550-580 Ma magmatism in Cape Breton Island (Nova Scotia,Canada): the product of NW-dipping subduction during the final stage of amalgamation of Gondwana

E L S E V I E R Precambrian Research 76 (1996) 93-113

Pre(umbriun Reseurth

550-580 Ma magmatism in Cape Breton Island (Nova Scotia, Canada)" the product of NW-dipping subduction during the final

stage of amalgamation of Gondwana

J. Dostal a, J.D. Keppie b, B.L. Cousens c, J.B. Murphy b "DepartmentofGeology, Saint Mary's University, Halifax, N.S. B3H 3C3, Canada

b Department of Geology, Saint Francis Xavier University, Antigonish, N.S. B2G 2W5, Canada c Department of Earth Sciences, Carleton University, Ottawa, Ont. KIS 5B6, Canada

Received 27 September 1994; revised version accepted 18 July 1995

Abstract

Cape Breton Island is host to widespread magmatic activity dated between ~ 550 and 580 Ma. In the south, its products include the Capelin Cove pluton and host Coastal Volcanic Belt; both are composed of bimodal, tholeiitic--calc-alkaline suites that were emplaced on a thin sialic crust in a volcanic arc system. Their trace element and Nd isotope variations are consistent with fractional crystallization accompanied by lower crustal contamination in a compositionally zoned magma chamber. In contrast, the Indian Brook and Kerrs Brook plutons and Price Point volcanic rocks in central Cape Breton Island have higher contents of strongly incompatible trace elements and resemble modem arc-related calc-alkaline rocks emplaced on a moderately thick continental crust. Comparison with available data for other 550-580 Ma igneous rocks across Cape Breton Island indicates that they were produced above a NW-dipping subduction zone. This convergence was synchronous with the final stage of global- scale orogenic activity that culminated with the final amalgamation of Gondwana. Available detrital zircon ages in late Neopro- terozoic ( ~ 650-545 Ma) sediments in the Antigonish Highlands (Avalon Composite Terrane) suggest derivation from the Amazon craton. It is proposed that the Avalon Composite Terrane formed off northwestern South America during the late Neoproterozoic,

1. Introduct ion

Neoproterozoic calc-alkaline igneous rocks are an important component of Pan-African and Brasil iano orogens. Those that occur in the Avalon Composite Terrane of Atlantic Canada (Fig. 1) have generally been correlated with northwest Africa (e.g., O 'Br ien et al., 1983). However, igneous activity in the Trans- Sahara orogen occurred between 800 and 600 Ma (Caby and Andreopoulos-Renaud, 1987), and between 700 and 580 Ma in the Reguibat Shield (Ducrot and Lancelot, 1978). In contrast, igneous activity in the

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Avalon Composite Terrane lasted from ~ 700 Ma until ~ 5 5 0 Ma (Keppie et al., 1989; Dunning et al., 1990;

Barr et al., 1990; Bevier et al., 1993). Such a span of Neoproterozoic calc-alkaline magmatism is also to be found in the Amazon craton (Teixiera et al., 1989) and is attributed to the final amalgamation of Gondwana. In this context, the 550-580 Ma magmatic arc rocks in the Avalon Composite Terrane probably formed during the closing stages of amalgamation of Gondwana.

In southern Cape Breton Island, Neoproterozoic rocks occur in several fault-bounded blocks termed (from southeast to northwest) the Coastal, Stirling,

94 J. Dostal et al. / Precambrian Research 76 (1996) 93-113

n NEOPROTEROZO,C __OAP _ ,'1 ROCKS ~ / - f 7 ~ O . ~ . ~ H H I " I U N / ~ .~\,4 i

~ , u , O ~ ISLAND j~j / o~,%,~ / • c,~ ~,~ " ~ J ~ , _ " ~ ' ~ [ - ~ " ~ ANTIGONISH / I J ~ /* G~'- [ ~ ~ ~ C~. HIGHLANDS ~ " ~ " ./~.O~Z~. * ~ /

NOVA S C O T I A ( A ) i 10-0 i~m ' /

~ CAMBRO-O.DOV,C,AN ( B ) OVERSTEP SEQUENCE

550-580 Ma VOLCANIC ROCKS 550-580 Ma PLUTONIC ROCKS 500-700 Ma IGNEOUS ROCKS NEOPROTEROZOIC CHETICAMP METASEDIMENTS i PLUTON

30 km

~,\LL

EASTERN CAPE

BRETON HIGHLAND: IGNEOUS

SUITE

PLUTON

~ STIRLING BLOCK

BOISDALE HILLS _ PLUTON

BLOCK

COASTAL BLOCK

CAPELIN COVE

PLUTON

MURRAY BROOK

gd I

2 km

JL_/Lj~.

MAIN A DIEU GROUP

~FOURCHU GROUP

Fig. 1. Simplified geological map of major Neoproterozoic tectonostratigraphic units of Cape Breton Island. (A) Sketch map of Nova Scotia showing the distribution of Neoproterozoic rocks. (B) Neoproterozoic tectonostratigraphic units in Cape Breton Island (modified after Barr, 1993 ). ( C ) Neoproterozoic tectonostratigraphic units of Eastern Cape Breton Highlands ( modified after MacDonald and Barr, 1985 ).

J. Dostal et al. /Precambr ian Research 76 (1996) 93-113 95

East Bay Hills and Coxheath blocks (Fig. 1B). The mafic volcanic rocks show a compositional zonation ranging from island arc tholeiitic along the southeastern coast to calc-alkaline inland to the northwest (Dostal et al., 1990). Farther west, in the Antigonish Highlands, the Neoproterozoic rocks consist of turbidites and interbedded calc-alkaline and rift tholeiitic volcanic rocks inferred to have been deposited in a back-arc or intra- arc basin (Murphy et al., 1990). Assuming that all these volcanic rocks were approximately contempora- neous, Keppie and Dostal (1991) attributed the arc/ back-arc geometry and the gradual change in geochemistry across southeastern Cape Breton Island to trans-arc variations above a NW-dipping subduction zone. Subsequent age dating has shown that the volcanic rocks range in age from ~681 +6_ 2 Ma in the Stirling block through ~ 620 Ma in the East Bay Hills and Coxheath blocks to ~ 560-575 Ma in the Coastal block (Bevier et al., 1993). This led Barr (1993) to question the model of Keppie and Dostal ( 1991 ). Thus, more detailed studies of rocks formed in narrower time intervals are needed to provide additional constraints on the various tectonic models for Cape Breton Island.

In this paper, we focus attention on 550-580 Ma volcanic and plutonic rocks that occur throughout Cape Breton Island (Fig. 1B). In the southeastern coastal area, they consist mainly of volcanic rocks intruded by a single Neoproterozoic pluton (Capelin Cove pluton). In central Cape Breton Island, they form an extensive belt of predominantly plutonic rocks with minor extru- sive units extending from the eastern Cape Breton Highlands to the Creignish Hills in southwestern Cape Breton Island. In western Cape Breton Island, this magmatic event is represented only by the Cheticamp pluton, 2 5 × 5 km in size (Fig. 1B).

