eu value assuming that there is no Eu anomaly) in the Port Askaig diamictites, indicating that neither clays nor zircon are responsible for Eu anomalies in the samples. There is no strong correlation between Eu/Eu* and any trace elements in the samples, suggesting that no single mineral controls the Eu anomaly in the diamictites. The REE values obtained from 21 samples of the diamictite matrices were re-calculated on a carbonate-free basis and then normalized to chondritic values reported by Masuda et al. (1973) and Taylor and McLennan (1985) (Fig. 7A and B). Chondrite-normalized REE data obtained from granite clasts associated with the diamictites (Fitches et al., 1996), are also plotted on the diagram. The granite clasts pattern is characterized by LREE enrichment, pronounced negative Eu anomaly and flat REE pattern. The REE data obtained from the diamictite matrices do not

91

A. Panahi, G.M. Young / Precambrian Research 85 (1997) 81-96

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A. PanahL G.M. Young / Precambrian Research 85 (1997) 81-96

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lO

i 0.1

. . . . . .

0.1

. . . . . . . . . . . . . . .

1

10

100

Sc (ppm)

Fig. 6. (A) La-Th and (B) Th-Sc diagrams.

directly reflect a granitic nature, but more possess the characteristics of a 'cratonic-type shale (e.g. PAAS)' component. It has been suggested (Taylor and McLennan, 1985, p. 102) that the Th/Sc ratio probably best reflects crustal level of the eroded source rocks because Th tends to be concentrated in more evolved igneous rocks, whereas Sc is much more compatible. In Fig. 8, Th/Sc ratios for all diamictite samples are plotted against Sc abundances. The majority of samples form a tight cluster around the line representing Th/Sc ratio of 1 but some samples, especially those from the upper part of the Formation, have somewhat higher values.

LaCe

i

N/

d

i

m

i

I

~

SmEuCatTb

~

i

J

m

I

I

Yb Lu

Fig. 7. Chondrite-normalized REE distributions in the Port Askaig Diarnictites. (A) REE distribution for the average Port Askaig diamictite from the lower part of the section. The stippled area represents ranges of variation. (B) REE pattern for the upper part of the section. Refer to Fig. 2 for subdivisions. Data for PAAS and average granite clasts are after Taylor and McLennan (1985) and Fitches et al, (1996), respectively.

The majority of the samples from the lower part of the Formation plot close to the estimated composition of PAAS (Taylor and McLennan, 1985). Average Proterozoic granite (Condie, 1993) is also plotted and the black dots show the results of mixing the granite and average shale in 20% increments. Apart from two samples with low Sc values (due to sorting?) the majority of the samples fall close to the mixing line. Samples with the highest Th/Sc values fall close to a point representing a 4:1 mixture of granite and PAAS.

A. Panahi, G.M. Young / Precambrian Research 85 (1997) 81-96

10.00

Average Proterozoic

I~ O

4gr:.ls~,l~

T

(,.Te~ lgr:4sh U_

1.00

~(ak

+ •

ThlSc

__

rh/sc__

= 1

~post-Archean shale

0 dllmlctlte matrix, upper part -.F dlamictlte matrix, middle part • dlamlctlte matrix, lower part

0.1

0.01

,

,

,

,

i

,

i,J

10

,

I

i

i

J I L l

100

Sc(ppm)

Fig. 8. Variation of Th/Sc versus Sc for the Port Askaig Diamictite samples. The majority of samples makes a tight cluster around the Th/Sc = 1 line. Average Proterozoic granite [after Condie ( 1993)] and average post-Archean shale are also plotted and the black dots show the result of mixing the granite and average shale in 20% increments.

of the present upper continental crust. The occurrence of significant Eu/Eu* anomalies (i.e. Eu/Eu*<0.85) in all diamictite samples suggests that intracrustal differentiation was an important process in the source areas from which the sediments were derived. It is well established that Archean granitoid rocks are depleted in HREEs compared to most post-Archean equivalents (Martin, 1986). Hence both the La/Yb and Gd/Yb ratios in detrital sediments should reflect the average age of their source rock (Condie and Wronkiewicz, 1990). The distinction between these two groups (i.e. Archean and post-Archean) is apparent when (La/Yb)N is plotted against YbN for Archean and post-Archean granodioritic to granitic rocks. Almost all Port Askaig diamictite samples fall into the postArchean granitoid area on a plot of this kind (Fig. 10). Theoretically, the La/Yb ratio in the diamictite samples should reflect the nature and the relative age of their source.

