Geochemical and isotopic characteristics of Early Silurian clastic sequences in Antigonish Highlands, Nova Scotia, Canada: constraints on the accretion of Avalonia in the Appalachian - Caledonide Orogen

J. Brendan Murphy, J. Duncan Keppie, Jarda Dostal, John W.F. Waldron, and Mary Pat Cude

Abstract: Avalonia is a terrane that accreted to Laurentia-Baltica during the development of the Appalachian-Caledonide Orogen. Interpretations of the timing of accretion have been constrained by comparing faunal affinities, overstep sequences, age and kinematics of inferred accretionary deformational events, and controversial paleomagnetic data. We show that the time of accretion of Avalonia may also be constrained by contrasts in the geochemical and isotopic signatures of its igneous rocks (which reflect the characteristics of the underlying continental basement and mantle) and sedimentary rocks (which reflect provenance). Early Silurian clastic sedimentary rocks of the Beechill Cove Formation, Antigonish Highlands, Nova Scotia, were deposited on Avalonian crust. The formation predominantly consists of approximately 80 m of siltstones and shales deposited in a nearshore environment and derived from the northeast. Their age is constrained by paleontological data and by directly underlying Late Ordovician - Early Silurian bimodal volcanic rocks that have typically Avalonian geochemical signatures. In comparison with typical Avalonian rocks, the Beechill Cove sediments are characterized by high SiO,, Ce/Yb, and initial 87Sr/86Sr, low FqO,, MgO, and TiO,, and strongly negative ~,,(ur). These characteristics cannot be attributed to erosion of underlying Avalonian basement or coeval volcanic rocks and are consistent with derivation via significant transport from radiogenically enriched continental crust. E, data are typical of Grenvillian basement compositions and suggest that the Beechill Cove sedimentary rocks were derived from an adjacent landmass with Grenvillian crust. The data, in conjunction with paleocontinental reconstructions and recent geochronological and structural data from the northern Appalachians, suggest that the Caledonide orogenic belt is the most likely source. Deposition of the Beechill Cove Formation is inferred to have occurred in an intracontinental basin associated with strike-slip tectonics during the oblique collision of the Avalon with Laurentia-Baltica.

R&um6 : Avalonia reprCsente un terrane qui fut accrktC i Laurentia-Baltica durant le dCveloppement de l'orogtne Appalachien-CalCdonien. Les interprktations portant sur la dCtermination de la ptriode de l'accrktion ont Ctk influenckes par la comparaison des affinitCs fauniques, les discordances de sequences stratigraphiques, l'lge et la cinCmatique des Cvhements de dkformation associks i I'accrCtion infCrCe, et par les donnks palComagnCtiques controversCes. Nous dkmontrons que d'autres facteurs peuvent contraindre la determination du moment de l'accrktion, ce sont : les contrastes qui apparaissent dans les signatures gkochimique et isotopique des roches ignks (reflktant les particularitks du socle continental sous-jacent et du manteau), et la composition des roches ~Bdimentaires (indiquant la provenance). Les roches saimentaires clastiques de la Formation de Beechill Cove, Silurien prkoce, dans les hautes terres d'Antigonish, ~ouve l le -~cosse , furent dkposBes sur la crotite avalonieme. Cette formation est compos6e principalement d'environ 80 m de siltites et de shales accumulBs en milieu infratidal, et dont les matCriaux constitutifs sont dCrivts du nord-est. L'lge de ces sdiments est contraint par les donnCes palContologiques, et par les roches volcaniques bimodales sur lesquelles ils reposent directement; ces roches volcaniques sont da tks de I'Ordovicien tardif - Silurien prkcoce, et elles montrent des signatures gkochimiques typiquement avaloniennes. Les skdiments de Beechill Cove, comparativement aux roches avaloniennes typiques, sont caractCrids par des valeurs Blevks de SO,, Ce/Yb et 87Sr/86Sr initiaux, par des teneurs faibles en Fe,O,, MgO et TiO,, et par des valeurs fortement nkgatives de ENd(ur). Ces particularitks ghchimiques ne sont pas le rksultat de 1'Crosion du socle avalonien sous-jacent ou des roches volcaniques contemporaines, cependant, elles sont compatibles avec une provenance impliquant un transport substantiel de matkriaux fournis par une crotite continentale enrichie d'C1Cments radiogkniques. Les valeurs de E,, sont typiques de

I Received February 25, 1995. Accepted November 22, 1995.