The aims of this paper are: ( 1 ) to evaluate the petrogenesis of the 550-580 Ma volcanic-plutonic asso- ciation in southern Cape Breton Island (Capelin Cove pluton and Coastal Belt volcanics) using major and trace element data and Nd isotopic ratios; and (2) to compare the geochemistry of these rocks with coeval igneous bodies across Cape Breton Island, particularly those in the central part of the island, in order to arrive at a tectonic model for the 550-580 Ma period and suggest its place relative to Gondwana.

2. Geological setting

The Coastal block extends for more than 80 km along the southeastern coast of Cape Breton Island (Fig. 1 ). It is underlain by two groups of rocks: Fourchu Group in the southwest, which is overlain by the Main-a-Dieu Group in the northeast. The Fourchu Group consists predominantly of felsic and mafic pyroclastic rocks interbedded with some flows, whereas the Main-a-Dieu Group is made up of bedded volcaniclastic rocks and flows and tufts that are mainly felsic with minor mafic components (Keppie et al., 1979; Barr, 1993). The pyroclastic rocks at the top of the Fourchu Group are mainly subaerial but pass upwards into shallow sub- marine rocks in the Main-a-Dieu Group. These rocks are cut by sills, dikes and plugs. The lava flows and associated sills and dikes are bimodal. The main primary mineral phases of mafic rocks, augite and plagioclase, have been variably replaced by secondary minerals, particularly albitic plagioclase, actinolite, epidote, chlorite and calcite (Dostal et al., 1990). The felsic lavas and sheets are composed of quartz and feldspars with subordinate sericite, chlorite, epidote and opaques. In the southwest, the Capelin Cove pluton underlies an area about 10 km 2 (Fig. IB) and is well exposed along a 3 km coastal section. There are very few outcrops inland. Although the main contacts are not exposed, an intrusive relationship is indicated by the occurrence of rafts and xenoliths of volcanic rocks within the pluton. Also, a small, foliated apophysis of the main pluton intrudes the volcanic rocks about 1 km east of the pluton. The pluton consists mainly of diorite and tonalite-granodiorite that are in sheared contact. The diorite, which is less abundant than tonalite-granodiorite, occurs mainly in the core of the pluton.

Tonalite-granodiorite (henceforth, tonalite) of the Capelin Cove pluton is typically composed of plagioclase (zoned andesine) and quartz with about 10% K- feldspar (microperthitic microcline) and less than 10% of mafic minerals, mainly biotite and less abundant amphibole. Accessory minerals include sphene, apatite, zircon, ilmenite and magnetite; secondary phases include abundant epidote, aibitic plagioclase and chlorite. Diorite to quartz diorite (henceforth, diorite) con- tains zoned plagioclase (andesine-labradorite) and hornblende with subordinate quartz and biotite. Acces- sory and minor phases are similar to those in the tonalite


including the occurrence of both magnetite and ilmenite (Starzynski, 1993).

The tonalitic part of the Capelin Cove pluton has yielded a U-Pb zircon age of 574_+ 3 Ma (Bevier et al., 1993). A felsic tuff in the Fourchu Group, 35 km northeast of the intrusion, yielded a U-Pb zircon age of 574+4_1 Ma (Bevier et al., 1993). The Main-a- Dieu sequence is constrained between a discordant, U - Pb zircon maximum age of ~ 563 Ma from a rhyolite near the base of the Main-a-Dieu and the overlying basal Cambrian units comparable with those defining the Precambrian-Cambrian boundary in eastern New- foundland (Landing, 1991; Bevier et al., 1993). Recent estimates place the age of the Precambrian~Cambrian boundary at ~ 545 Ma (Keppie et al., 1990). Thus, the volcanic activity in the Coastal block appears to have lasted from at least 580 to 550 Ma. Although the Cap- elin Cove pluton appears to intrude the host Fourchu Group, it probably represents a plutonic equivalent of the Fourchu volcanic edifice.

The Neoproterozoic igneous rocks from central Cape Breton Island analyzed in this study are part of an extensive belt of granitoid plutons that underlie the southeastern Cape Breton Highlands. U-Pb age deter- minations range from ~ 550 to 600 Ma (Dunning et al., 1990; Dallmeyer and Keppie, 1993). Similar Neo- proterozoic plutons occur in pre-Carboniferous basement blocks throughout central Cape Breton Island, but Neoproterozoic volcanic rocks are uncommon in this part of the island; the most prominent volcanic rocks are those from the Price Point Formation (MacDonald and Barr, 1985).

The Price Point Formation occurs at the southeastern end of the Eastern Cape Breton Highlands (Fig. 1C). It consists predominantly of andesitic tufts and flows intruded by mafic dikes. The volcanic rocks locally display a well preserved magmatic texture; however, mineral assemblages are mainly the result of greenschist-facies metamorphism. The mafic and intermediate types only rarely contain relics of primary clinopyroxene and hornblende. The Price Point Formation is undated, but it is intruded by the Indian Brook granodiorite (dated at 575-572 Ma: 2°6pb-235U ages of slightly discordant zircons; and 564 ___ 5 Ma: U-Pb concordant titanite age; Dunning et al., 1990). The Indian Brook granodiorite (Fig. 1C) and the correlative Kerrs Brook granodiorite (Raeside and Barr, 1992) have variable compositions although granodiorite is predomi-

nant. The rocks are composed mainly of primary plagioclase (oligoclase-andesine), perthitic microcline, quartz, hornblende and biotite. Minor and accessory minerals include sphene, Fe-Ti oxides, zircon and apatite. Greenschist-facies alteration is widespread. As is shown below, these volcanic and plutonic rocks share a common magmatic arc signature suggesting that they are related.

3. Geochemistry--analytical methods and alteration

In this study, we analyzed the following representative samples: 26 samples from the Capelin Cove pluton, 10 volcanic rocks from the Coastal block (the latter selected from the suites studied by Dostal et al., 1990, 1992), 19 volcanics from the Price Point Formation and 13 samples from the Indian Brook and Kerrs Brook granodiorites. In addition, some of the graphs (Figs. 2-5) also show data from Capelin Cove pluton of Star- zynski (1993) and the Coastal Belt of Dostal et al. ( 1990, 1992). As the rocks were affected by secondary processes which appear to have modified their composition, the samples were petrographically and chem- ically screened and those which were strongly altered were discarded. To further reduce the alteration problem, the study focuses on trace elements which are thought to be relatively 'immobile' during low-grade metamorphic processes (e.g., Winchester and Floyd, 1977).

The samples were analyzed by X-ray fluorescence for major and some trace elements (Rb, Sr, Ba, Zr, Nb, Y, Cr, Ni) and by the inductively coupled plasma-mass spectrometry (ICP-MS) or instrumental neutron acti- vation (INAA) for Hf, Th and the rare earth elements (REE). Precision and accuracy are discussed in Jenner et al. (1990) and Dostal et al. (1986, 1994), respectively. In general, precision is better than +5% for major elements and 2-10% for trace elements. Three samples were analyzed by both ICP-MS and INAA to test the comparability of the data sets; agreement between the methods was good.