Ratios of (Gd/Yb)N (subscript N represents chondrite normalized values) in the diamictites are virtually all in the range of 1.0-2.0 (Fig. 9), indicating that the chondrite-normalized HREE patterns are nearly flat. Eu/Eu* is variable with values ranging from ,,~0.8 to ~ 0.6, values characteristic 1.4

E

120

(La/Yb).

eo

Archean TTG

Gd./Yb.=2.0

1.2

1.0

93

volcanically active tectonic settings | (post-Archean)

40

I

Archean sedimentary rocks Eu/Eu*

0.8

Eu/Eu*=0.85 4

r

8

12

16

20

(Yb). 0.6 (post-Archean) f-r.~ 0.4

.~.

I

I

1 Gd./Yb.

Fig. 9. Plot of Eu/Eu* against (Gd/Yb)N for the diamictite samples. Fields are after Taylor and McLennan (1985).

Fig. 10. (La/Yb)N versus (Yb)N diagram showing change in REE content in granitic rocks with time. Two groups are: (1) Archean TTG (trondhjemite, tonalite, granodiorite) with high (La/Yb)N ratios (from 5 to >150) and correlated low Yb content (0.3 < Yb N< 8.5); and (2) post -2.5 Ga granitic rocks with low (La/Yb)N ratios (<20) and higher Yb content (4.5
94

A. Panahi, G.M. Young / Precambrian Research 85 (1997) 81-96

6. Conclusions The major and trace element chemistry of matrix materials of diamictites in the Port Askaig Formation suggests unroofing of a sedimentary terrain and incorporation of unweathered basement materials in the upper part of the formation. Diamictites in the lower part of the formation have relatively high CIA values, which are consistent with field observations suggesting derivation from an older sedimentary succession that had been weathered in a previous cycle. In the upper part of the Formation, diamictites have much lower CIA values, suggesting that they were largely produced by physical erosion of an unweathered basement terrane. Because ice sheets depositing glacial diamictites may sample large areas, the diamictites should provide a good index of average major and trace element distributions in the source regions. The trace-element geochemistry of the Port Askaig diamictites suggests that these sediments were derived from areas with cratonic-type shales developed on a crystalline basement. The basement from which these sediments were derived must have been post-Archean according to the (La/Yb)N ratios. The geochemical data presented in this report are insufficient to assign a specific provenance area to the Port Askaig diamictites. Paleocurrent indicators are not sufficiently abundant to draw a detailed paleogeographic picture, but, it has been proposed that a large ice sheet, centred over Eastern Europe, flowed out onto a broad continental shelf over East Greenland, Svalbard, Scotland and Scandinavia (assuming that no Iapetus ocean existed at that time) and onto another shelf in the western Urals. Chumakov (1981), Hambrey (1983) and Spencer (1975) postulated that an icesheet covered the Russian Platform and Baltic Shield area during the Varangian ice age. The thickness of glacial deposits in these areas in relation to the thickness of the sequences in which they lie suggests that the Varangian ice age lasted for at least 10-30 Ma (Spencer, 1975). During this time span, unroofing of sedimentary cover rocks exposed the underlying crystalline rocks. The striking similarity of granitic stones in the Port Askaig diamictites to Rapakivi-

type granites of Sweden and Finland may indicate that the ice passed over these areas and moved radially towards a broad gulf (Anderton, 1985) separating the emergent parts of the Laurentian continent and the Baltic shield. Alternatively these exotic clasts may have been derived from an area in the southeast, possibly northwestern South America (Dalziel, 1991; Wasteneys and Clark, 1995). Rare earth elements patterns closely resemble that of PAAS, with a significant Eu anomaly and typical enrichment in LREE. In spite of source differences inferred from both field observations and CIA calculations, all of the sampled diamictite matrix materials have very uniform chondritenormalized REE distributions, suggesting that the sedimentary rocks from which the lower diamictites were derived, were themselves formed from a terrain that was compositionally similar to the source rocks for the upper diamictites. The ubiquitous presence of negative Eu anomalies suggests that the detrital materials in all of the sampled diamictites were derived from source areas that had undergone considerable intracrustal differentiation.

Acknowledgment GMY would like to acknowledge financial support from the Canadian Natural Sciences and Engineering Research Council (NSERC). We are grateful to C. Wu, who carried out the chemical analyses reported in this paper at laboratories at the University of Western Ontario. We are also grateful to H.W. Nesbitt for discussions and helpful insights and comments on an earlier version of this manuscript and to C.W. Fedo for discussions on geochemical problems.

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