I J.B. Murphy,' J.D. Keppie, and M.P. Cude. Department of Geology, St. Francis Xavier University, P.O. Box 5000, Antigonish, NS B2G 2W5. Canada.

I J. Dostal and J.W.F. Waldron. Department of Geology, St. Mary's University, Halifax, NS B3H 3C3, Canada.

Corresponding author (e-mail: bmurphy@stfx.ca) .

Can. J. Earth Sci. 33: 379-388 (1996). Printed in Canada 1 Imprim6 au Canada

380 Can. J. Earth Sci. Vol. 33, 1996

la composition des roches du socle grenvillien, ce qui 6voque la possibilitk que les matkriaux des roches saimentaires de Beechill Cove proviement d'un terrain adjacent i la cro~te grenvillieme. Nos rhltats, joints i ceux des reconstructions palwcontinentales, et des 6tudes g6ochronologiques et structurales rkentes des Appalaches du Nord, sugghrent que la Zone de plissement de l'orogkne Calaonien reprksente la source des skdiments la plus plausible. Le milieu de sdimentation des mat6riaux de la Formation de Beechill Cove peut &re consid6r6 comme 6tant un bassin intra-continental, associ6 ii une tectonique de dkrochements active durant la collision oblique de 1'Avalonia avec Laurentia - Baltica. [Traduit par la rhction]

Introduction

The Appalachian-Caledonide Orogen was formed during the accretion of suspect terranes to Laurentia and Baltica at various times through the Paleozoic (e.g., Williams and Hatcher 1983; Keppie 1985, 1989, 1993a; Gee and Sturt 1985; McKerrow and Scotese 1990). The timing and kine- matics of accretion of many of these terranes are imprecisely understood.

Avalonia is the largest suspect terrane in the northern Appalachian Orogen. It is variously interpreted as a com- posite (Avalon Composite Terrane, Keppie 1985) or a super (Avalon superterrane, Gibbons 1990) terrane, depending on interpretations of its Late Proterozoic and early Paleozoic tectono-thermal history and genetic relationships with Laurentia and Gondwana. Avalonian rocks occupy much of the southeastern flank of the Appalachians, and correlative rocks occur in Ireland, Britain, and continental Europe (Fig. 1). Avalonia is characterized by a tectono-stratigraphy consisting of 630-570 Ma arc-related volcanic and sedi- mentary successions and cogenetic plutons, locally devel- oped latest Proterozoic (ca. 570 - 540 Ma) wrench-type sedimentary and bimodal igneous rocks, which are conform- ably to unconformably overlain by Cambrian-Ordovician platformal successions containing Avalonian fauna (e.g., Keppie 1985; Dostal et al. 1990). The component of the ter- rane that is presently attached to North America is known as West Avalonia; the component of the terrane that occurs in western Europe is known as East Avalonia (Fig. I), where it is in contact with the north Atlantic Caledonides and Baltica.

In general, paleomagnetic and faunal data indicate that Avalonia was located along the periphery of Gondwana during the Late Proterozoic - Early Ordovician (Cocks and Fortey 1982; Johnson and Van der Voo 1986; Landing and Murphy 1991; Theokritoff 1979; Van der Voo 1988), whereas by Early Devonian times it had migrated northwards and was located adjacent to Laurentia and Baltica (Pickering et al. 1988; Miller and Kent 1988; Cocks and Fortey 1982, 1990; Scotese and McKerrow 1990; Soper and Woodcock 1990; McKerrow et al. 1991; Trench and Torsvik 1992; Potts et al. 1993; Hodych and Buchan 1994; Golonka et al. 1995). However, conflicting paleomagnetic data hinder more detailed analysis of the time of accretion.