Nd isotopic ratios in 16 samples of the Capelin Cove, Coastal block and Price Point Formation were determined at Carleton University using techniques described in Cousens et al. (1994). 147Sm/144Nd ratio was determined by isotope dilution mass spectrometry.

J. Dostal et al. /Precambrian Research 76 (1996) 93-113 97

Cape/in / Coastal 80 80

75.

70.

6 5

o ~ 60 v

OJ O

55

50

4 5

it Capelin Diorite

• Capelln Tonallte

O Coastal Marie

X Coastal Felsic

Rhyolite

Rhyodacite / Daclte • • •

× A

Andeslte

75-

70-

° i

65-

• / A .-~'~",~ 60-

Phonollte 55-

o O / /

o O/O/ / Basalt O ~ Bas / Trach I Neph I

Alkaline Basalt

45-

40 . . . . . . . . , . . . . . . . . , t 40 0001 0.01 0.1 0.2 0001

Z r / TiO 2

Indian Brook/Price Point

[3 Indian Brook I~

• Indian Brook (F&B) Rhyolite

+ Price Point Dike 9

ix Price Point Lava []

Rhyodaclte / Daclte ~ O ~ _ _ _ _ ~

. / / J ~ Phonollte

'he +/

Alkaline Basalt

. . . . . . . oo71 . . . . . . . . , 0.1 0.2

Zr / TiO 2

Fig. 2. Zr/TiO2 versus SiO2 (%) diagram of Winchester and Floyd (1977) for (A) the rocks of Capelin Cove pluton, Coastal Volcanic Belt, and (B) Price Point and Indian Brook (Indian Brook and Kerrs Brook plutons) formations. The Indian Brook data (marked F&B) are from Farrow and Barr (1992).

The 143Nd/L44Nd was normalized to t46Nd/ 144Nd = 0.7219. The La Jolla Nd standard analyzed as part of every run yielded average 143Nd/ 144Nd=0.51187+1. Initial ratios were calculated using available U-Pb zircon ages of Bevier et al. (1993) for the Capelin Cove and Coastal Belt. Epsilon values (eNd) were calculated based on the above ages and modern 143Nd/144NdcHuR = 0.512638, and 147Nd/ 144NdcHuR = 0.1967. Four runs of USGS standard rock BCR-1 yielded Nd = 29.02_ 0.15 ppm, Sm=6 .70+0 .04 ppm, Sm/Nd=0.231 and t43Nd/ 144Nd = 0.512659 -t- l0 (all quoted errors are two standard deviations of the mean). Samples were not normalized to the La Jolla standard, since the average determined over this time period is well within the uncertainty of the La Jolla ratio. Uncertainties in the ~47Sm/t44Nd are less than 1% and duplicate analyses of samples shows that uncertainties in the 143Nd/144Nd

are less than 0.00002. The uncertainty in initial end is ___ 0.4 epsilon units. Depleted mantle model ages (TDM) were calculated assuming modern depleted mantle (DM) with 143Nd/144Nd=0.513114 and 147Sm/ l~Nd : 0.213.

4. Coastal belt

4.1. Plutonic rocks

The two petrographically distinct rock types of the Capelin Cove pluton differ in chemical composition (Table 1). The diorites have SiO2 contents (on LOI- free basis) ranging between 56 and 60% whereas the tonalites have concentrations of SIO2>68%. The majority of tonalite samples are within the SiO2 interval of 71 to 75% (Fig. 2). According to the Fe and Ti

98 J. Dostal et al. / Precambrian Research 76 ( l 996) 93-113

;>

o r..)

"a

g

~ . . . . . . . o - - - d d d d ~ 2 d ~

~- ~ , - ~ - ' 2 ~d~ • ~ t ~ ~ ~ , ~ , D ~ , o . . , ~

1",-i

oo

_ o . . . . ¢¢3, ~

J. Dostal et al. / Precambrian Research 76 (1996) 93-113 99

1.6

1 .4

1 2

1.0

0.8

o_ t--

1.8

A CapeUnDiorite ] / Thole.tic

O_ Coastal Mafic J / O O

/ O

0 0 0 t~)

o o / o ,

Calc-alkaline 0.6 , , , , , . . . . ,

0.0 0,5 1.0 1.5 2.0 2.5 3.0 3.5

FeO t / MgO

Fig. 3. FeOJMgO versus TiO2 (%) for mafic rocks of the Capelin Cove pluton and Coastal Volcanic Belt. The vectors depict tholeiitic and calc-alkaline trends, respectively.

ment patterns of the tonalites are also similar to those of the diorites. They exhibit a decrease from Th to Eu with distinct negative anomalies of Nb and Ti (Fig. 6). This distribution of incompatible trace elements and high Na20/K20 resembles those of felsic rocks from volcanic arc systems (Condie, 1989).

The 143Nd/ln4Nd ratio has been measured in three diorites and two tonalites (Table 2). All the analyzed samples have positive ENd, and their Nd isotopic signatures approach those of depleted mantle at 570-575 Ma (DePaolo, 1988). The diorites have a small range in isotopic composition, with end between + 5.1 and + 5.3; the low-silica tonalite (18a) has an ENj of + 5.5, whereas the tonalites have higher er~d of + 7.4 to + 7.9. Thus, there is a rough correlation between SiO2 content and eNO, with the lowest end in the low-silica rocks.

variations, the diorites are transitional between calc- alkaline and tholeiitic liquid lines of descent (Fig. 3). Both rock types form coherent trends for numerous elements including Ti, Fe, Ca and V with increasing of SiO2 (Fig. 4), suggesting that fractional crystallization was a dominant process in the liquid line of descent within each group.

The dioritic rocks have high FeOJMgO ratios (Fig. 3 ), low transition element contents and high CaO (Fig. 4). Their REE patterns show fiat heavy rare earth element (HREE) profiles accompanied by moderate light rare earth element (LREE) enrichment (Fig. 5). La/ Yb ratios range from 4 to 7 and there is no Eu anomaly. The mantle-normalized incompatible element profiles are characterized by a moderate enrichment of strongly incompatible elements (Th, La) and negative anomalies for Nb, Zr, Hf and Ti (Fig. 6). These patterns resemble those of modern island arc basaltic andesites and andesites (Pearce, 1982, 1983), and suggest that the rocks are the result of subduction-related processes.

The tonalites have high Na20/K20 ratios, usually > 2. The chondrite-normalized REE patterns of these rocks (Fig. 5 ) exhibit moderate LREE enrichment and a flat HREE segment, with a La/Yb ratio of about 4 - 7. The REE patterns of diorites and tonalites are similar in shape, but tonalites have slightly steeper LREE profiles and some of them have negative Eu anomalies. The tonalites with higher total REE abundances have negative Eu anomalies (Fig. 5 ) and low concentrations of Sr (Fig. 4), indicating significant feldspar fractionation. The mantle-normalized incompatible trace ele-

4.2. Volcanic rocks

The volcanic rocks of the Coastal block are also distinctly bimodal (Fig. 2). The mafic rocks have SiO2 contents (on LOI-free basis) mainly < 55% while the felsic rocks have >69% S i Q (Fig. 2A). The mafic rocks are tholeiitic showing a Fe-enrichment trend as well as a slight increase of TiO2 with increasing FeO,/ MgO (Fig. 3). The ranges of the FeOt/MgO ratio, Ni, Cr, V and other elements (Fig. 4; Table 1) imply that the mafic rocks underwent variable degrees of fractional crystallization. They exhibit slight LREE enrichment and have flat HREE patterns (Fig. 5), whereas mantle-normalized trace element patterns are characterized by a Th enrichment accompanied by negative anomalies of Nb and Ti (Fig. 6). The basalt patterns resemble those of tholeiites from island arcs (Pearce, 1982). The contents of strongly incompatible elements including LREE together with the presence of abundant felsic rocks are consistent with their emplacement on thin sialic crust in a moderately evolved volcanic arc system (Dostal et al., 1990). The distribution of incompatible trace elements in these rocks is similar to that of the diorites (Figs. 5 and 6). However, compared to the diorites, the mafic volcanic rocks are more primitive (lower FeOt/MgO, La/Yb and Th/Yb ratios; Figs. 3 and 7).

The felsic volcanic rocks of the Coastal block are comparable to modern island arc rhyolites; in particu- lar, they resemble tholeiitic/low-K types (Dostal et al., 1992). Their chondrite-normalized REE patterns are

I O0 J. Dostal et al. / Precambrian Research 76 (1996) 93-113

14 OO 0

12- 0 0 ~ 0 0 0

10- OO

8- Iiii " 6-

4- 0

a- FeO t (%) 1 2 = , , , I , , , , I , J L , I * L . . I

o o 10-

8- ~ c~O 0 o

8- o o%&•

4-

2- CaO (%) ~ " I . . . . t . . . . I

o o "A 50o2. o o ~•

0 400- 0 0 0 0 0

300- o o

200- o

I00- Sr (ppm)

45 50 55 60

• Capelin Diorite

• , CapelJn Tonalite

O Coastal Marie

x Coastal FalsJ¢

xx

x

x

65 70 75 80

0 0 0

0 (30

o

TiO 2 (%)

o o

o

o

o "~* MnO (%)

o o

o o

o O o

O

@

x

45 50 55 60 65 70 75 80

SiO 2 (%)

Fig. 4. FeOt ( % ) , TiO2 ( % ), CaO (%) , MnO (%) , Sr (ppm) and V (ppm) versus SiOz (%) for the rocks of the Capelin Cove pluton and the Coastal Volcanic Belt. Fractional crystallization (FC) and partial melting (PM) vectors are shown. The FC vectors of FeO,, TiO, and CaO were calculated for assemblage composed of plagioclase (An20AbToOq o; 60%), clinopyroxene (35%) and Fe-Ti oxides (5 %). The Sr vectors assume a bulk partition coefficient of 1.5. Note that for plagioclase in the residuum, Sr contents in the melt increase with increasing degree of melting.

slightly enriched in LREE. They have unfractionated HREE and display a negative Eu anomaly (Fig. 5). The mantle-normalized trace element pattern of these rocks (Fig. 6) is relatively flat with only a small enrichment of Th relative to high-field-strength elements (HFSE) and HREE, and exhibits only small negative anomalies of Eu. The felsic volcanic rocks of the Coastal block are geochemically similar to tonalites of the Capelin Cove pluton.

Nd isotopes have been determined in four mafic and five felsic volcanic units from the Coastal Belt (Table 2). The mafic volcanic rocks range in end from + 3.9 to ÷ 6.1, and the felsic volcanic rocks range from + 4.1 to + 5.3. Thus, the Capelin Cove diorites and low-silica tonalite are isotopically similar to the volcanic rocks, whereas the Capelin Cove tonalites have higher ENj.

4.3. Petrogenesis

Overall compositional similarities between the dioritic and basaltic rocks and modern island arc rocks suggest a similar origin. Like other mafic and intermediate rocks of the orogenic zones, the parental magma of the basalts and diorites was probably derived from an upper mantle source overlying a subduction zone, The relatively low La/Yb and Th/Yb ratios indicate that the rocks were emplaced in a moderately evolved island arc (Fig. 7) . The flat HREE patterns also suggest that garnet was not involved in the genesis of these rocks, so the magma was generated at shallow depth.

The range of Nd isotope ratios of the Coastal block rocks is typical of volcanic arcs at continental margins.


8 0

ffl 0 L , .

"10 C 0 e-

0 0

1 0

A Capelin Cove Pluton --II-- C-1 5 Dior

C-9 Dior

C-12 Ton

-~-- 0-16 Ton

9 0 I I I ] I I I I I I I I I

B Coastal Belt -~- 257 Bas

Basalts & Rhyolites -~- 252 Bas

-'e-- 235 Rhy

~ 234 Rhy

6 I I I I I I I F I I I r I I

La Ce Pr Nd Sm Eu Gd Tb Dy He Er Tm Yb Lu

Fig. 5. Chondrite-norma]ized REE abundances in the rocks of the Capelin Cove pluton (A) and Coastal Volcanic Belt (B). Normal- izing values after Sun (1982). Black and white symbols correspond to mafic and felsic rocks, respect ively.

It probably is due to subduction processes which modified the mantle beneath the arc (e.g., White and Patch- ett, 1984; Cousens et al., 1994). Variations in end as well as in Th/Yb ratios within the mafic igneous rocks may reflect different inputs of subduction components (slab-derived fluids, sediments) into their mantle source, as well as potential interactions with the crust prior to eruption. Based on the overlap of isotope ratios, the felsic plutonic and volcanic rocks had sources similar to those of the mafic rocks, suggesting that the felsic rocks evolved primarily by crystallization of the mafic types. It is unlikely that the felsic rocks were partial melts of older crust. This is also consistent with trace element characteristics of the rhyolites and tonalites which are similar to those of the associated basalts and diorites, respectively. The fiat or slightly LREE- enriched patterns of the volcanic arc tholeiitic basalts (Fig. 5) are broadly comparable to the patterns of the accompanying rhyolites. Compositional similarities among the Coastal block igneous rocks suggest that the

mafic and felsic rocks in each suite are genetically related.