In the Appalachian Orogen, several lines of evidence point to accretion of West Avalonia to Laurentia prior to the Early Silurian. Chandler et al. (1987) inferred that Lower Silurian rocks represent outliers of an overstep sequence deposited across the Appalachians from Laurentia to Avalonia. Dunning et al. (1990), Cawood et al. (1994), and Keppie (1993a, 1993b) interpret the Late Ordovician - Silurian structures in Avalonia of Newfoundland and Cape Breton

Island in terms of accretionary deformation related to colli- sion between Laurentia and Avalonia, an event assigned to the Salinic orogeny (Cawood et al. 1994).

As noted by Soper and Woodcock (1990), the time of docking of Avalonia with Laurentia or Baltica may be more confidently constrained when sediments from one plate over- step the terrane boundary. In this paper, we test the sensitiv- ity of geochemical and isotopic data from clastic rocks in helping to identify provenance and constrain times of terrane accretion in the Appalachian orogen. We present geochemi- cal and isotopic data for Avalonian sedimentary rocks of the early Llandovery Beechill Cove Formation of the Arisaig Group in the Antigonish Highlands, Nova Scotia. Identifica- tion of the source area of these rocks helps to resolve some of the conflicting paleocontinental recons&ctions. The age of these rocks is well constrained by paleontological data (Pickerill and Hurst 1983), and they immediately overlie the bimodal volcanic rocks (variously known as Dunn Point and Bears Brook formations) from which paleomagnetic data (Van der Voo 1988) have been used to determine the Late Ordovician - Early Silurian position of West Avalonia. In this paper, we show that, although the Beechill Cove Forma- tion was deposited on Avalonian basement, it displays no sig- nificant chemical contribution from underlying Avalonian rocks. The data indicate derivation from a Grenvillian crust and provide general support for interpretations in which the portion of the Iapetus Ocean separating Avalonia from either Laurentia or Baltica had virtually disappeared by Early Silurian time.

Geological setting

The Late Ordovician - Early Devonian Arisaig Group occurs along the flanks of the Antigonish Highlands of northern mainland Nova Scotia. It is predominantly underlain by Late Proterozoic (ca. 615 Ma) arc - back-arc volcanic and sedi- mentary rocks of the Georgeville Group that are typical of Avalonian Late Proterozoic arc-related rocks (Murphy et al. 1990). These rocks are overlain by a Cambrian - Early Ordovician succession of bimodal, intracontinental, volcanic rocks, and terrestrial to shallow-marine clastic rocks and limestones that contain typically Avalonian Acado-Baltic fauna (Theokritoff 1979; Landing and Murphy 1991).

The Arisaig Group consists of a relatively continuous, 1400- 1500 m stratigraphic succession, dominated by silici- clastic rocks, that unconformably overlies the Cambrian - Early Ordovician succession. The base of the succession consists of about 80 m of bimodal intracontinental volcanic rocks, known as the Dunn Point Formation, that were deposited in a subaerial environment. At present, the only available geochronological data from this formation is an Rb-Sr whole-rock isochron, which yields 421 + 11 Ma

Murphy et al. 381

Fig. 1. Late Paleozoic reconstruction of the circum-North Atlantic area (modified from Keppie and Dallmeyer 1989). Avalonian and Cadomian terranes are shown in black; the Meguma Terrane is lined. The location of the Antigonish Highlands (AH) is shown north of the boundary between West Avalonia and the Meguma Terrane, in mainland Nova Scotia.

(Fullager and Bottino 1968; recalculated in Keppie and Smith 1978). This date is considered unreliable, given that these rocks demonstrably underlie sedimentary rocks with Early Silurian fossils. The Dunn Point Formation is disconform- ably overlain by shallow-marine conglomerate, sandstone, and siltstone of the Beechill Cove Formation. Paleontologi- cal evidence indicates that the Beechill Cove Formation is early Llandovery in age (Boucot et al. 1974; Pickerill and Hurst 1983). The formation grades upwards into the middle to late Llandovery black shale, muddy siltstone, tuff, and arenaceous limestone of the Ross Brook Formation (Boucot et al. 1974; Hurst and Pickerill 1986). These rocks are con- formably overlain by a thick succession of middle to Late Silurian rocks dominated by greenish-grey mudstone and interbeds of siltstone and sandstone that were deposited mainly in a nearshore environment. Current directions indi- cate that the sediments were derived from the northeast throughout the Early Silurian (Boucot et al. 1974).

The Beechill Cove Formation is described in detail in Pickerill and Hurst (1983). It is about 80 m thick, and con- tains six recognizable facies. These are (i) a conglomerate facies, which represents a transgressive marine lag; (ii) a red shale facies, deposited in a nearshore environment; (iii) a mottled mudstone facies displaying extensive bioturbation, which is indicative of shallow subtidal areas and low sedi- mentation rates; (iv) regular layered facies with shelf turbidites generated by storm activity superimposed on a quiescent subtidal environment; (v) a lenticular facies induced by storm activity and deposited in a shallow subtidal environ- ment; and (vi) laminated shale facies produced by low- energy deposition from suspension.

Geochemical and isotopic signature of the Beechill Cove Formation

Representative samples of medium-grained sandstones and siltstones from facies iv and v of Pickerill and Hurst (1983)

were selected for analysis (Table 1). They generally consist of angular to subangular quartz, plagioclase that displays moderate alteration to sericite, detrital muscovite, and chlorite. The samples were analyzed for major and selected trace elements by X-ray fluorescence at the Nova Scotia Regional Geochemical Centre at St. Mary's University, Halifax. Details of the analytical methods are given in Murphy et al. (1991). Five representative samples were ana- lyzed for rare earth elements (REE) by inductively coupled plasma mass spectrometry at Memorial University of New- foundland (Table 1). The five samples were also analyzed for Sm -Nd and Rb -Sr isotopic compositions at the Atlantic Universities Regional Isotopic Facility (AURIF) at Memorial University of Newfoundland (Table 2). The accuracy and precision of both the REE and isotopic analyses are generally better than 10%.

On a volatile-free basis, Si02 ranges from about 65 to 90 wt. %, M203 from 4.1 to 20 wt. %, CaO from trace amounts (<0.05) to 3.7 wt.%, F q 0 3 from 1.7 to 7.0 wt.%, and Ti02 from 0.42 to 1.02 wt. %. On plots such as log Si02/A1203 versus log Na20/K20 (Fig. 2a) (after Bhatia 1983), the rocks display a strong positive correlation and plot in the "arkose to subarkosic field," and on a Si02 versus K20/Na20 plot, the analyses straddle the divide between "passive and active continental margin" rocks (Fig. 2b) (after Roser and Korsch 1986). The relatively low Ti02 (Table I), Fez03 + MgO, and M2O3/SiO2 (Figs. 2c-2e) (after Bhatia 1983) are typical of upper crustal rocks, as are the K/Rb ratios of about 200 (Fig. 3) (ave. 230; Taylor and McLennan 1985), and suggest that the contribution from the underlying mafic rocks was very limited. In general, any inferences based on the discrimination diagrams are con- sidered suspect. For example, the apparent passive margin signature implied by Figs. 2b and 2c could equally be attributed to a source region lacking mafic rocks, and (or) to a degree of maturity and reworking related to significant dis- tances of sediment transport.

Tab

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yses

of

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(PP

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Sr

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Pb

Ga

Zn

Cu

Ni v Cr

La

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Nd

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and

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ith the e

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an 1

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are

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Murphy et al.

Table 2. Isotopic data from the Beechill Cove sedimentary rocks.