Both diorites and tonalites underwent fractional crystallization, including plagioclase and subordinate ferromagnesian minerals in the diorites, and feldspar in the tonalites. The negative correlations of the FeOJ MgO ratio with the Ni and Cr contents and Co/Ni ratios > 1 indicate the fractionation of ferromagnesian minerals in the diorites. A decrease of A1, Ca and Sr with increasing SiO2 (Fig. 4), accompanied by a decrease of Eu relative to other REE with differentiation, implies the fractionation of plagioclase in both rock types. Var- iations ofFe, Ti and V in the tonalites (Fig. 3) point to the crystallization of Fe-Ti oxides, whereas the gradual decrease of Zr, Hf and P, with increasing SiO2, suggests that zircon and apatite were also fractionating phases in the tonalites. Similar variation trends char- acterize the Coastal Belt volcanic rocks (Dostal et al., 1990) and have been attributed to crystallization of plagioclase and mafic minerals, particularly clinopyroxene. However, the bimodal nature of the suites cannot be readily explained by simple fractional crystallization.

8 0 - 3

i A Capelin Cove Pluton -e- | 8a Oior Diontes & Tonalites ..~-. ~4a Oior

/ --e-- 20a Ton

"~ B Coastal Belt 0 rl" ~ ~ ° lO- , Basalts & Rhyolites

ThNb LaCe Pr Nd Zr HtSmEu 33 GdTb Dy y He ErTmYb Lu

Fig. 6. Mantle-normalized incompatible trace element patterns for the rocks of the Capelin Cove pluton (A) and Coastal Volcanic Belt (B). Normalizing values after Sun and McDonough (1989). Black and white symbols correspond to mafic and felsic rocks, respectively.


The or ig in o f b i m o d a l sui tes is c o m m o n l y a t t r ibuted

to var ious fac tors ( C o n d i e and Shadel , 1984) : source

d i f fe rences , f rac t iona l mel t ing , i m m i s c i b l e l iquid seg-

regat ion, f rac t iona l c rys ta l l i za t ion and l iquid f rac t ion-

at ion. F rac t iona l m e l t i n g and l iquid immisc ib i l i t y do

not appea r to be feas ib le m e c h a n i s m s for large m a g m a

vo lumes ( C o n d i e and Shadel , 1984) . S imi la r Nd iso-

topic ra t ios in all rocks , the con t inu i ty o f g e o c h e m i c a l

t rends in te rpo la ted b e t w e e n dior i tes and tonal i tes (Fig .

4) and c lose s imi la r i t i es in the d i s t r ibu t ion o f i ncom-

pa t ib le t race e l e m e n t s b e t w e e n the two rock types

(Figs . 5 - 7 ) a rgue aga ins t a two- sou rce mode l and sug-

ges t c o m m o n s o u r c e ( s ) and gene t ic processes .

A m e c h a n i s m w h i c h can sa t i s fac tor i ly expla in the

b imoda l d i s t r ibu t ion o f gene t i ca l ly re la ted rocks

o b s e r v e d in the Cape l i n C o v e p lu ton and the Coas ta l

Bel t vo lcan ics i n v o l v e s f r ac t iona t ion in a zoned m a g m a

c h a m b e r ( M c B i r n e y , 1980; Turner , 1980) . M c B i r n e y

et al. ( 1 9 8 5 ) s h o w e d that l iquid f rac t iona t ion accom-

p a n y i n g s ide-wal l c rys ta l l i za t ion can p roduce cont ras t -

ing l iquids in a zoned m a g m a chamber , As a

h o m o g e n e o u s maf ic pa ren t m a g m a cools , a fels ic l iquid

. .0 >-

,,M

1 001

• Capelin Diorite

• Capelln Tonalile

0 Coastal Mafic

X Coastal Felsic

[~ Indian Brook

• rndian Brook {F&B)

-F Price Poinl Dikes

Price Poinl Laves

0 0

01

Andean

C o n t i n e n t a ; r ~ E~ Margin A r ~ l

? Island ~ ' ~

0 / / Primitive y f,sI:n, Arc 1 10

Th / Yb

Fig. 7. Th/Yb versus La/Yb for the rocks of the Capelin Cove pluton, the Coastal Volcanic Belt, the Price Point Formation and Indian Brook Formation (Indian and Kerrs brooks bodies). The fields are after Condie (1989). The Indian Brook samples (marked F&B ) are from Farrow and Barr (1992).

may separa te and a c c u m u l a t e in an ex te rna l ( u p p e r )

par t of the chamber , whe reas a more mafic l iquid will

Table 2 Neodymium isotopic data for the 550-580 Ma volcanic and plutonic rocks from Cape Breton Island

Sample Type Sm (ppm) Nd (ppm) ta7Sm/144Nd J43Nd/144Nd., Age (Ma) 143Nd/l~Ndi end TDM

Capelin Cove Pluton 8a Diorite 6.26 25.95 0.146 0.51271 574 0.51216 +5.07 819 14a Diorite 5.80 23.29 0.150 0.51274 574 0.51217 +5.35 803 I la Tonalite 4.00 18.47 0.131 0.51280 574 0.51230 +7.91 575 18a Tonalite 4.19 17.06 0.148 0.51274 574 0.51218 +5.47 783 20a Tonalite 3.46 15.90 0.131 0.51277 574 0.51228 + 7.44 575 Coastal Belt 406 Basalt 4.35 16.97 0.155 0.51268 570 0.51210 +3.90 985 440 Basalt 5.36 21.07 0.154 0.51274 570 0.51216 + 5.09 843 257 Basalt 2.81 11.17 0.152 0.51278 570 0.51221 +6.05 727 252 Basalt 4.70 16.68 0.170 0.51280 570 0.51217 +5.17 914 234 Rhyolite 5.65 22.16 0.154 0.51271 570 0.51214 +4.56 904 235 Rhyolite 6.89 27.80 0.150 0.51268 570 0.51212 +4.23 921 419 Rhyolite 5.31 22.18 0.139 0.51270 570 0.51218 +5.34 770 420 Rhyolite 6.14 24.41 0.152 0.51268 570 0.51212 +4.16 939 421 Rhyolite 9.20 36.92 0.151 0.51271 570 0.51214 +4.68 876 Price Point P-20A Basalt 5.34 22.93 0.141 0.51251 565 0.51199 + 1.55 1137 P-24A Basalt 6.86 28.59 0.145 0.51249 565 0.51195 + 0.75 1247 P-25B Rhyolite 2.82 17.43 0.098 0.51228 565 0.51192 +0.11 1030

Age (Ma) = assumed crystallization age (see text); ]43Nd/]44Nd m and 143Nd/144Nd~ - measured and initial Nd isotopic ratios, respectively; TDM - Nd model age (in Ma) for fractionation from depleted mantle (DM) to the crust; eNo = the fractional difference between the ~43Nd/~44Nd of rock and the bulk earth at the time of crystallization.