Sample Sm Nd I ~ ~ s ~ / ' " N ~ (24 '43Nd/'"Nd (24 EM^ TDM 87Sr186Sro

Notes: Chemical separations and isotopic analyses were determined at the Atlantic Universities Regional Isotopic Facility, Memorial University of Newfoundland. Nd and Sm were determined by inductively coupled plasma mass spectrometry. The '43~d / '"~d ratios are measured by thermal ionization mass spectrometry, after chemical separation of Nd from Sm and other REE by ion-exchange chemistry. ' 47~m/ '"~d ratios were determined by isotope dilution. La Jolla Nd standard gave an average value of 0.511862 + 0.000016. cNd values are relative to ' 4 3 ~ d / ' 4 4 ~ d = 0.512638 and ' 4 7 ~ m / 1 4 4 ~ d = 0.196593 for present-day CHUR (Jacobsen and Wasserburg 1980), and As, = 6.54 x 10-'~/~ear. TD, are calculated using the model of DePaolo (1981, 1988). Isotopic data for the Georgeville Group sedimentary rocks may be found in Murphy and MacDonald (1993) and for the Late Proterozoic to Early Silurian volcanic rocks in Murphy et al. (1996).

Rare earth elements (REE) are generally a more reliable indicator of provenance and tectonic setting because they are relatively unaffected by sedimentary processes such as weathering, sorting, or diagenesis (e.g., McLennan et al. 1980; Bhatia and Crook 1986) and because they have short residence times in seawater (Holland 1978). Chondrite- normalized REE patterns (Fig. 4) are moderately sloping, and display moderate light REE (LREE) enrichment (La,,/ Yb, = 4.5 - 11; Cen/Ybn = 7.3 - 19). These profiles exhibit some important distinguishing characteristics, such as high normalized Ce relative to La, low Yb and Lu, and a slight negative Eu anomaly. The high Cen/Lan is unusual, and its origin is unclear. In general, the Beechill Cove pro- files are similar to those of modern intracontinental rifts (see McLennan et al. 1990). One sample (BC-58) with relatively high REE and relatively flat LREE occurs in rocks with unusually high P2O5 (0.75 wt.%) and Y (93 ppm), suggest- ing that its chemistry may be influenced by the presence of authigenic apatite.

Rb - Sr and Sm - Nd isotopic data for sedimentary rocks of the Beechill Cove Formation are listed in Table 2. The sedimentary rocks are characterized by initial 87Sr/86Sr ratios ranging from 0.7070 to 0.7095 and strongly negative E ~ ~ ( u ~ ) (Table 2). Although Rb-Sr isotopic systematics are generally sensitive to intracrustal processes, SmINd ratios are rarely affected (e.g., Nelson and DePaolo 1988) and for sedimentary rocks may yield valuable information on the source area. With the exception of sample BC-58, 6 ~ d and TDM(Nd) model ages are tightly clustered, ranging from -5.66 to -6.08 and 1.422 to 1.731 Ga, respectively (cal- culated for a 430 Ma depositional age). This Nd isotopic sig- nature is interpreted to represent the weighted average of EN^ values for detrital contributions from the source area and the range in values may be attributed to isotopic hetero- geneities of the source rocks (see Nelson and DePaolo 1988). Sample BC-58, however, has significantly lower E~~ and higher TDM. Given the high P2O5 of this sample, and the possible influence of authigenic apatite, the data for this sample are viewed with caution.

Comparison with typical Avalonian rocks

The Beechill Cove Formation overlies a typically Avalonian late Precambrian to earliest Silurian succession. To assess

the contribution of underlying Avalonian rocks to the geo- chemistry and isotopic signature of the Beechill Cove Forma- tion, two representative suites were selected for comparison (Figs. 2 -5): Late Proterozoic sedimentary rocks (George- ville Group) and Late Ordovician - Silurian volcanic rocks (Dunn Point and Beechill Cove formations). The ca. 615 Ma Georgeville Group sedimentary rocks underlie most of the Antigonish Highlands and are an example of sedimentary rocks derived almost exclusively from erosion of adjacent coeval Avalonian volcanic rocks (Murphy and MacDonald 1993). The Late Ordovician - earliest Silurian volcanic rocks at the base of the Arisaig Group which immediately underlie the Beechill Cove Formation, also display a typi- cally Avalonian signature (Murphy et al. 1996). These data relate to the lower crustal - mantle source of the magmas rather than near-surface sedimentary sources. However, once extruded, they could provide a potential source for the overlying sediments.

The geochemical and isotopic signatures of the Beechill Cove Formation show such fundamental geochemical and isotopic differences from both representative suites that a sig- nificant chemical contribution from either of these suites is unlikely. For example, the Georgeville Group rocks contain higher Fe203 and MgO and lower Si02 and consistently plot in active continental margin or arc-related fields on dis- crimination diagrams. KIRb ratios (Fig. 3) range from 200 to 600 in the Georgeville Group rocks (compositional limits shown by the slopes in Fig. 3), are higher than typical upper crustal rocks, and reflect a more significant contribution of mafic detritus to the geochemistry of the sediments (Taylor and McLennan 1985). Georgeville Group and Beechill Cove Formation REE profiles show some important distinguishing characteristics. In comparison, Georgeville Group profiles do not display an Eu anomaly, are enriched in normalized La relative to Ce, and have higher heavy REE (HREE). These features are exemplified by the significantly lower Cen/Yb,, which ranges from 3.8 to 5.6, in contrast with the Beechill Cove Formation values, which ranges from 7.3 to 19.0. In comparison with the Beechill Cove Formation, Georgeville Group sedimentary rocks display higher 6Nd(ur) (ranging from +O. 16 to +4.39, calculated for a 615 Ma depositional age) and lower TDm(Nd) model ages (0.959 - 1.145 Ga) that are similar to the TDM ages of the coeval Precambrian mafic and felsic volcanic rocks (Murphy and MacDonald 1993;

384 Can. J. Earth Sci. Vol. 33, 1996

Fig. 2. Geochemical plots showing the contrasting compositions of Early Silurian Beechill Cove Formation (open squares) and Late Proterozoic Georgeville Group (solid squares) sedimentary rocks. (a) Log Na20/K20 vs. log Si021A1203 (after Bhatia 1983). Fields for sediment compositions: arc, volcanic arc; acm, active continental margin; pm, passive margin. (b) K,O/Na,O vs. SiO, (after Roser and Korsch 1986). (c) A1,03/(Ca0 + Na20) vs. Fe203 + MgO (after Bhatia 1983). (d) A1,03/Si02 vs. Fe203 + MgO (after Bhatia 1983). (e) K,O/Na,O vs. Fe203 + MgO (after Bhatia 1983). Compositional fields of coeval felsic rocks (crosses) and basalts (stipples) from the immediately underlying Dunn Point and Bears Brook formations are also shown. The Beechill Cove Formation data are given in Table 1, the Georgeville Group sediment data in Murphy and MacDonald (1993), and the Bears Brook and Dunn Point formations data in Murphy (1987) and Keppie et al. (1979), respectively. In (b)-(e), fields for sediment compositions are as follows: 1, ocean island arc; 2, continental island arc; 3, active continental margin; 4, passive margin.

log Si02/A1203

Fe203 t MgO (wt.%)

r 5 10 15 20

Fe203 + MgO (wt,%)

Murphy et al. 1996). Although the Beechill Cove rocks have significantly higher initial 87Sr/86Sr ratios (Georgeville rocks range from 0.7021 to 0.7039), the significance of this in terms of provenance is unclear, as Beechill Cove values are comparable to that of Early Silurian seawater (approx.

00 h 0 5 10 15 20 25 30

F 2 4 + MgO (wt.%)

0.7080; e.g., Burke et al. 1982). However, if Avalonian detritus were abundant in the Beechill Cove rocks, one would expect initial 87Sr/86Sr ratios between those of the George- ville Group and 0.7080.

Similar contrasts between the Beechill Cove Formation

Murphy et al.