10

ENd t

8

6-

4-

2

0 0.12

r=0.98

r=0.75 r-0.50 X

• Capalin Dloflle

• Capalin Tonalite

0 Coastal Msfic

X Coastal Felsic

-}- Price Point Dike

0 X ×

× X 0

+~+ Osedld Crust, iments

0214 0.15 0.16 0.13 0.17

147Sm / 144N d

Fig. 8. eNo versus ~47Sm/~Nd for Capelin Cove and Coastal Belt rocks. Three assimilation/fractional crystallization (AFC; DePaolo, 1981) curves with different values of r (mass assimilated/mass crystallized) are shown for mixing of a diorite magma with a 30% partial melt of lower crust (composition of Taylor and McLennan, 1985). Values of F (mass final magma/mass initial magma) are indicated by tick marks on the curves, spaced at 0.1 increments. The r = 0.98 curve shows an additional tick for F = 0.95.

stay in the internal (deeper) part of the chamber. Tap- ping different parts of the magma chamber could pro- duce the bimodal suites such as those forming the Capelin Cove pluton and the Coastal Volcanic Belt. The side-wall interaction with the magma would also lead to magma contamination and could explain subtle differences in the Nd isotopic composit ion of the diorites and tonalites. The higher EN0, lower 1478m/14aNd and similar REE contents of the tonalites relative to the diorites of the Capelin Cove pluton suggest that magmas parental to more evolved rocks interacted with material having a more depleted isotopic signature, perhaps in the lower crust.

We have model led (Fig. 8) the assimilation of lower crust by a Capelin Cove dioritic magma using an assim- i lat ion/fract ional crystall ization ( A F C ) model (De- Paolo, 1981). The model assumes that feldspar is the major fractionating phase, along with minor pyroxene, iron oxides, apatite and zircon, that calculated sol id / liquid bulk partition coefficient (D s°l/liq) values for Nd

and Sm are ~0 .1 and 0.12 (Arth, 1976), and that the lower crust has 12 ppm Nd, 3 ppm Sm, and eNa = + 9. Assuming a degree of partial melting of the lower crust of 30% (generat ing a melt with N d = 2 4 . 3 ppm, S m = 5.13 ppm) , the ratio of mass assimilated to mass crystall ized ( r ) must be between 0.75 and 1 in order

to evolve from a Capelin Cove diorite to a tonalite. Crust /mant le (mass ass imi la ted/mass mantle) ratios are ~ 2. This estimate of the amount of mafic crustal assimilation required to explain the coupled isotope/ trace element shift of the tonalites requires a significant input of heat into the crust, perhaps by regional under- plating of mafic magma, or by prolonged heat loss from a recharged magma chamber. If the Coastal Belt rhyolites are related by a similar AFC process, the roughly constant to slight increase in end and significant decrease in S m / N d can be modelled by assimilation of a 10% partial melt of lower crust, with r = 0 . 1 5 and crust /mant le ratios of 0.1 to 0.15. Note that the lower REE concentrations in the tonalites relative to the diorites (Fig. 5) requires fractionation of an REE-rich phase, such as apatite, consistent with the decrease in P205 from diorite to tonalite (Table 1 ).

The alternative to the AFC model described above is that the Capelin Cove tonalites had a parental magma derived from a mantle source that was more depleted (N-MORB-l ike with high ENd) than the diorite mantle source. However, the patterns of the tonalites have large Nb depletions, Th enrichments, and similar incompatible element signatures as the diorites (Figs. 5 and 6). These characteristics are not consistent with a more- depleted mantle source. Ratios of incompatible elements, such as Th/Nd, Rb /Nd , and La /Nd , increase from diorite to tonalite, whereas N b / N d remains constant. These changes in ratios are consistent with assimilation of lower crust (except for Rb, whose concentrations in the plutonic rocks have been modified due to alteration).

The bimodal suite of the Capelin Cove pluton is composit ionaily similar to the volcanic rocks of the Coastal block and could be their plutonic equivalent. The combined isotope-trace element characteristics of felsic rocks from the Coastal Belt and Capelin Cove pluton are consistent with assimilation of lower crust with a depleted mantle isotopic composition. The crust /mantle mass ratios are an order of magnitude greater in the plutonic rocks than in the volcanic rocks, and thus, small trace element variations in the tonalites are accompanied by large isotopic shifts, whereas the reverse is true in the rhyolites. The rhyolites evolved primarily by fractional crystallization, but assimilation of crustal wall rocks had an effect on trace element characteristics (which is especially evident in the middle R E E / H R E E ratio) and eNj.


Table 3 Major and trace element composition of representative rocks of the Price Point and Indian Brook Formations

Price Point Formation Indian Bk. Formation

lavas dikes Indian Bk. Kerrs Bk.

Sample: P-7 P-9 P-6 P-10 P-15 P-17 P-20A P-24A P-8 S-28 S-29 S-4 S-5

Si02 (%) Ti02 Al20a F%0:~ Mn0 Mg0 Ca0 Na:0 K20 P20~ LOI Total

51.29 54.29 54.39 54.58 56.93 63.33 45 .11 47.92 58.64 68.39 6 8 . 9 1 69.08 69.44 0.97 0.82 0.86 0.86 0.61 0.55 0.97 0.98 0.71 0.40 0.34 0.33 0.34 16.88 1 7 . 1 0 17.75 17.84 20.29 16 .54 17 .07 1 7 . 8 6 15 .77 14 .14 1 4 . 3 7 14 .94 14.95 t0.62 8.60 8.55 9.30 5.11 4.47 1 2 . 9 6 11.62 7.75 3.69 3.16 3.[4 2.92 0.19 0.15 0.16 0.16 0.12 0.10 0.30 0.30 0.16 0.08 0.10 0.11 0.10 4.86 4.79 4.42 3.79 2.43 2.18 7.28 6.04 3.62 2.10 1.73 1.77 1.74 9.61 5.28 4.58 6.22 6.49 3.51 8.63 6.46 5.21 2.14 1.93 2.27 1.73 2.79 3.42 3.79 2.86 3.54 3.28 2.22 3.46 3.31 3.24 3.86 3.85 3.55 0.35 2.13 1.63 1.62 2.96 3.65 I ~30 1.05 3.00 4.41 4.24 2.84 3.13 0.23 0.23 0.25 0.29 0.27 0.17 0.30 0.32 0.25 0.11 0.09 0.12 0.12 2.80 2.70 2.90 2.00 1.60 [.40 2.90 3.60 1.50 1.30 1.30 0.90 1. I 0 100.59 99.61 99.28 99.52 100.35 99.18 99.04 9 9 . 6 1 99.92 100.00 100.03 99.35 99.12

FeO*/MgO 1.97 1.62 1.74 2.21 1.89 1.84 1.60 1.73 1.93 1.58 1.64 1.60 1.51 Cr (ppm) Ni V Zi1 Rb Ba Sr Ta Nb Hf Zr Y Th La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

27 20 350 159 8 109 551

53 46 41 29 33 32 33 38 34 46 36 35 51 35 12 8 8 34 26 9 6 5 6 l 232 249 266 124 96 414 380 209 78 63 58 52 106 83 140 87 68 288 190 96 62 41 47 44 62 55 54 98 II1 57 41 71 149 147 83 84 472 224 418 466 562 237 141 544 530 530 473 461 423 470 565 671 470 462. 499 475 289 258 343 334

0.22 0.17 0.24 0.58 0.51 0.70 O.ll 0.15 0.54 1.03 0.88 0.51 0.60 1.0 1 ~0 1.0 8.0 6.0 I 0.0 2.4 4.0 7.0 9.0 8.0 1.0 8.0 2.50 2.20 2.20 4.10 3.40 4.80 1.29 1.99 3.80 4.70 4.70 3.20 3.60 109 97 102 184 157 203 47 83 159 161 145 117 129 18 18 20 33 20 25 18 19 25 24 20 17 21 2.85 0.72 0.92 10 .30 8.82 13.10 1.30 2.20 9.76 1 8 . 9 0 17.40 8.19 10.80 18.20 1 3 . 2 0 13.80 48.20 31.60 37.80 13 .14 1 6 . 5 3 31.70 69.50 38.70 18 .60 27.70 39.10 29.80 31.70 90.20 64.30 80.40 31.14 40.08 68.30 123 .0 74.30 41.50 58.30

4.15 5.25 20.10 1 4 . 8 0 16.20 46.40 27.70 31.20 18 .65 23.18 28.60 35.30 20.90 17 .50 21.70 5.16 3.70 3.99 9.77 5.54 6.21 4.45 5.28 6.24 5.74 3.72 3.77 4.69 1.39 1.14 1.20 1.86 1.40 1.38 1.40 1.48 1.41 1.02 0.81 0.92 1.01

4.31 4.67 0.84 0.65 0.67 1.32 0.67 0.80 0.57 0.64 0.81 0.65 0.48 0.55 0.64

3.63 4.03 0.67 0.77 1.89 2.04 0.26 0.30

2.12 1.63 1.90 3.44 1.98 2.40 1.81 1.86 2.37 2~21 1.79 1.59 1.83 0.36 0.30 0.30 0.56 0.31 0.38 0.26 0.30 0.40 0.39 0.35 0.29 0.33

5. C e n t r a l C a p e B r e t o n

5.1. P lu ton i c rocks

T h e p l u t o n i c r o c k s o f the I n d i a n B r o o k and Ker f s

B r o o k p l u t o n s s h o w va r i a t ions in SiO2 f r o m a b o u t 6 0 %

to 7 5 % ( L O I - f r e e ; T a b l e 3 ) . Bo th b o d i e s h a v e ove r -

l a p p i n g c o m p o s i t i o n and c o m p a r a b l e va r ia t ion t rends .

Un l ike the C a p e l i n C o v e sui te , the c o m p o s i t i o n a l var-

i a t ions are c o n t i n u o u s (F ig . 2 ) , W i t h i n c r e a s i n g S i Q ,

1000 Ioo ,oBoo 1 • Indian Brook (F&B) SYN-COLG


leo wPG v y rr

10

VAG ORG

. . . . i 5 " loDe

Y + Nb (ppm)

Fig. 9. Rb (ppm) versus ( Y + Nb) (ppm) diagram o f Pearce et al.

(1984) for granit ic rocks o f the Indian Brook Formation. The Indian

Brook samples (marked F & B ) are f rom Farrow and Barr (1992).

Fields: ORG=ocean ridge granites; VAG=volcanic arc granites; WPG = within-plate granites; SYN-COLG = syn-collisional granites.

300

Kerr Brook &

lOO

Price Point / ~ P-24A Dike / ~ . . . Oikes&Lavas I ~ ; i : 1

5 ~ La Ce Pr Nd Sm Eu Gd Tb Dy He Er Tm Yb LU

Fig. 10. Chondrite-norma]ized REE patterns for ( A ) the rocks o f the Indian Brook (IB) and Kerrs Brook (KB) plutons and ( B ) the Price

Point Formation (black and white symbols correspond to dikes and lavas, respectively). Normalizing values after Sun (1982).

'~ 10

"~ 3oo~ o

O O 100

r r

the rocks have rather constant FeOt/MgO ratios, suggesting calc-alkaline affinities. The relation of Rb ver-

sus ( Y b + N b ) and Y versus Nb (Pearce et al., 1984) implies that the rocks of the Indian Brook and Kerrs Brook intrusions were emplaced in a volcanic arc setting (Fig. 9). REE patterns of the rocks display distinct enrichment of LREE and rather fiat HREE, and lack a distinct negative Eu anomaly (Fig. 10). Mantle-normalized patterns of the rocks are subparallel and are characterized by a negative slope and distinct negative anomalies of Nb and Ti (Fig. 11 ).

5.2. Volcanic rocks

The volcanic rocks of the Price Point Formation (MacDonald and Barr, 1985; Raeside and Barr, 1992) have SiO2 contents ranging typically from 50 to 67% (Fig. 2). Many of the marie rocks are dikes which appear to be feeders for the volcanics. Compositionally, the dikes resemble associated basalt lavas. The mafic and intermediate rocks are characterized by low Ni and Cr but high AI contents (Table 3). The rocks show calc-alkaline affinities and, according to various dis-

300-

A Kerr Brook & -~- KB S-4

IB S-28

- l c

O ~ B [ -B-- P-2°A Dike 1 Price Point I + P-24A Oike j

f~ 100~ ~ Dikes & Lavas I -4~-- P-9 /

lo- \ ~ 1

Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y He Er Tm Yb Lu

Fig. 11. Mantle-normalized incompatible trace element patterns for (A) the rocks of the Indian Brook (IB) and Kerrs Brook (KB) plutons and (B) the Price Point Formation (black and white symbols correspond to dikes and lavas, respectively). Normalizing values after Sun and McDonough (1989).


H f / 3

D Zx

C -~- Price Point Dike

Price Point Lava

T h ' Y a

Fig. 12. Th-Ta-Hf diagram of Wood (1980) for the volcanic rocks of the Price Point Formation. Fields: A = N-type MORB; B = E-type MORB; C = alkaline within-plate basalts and differentiates; D = destructive plate margin basalts and differentiates.

crimination diagrams (e.g., Wood, 1980; Pearce, 1982), correspond to volcanic arc rocks (e.g., Fig. 12).

Their REE patterns display enrichment in LREE and lack a negative Eu anomaly (Fig. 10). Mantle-normalized patterns of these rocks (Fig. 11) show a negative slope and negative Nb, Ti, and even Zr and Hf anomalies. Compared to the Coastal Belt volcanic rocks, they have higher contents of incompatible trace elements and more fractionated REE and incompatible trace element patterns (Fig. 11 ).

5.3. Petrogenesis

Both the plutonic (Indian Brook and Kerrs Brook bodies) and volcanic (Price Point Formation) suites resemble modern calc-alkaline volcanic rocks of continental margins emplaced on a moderately thick continental crust (Fig. 7). The continuous compositional variations within the suites are mainly due to fractional crystallization of plagioclase and mafic minerals. Although the Price Point volcanic rocks display an

overall similarity to the plutonic rocks of the Indian Brook and related bodies, the differences in the distribution of incompatible trace elements (e.g., the shape of the HREE patterns) suggest that they were derived from a different source. Fractional crystallization and melting of a heterogeneous mantle source overlying a subduction zone can account for many compositional variations of the volcanic rocks, as well as the differences with rocks of similar S i O 2 c o n t e n t s .