Fig. 3. K20 vs. Rb plot for the Beechill Cove sedimentary rocks. The average slope of the continental crust (KIRb = 230; Taylor and McLennan 1985) is also shown. Compositional fields of the immediately underlying felsic (crosses) and mafic volcanic rocks (stipples) of the Late Ordovician - Early Silurian Durn Point and Bears Brook formations are also shown. The Georgeville Group sedimentary rocks lie between KIRb 200 and 600. For details see Fig. 4 of Murphy and MacDonald (1993). Symbols as in Fig. 2.

250

I - zoo D IT

CONTINENTAL

100

50

0 0 1 2 3 4 5 6 7 8

Fig. 4. Chondrite-normalized rare earth element (REE) patterns for representative samples of the Beechill Cove sedimentary rocks (see Table 1). The range in normalized REE compositions for the Georgeville Group turbidites is shown between the heavy lines (after Murphy and MacDonald 1993).

and the immediately underlying subaerially deposited Dunn Point Formation volcanic rocks are evident in Figs. 2 -4. In general, Beechill Cove compositions do not lie between the compositions of these volcanic rocks, implying that they are not simply derived from them. In comparison with the Beechill Cove Formation, Arisaig Group volcanic rocks have higher L ~ ~ ( u ~ ) (ranging from - 0.1 1 to + 5 .O8, calcu- lated for a 430 Ma extrusive age) and lower TDM(Nd) model ages (0.878-1.046 Ga, Murphy et al. 1996).

Fig. 5. Initial E~~ values plotted against age for the Beechill Cove Formation sedimentary rocks (open squares). Georgeville Group sedimentary rocks are shown for comparison (solid squares). Isotopic data for these rocks may be found in Murphy and MacDonald (1993). Solid line outlines the envelope for Nd isotopic composition for Avalonian rocks in mainland Nova Swtia (Murphy et al. 1996; Nance and Murphy 1994). The isotopic data for the Late Proterozoic - Early Silurian volcanic rocks in the Antigonish Highlands may be found in Murphy et al. (1996). Broken lines outline the Grenvillian envelope (see Barr and Hegner 1992) from the data of Patchett and Ruiz (1989). The depleted mantle curve shown is an average upper limit for the depleted mantle.

/' CHUR ,'

/ ,'

/ /

/ /'

/

/ / 1 I I r I I

02 04 0.6 0.8 1.0 12 1.4 1.6

Age (Fa)

Significantly, the Beechill Cove sedimentary rocks lie well outside the envelope of typical Avalonian igneous and sedimentary rocks (Fig. 5) (Murphy et al. 1996; Nance and Murphy 1994). This envelope includes the data from the immediately underlying volcanic rocks. However, they com- pare favourably with Nd isotopic compositions expected for rocks derived from Grenvillian crust (Fig. 5).

Provenance and tectonic implications

One of the main difficulties in interpreting the tectonic set- ting of ancient sedimentary rocks is that the source area is often displaced from the sedimentary sequences by subse- quent tectonic activity. This paper suggests that sediment geochemistry may be a useful tool in tectonic syntheses and is especially powerful when sedimentary samples are com- pared with igneous rocks in the same terrane. The com- parison facilitates evaluation of the overstepping of terrane boundaries by sedimentary rock units.

The tectonic setting of the Beechill Cove Formation sedi- mentary rocks is constrained by its geochemical and isotopic signatures. These signatures are thought to represent the average values of the detritus from the source regions and are inconsistent with derivation from underlying Avalonian source rocks. Although the Rb - Sr isotopic system is highly

Can. J. Earth Sci. Vol. 33, 1996

Fig. 6. An example of a Silurian reconstruction, showing the relative positions of Avalonia, Laurentia, Baltica, and South America (after Keppie et al. 1996). G and M, Gander and Meguma terranes, respectively. The heavy line approximates the axis of Late Ordovician - Early Silurian orogenesis in Laurentia - Baltica.

MID-SILURIAN

sensitive to a wide variety of intracrustal processes, the con- trast between the Georgeville Group and Beechill Cove For- mation sedimentary rocks supports the conclusion that the Beechill Cove Formation was predominantly derived by erosion of radiogenic non-Avalonian crust.

The similarity of isotopic data from the Beechill Cove Formation and the Grenville is compatible with a Grenvillian source for the sediments. Available paleocontinental recon- structions provide further constraints (e.g., Fig. 6). Grenville- age rocks are present in the Sunsas Orogen along the southwestern side of the Amazonian craton, possibly in the Guyana craton, in northwestern Cape Breton Island (Blair River Complex), along the eastern margin of Laurentia, and along the southwestern margin of Baltica. To discriminate between these potential sources, additional geological data must be considered. The northeast to southwest current direc- tions in the Beechill Cove Formation (Boucot et al. 1974) argue against derivation from the Sunsas Orogen or the Guyana craton. The Blair River Complex is an unlikely source for the Beechill Cove sediments because Keppie et al. (1992) have inferred that the complex underwent high-grade metamorphism during the Early Silurian and was not exhumed until the Early Devonian. This leaves eastern Laurentia or western Baltica as the most likely source of the Beechill Cove sediments. Laurentia and Baltica had very similar geological histories throughout the Proterozoic, sug- gesting evolution as a single continental plate (e.g., Gower et al. 1990). Thus, Paleozoic sediments derived from either Grenvillian landmass could provide detritus with the geo- chemical and isotopic composition of the Beechill Cove For- mation. However, as noted by McKerrow et al. (1991), Laurentia was covered by a carbonate platform throughout much of the Ordovician, and a strictly Laurentian source for the Beechill Cove clastic rocks is unlikely. Reconstructions

illustrating the closure of Iapetus (Fig. 6) suggest that the sediments may have been shed from the Caledonide orogenic belt, formed by the collision between Laurentia and Baltica. Derivation from this belt could provide clastic rocks of the appropriate chemical composition and would be consistent with the southwesterly directed paleocurrent data in the Beechill Cove Formation.

Recent geochronological data (Dunning et al. 1990; Cawood et al. 1994) suggest the importance of an Early Silurian (Salinic) orogeny along both the Laurentian and Avalonian margins in the Newfoundland Appalachians. Keppie (1993~) and Keppie and Dostal (1996) infer that Late Ordovician - Early Silurian structures in the northern Appa- lachians represent accretionary deformation. These interpre- tations support collision between Avalonia and Laurentia by the Early Silurian. The proposal of Chandler et al. (1987) that Early Silurian volcanic rocks form part of an overstep sequence across the entire Appalachian Orogen is consistent with this interpretation.

In summary, the time of accretion of Avalonia can be con- strained by comparing the geochemical and isotopic signa- tures of clastic sedimentary rocks (which reflect provenance) with underlying sequences (which reflect the chemistry of the underlying crustal and mantle sources). The geochemical and isotopic signature of the Early Silurian Beechill Cove Formation cannot be derived from the underlying Avalonian crust. The formation is derived from an adjacent continental landmass with Grenvillian crust, supporting the general infer- ences from paleocontinental reconstructions that suggest shrinking of the Iapetus Ocean during the Ordovician. The limitations of the data are also evident. The data, on their own, cannot distinguish between Laurentia and Baltica as source areas, because Laurentia and Baltica have had similar Proterozoic histories and are likely to yield sediments with similar average chemical composition. However, paleo- continental reconstructions suggest the Caledonide orogenic belt as the most likely source.

Acknowledgments

This work was supported by Natural Sciences and Engineer- ing Research Council of Canada general grants and Litho- probe grants to J.M.B., J.D., and J.W.F.W., the James Chair of Pure and Applied Sciences at St. Francis Xavier University to J.D.K., Canada - Nova Scotia Mineral Devel- opment Agreements, the Geological Survey of Canada, and the University Council for Research at St. Francis Xavier University. We are grateful to Stuart McKerrow, Mike Melchin, and Kevin Pickering for discussions, Ken Currie and Brian Fryer for constructive reviews, Pat Horan at the Atlantic Universities Radiogenic Isotopic Facility, Memorial University of Newfoundland, and Troy Wilcox for technical assistance.

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