The high MgO and low SiO2 contents of the Price Point dikes (Table 3) argue against significant crustal input, and thus, their negative HFSE anomalies and low end might be a characteristic of their source. The source of the dike rocks most likely is mantle which has been fluxed by a larger subduction component relative to the source of Coastal Belt basaltic lavas. Com- pared to the Coastal block rocks of a given SiO2 content, both plutonic and volcanic rocks of central Cape Breton Island have higher K20 and incompatible trace elements. They are also more enriched in LREE and Th relative to HFSE and HREE and have higher

J. Dostal et al. /Precambrian Research 76 (1996) 93-113 107

3.5

3.0-

2,5-

2.0-

1.5-

1,0

0.5

0.0 45

• Capelin Cove

O CoastaJ BeR

D Indian BROOK

A Pdce Point

c~ NS Pluton$

A [] I

W e s t e m . . ~ _ c _ H _

C B I ~ H

w M /

Cen t ra l ~ ~ J ~ C B I ~ ~ ~ ~

~ O

Coastal Block

50 55 60 65 70 75

sio 2 (%)

Fig. 13. K20 (%) versus SiO2 (%) for the averages of arc-related igneous rock bodies of the 550-580 Ma age from the Cape Breton Island. Symbols for NS plutons: SH= Shunacadie pluton (Barr and Setter, 1986). Eastern Cape Breton Highlands plutons (Farrow and Barr, 1992) : K = Kathy Road; T= Timber Lake; G = Gisborne Flow- age; W = Wreck Cove; IR = lngonish River. North Mountain plutons (Justino, 1991): B=Big Brook; M=Marble Mountain. CR = Creignish Hills pluton ( White et al., 1990); CH = Cheticamp pluton ( Ban" et al., 1986). Indian Brook data from Farrow and Barr (1992). The dashed line separates the averages of the Coastal block from those of central and northwestern Cape Breton Islands. Coastal Volcanic Belt and Capelin Cove pluton symbols are averages for the mafic and felsic rocks, respectively. Volcanic rocks of the Price Point Formation are (with increasing silica contents) averages of basalt lavas, basalt dikes, andesites and dacites.

La /Yb , L a / S m and T h / Y b ratios (Fig. 7) . This chemical distinction between Coastal and Central Cape Breton volcanic rocks mimics the distinction between island arc and established continental arc environ- ments. The higher proportions of intrusive versus extru- sive rocks in central Cape Breton relative to the Coastal block is consistent with a larger crustal thickness (Hil- dreth and Moorbath, 1988) in the former. A greater depth of melting, a higher proportion of subducted components in the source, and greater interaction between magmas and crust may be important factors in the evolution of Central Cape Breton volcanic rocks.

6. Tectonic implications

The volcanic arc nature of the 550-580 Ma magmatism in the Coastal block (Capel in Cove pluton, Fourchu and Main a Dieu groups) and in the southern Cape Breton Highlands (Indian Brook and Kerrs Brook

plutons and Price Point Formation) indicates that they originated above a subduction zone. Given the lack of evidence for a plate boundary between the two regions, we assume that only one subduction zone was present at that time. The polarity of the subduction zone may be derived by comparing various geochemical para- meters with those of other 550-580 Ma arc-related igneous bodies exposed elsewhere on Cape Breton Island (Fig. 1). These suites include: (1) Shunacadie pluton (564+3_ 2 Ma: U - P b zircon age; Barr et al., 1990); (2) Eastern Cape Breton Highlands plutons (Kathy Road, Timber Lake, Gisborne Flowage, Wreck Cove, Ingonish River and Indian Brook) ranging in age from 564_+5 to 556_+4 Ma ( U - P b zircon and 4°Ar- 39Ar hornblende ages; Dunning et al., 1990; Dal lmeyer and Keppie, 1993); (3) North Mountain plutons (Big Brook and Marble Mountain plutons) (555-+5 to 545 _+ 6 Ma: 4°Ar-39Ar hornblende ages; Keppie et al., 1990); (4) Creignish Hills plutons (560-550 Ma: U - Pb zircon ages; authors' unpubl, data) ; and (5) Che- ticamp pluton in western Cape Breton Island ( 5 5 0 _ 8 Ma: U-Pb zircon age; Jamieson et al., 1986). Most of these plutons are typically I-type and calc-alkaline (Barr, 1990), except for the Cheticamp pluton which is peraluminous (Barr et ai., 1986). Keppie et al. (1990) and Dal lmeyer and Keppie (1993) argued that because most of these plutons were intruded at high levels in the crust, the 4°Ar-39Ar plateau hornblende ages closely post-date the time of intrusion. This is

J~ n "

0.1

• Capelin C o v e

O Coastal Belt

E] Indian Brook

A Pr ice Point

NS Plutons

l p % Western B / CBI

IR ¢ ,W

,>a~ /nc r e \ \ K asing \ \ / ArcMaturity

\ ¢ 'M & A \ \

\ 0

\ Central CBI \ \

Coastal Block

;0 50

Nb Fig. 14. Rb/Zr versus Nb (ppm) plot (after Brown et al., 1984) for the averages of arc-related igneous rock bodies of 550-580 Ma age from the Cape Breton Island. Symbols are the same as in Fig. 13.


10

6

~Nd t 4-

.

.

2-

-41 0

~ _ Depleted ~ ' ~ ~ n t l e

I I

0 # ..::ii

• ~ Oaleozoic :~iiiiii~i~iiiiiiiiiiil / / ~ A v a l o n :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

/ Felsic Rocks . . . . . . . . . . . . . . . . . . . . : . . . . . . . ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

/ ' / .... iiii::iiii~iii~i!!!ii!iiiii~i i;!;!~i!~ & CapeUn Diorite ii~i~::~i~i~::~::~::~:::::::::::::::::::::::::::

/ .:iii ::~::i!Ci~ ::::::::::::::::::::::::::::::: I + .... ~:~:~!!~i!!ii;~iiii!~i~!ii~i~i~iii~i!~iJ~i~!i~i~ii!~i~!iii~iiii~ii!~!i~i!i~!ii~iiii~iiiiiii + Price Point Dike iiiii!ii!iiiiiiiiiiiiiii

,~!i!i::iiiiiiii!iii!iiiii!i!ii Grenvi l le iiiiiiiili . Cape,nTonaUte ~ili~i~i~ ......... iiii!ili!i!iiiiiii

.: ii iii iiiiiiiiiiiii ii iiiii ii iii iiiiiiiiiiiiiiiiiii i ii iii iiiiiiii iiiiiiiii ~!!!~!!::!i~::i::~::~::~i!i~!i::!~i~ii::i::~::!::i::i::!i!ii::~::i~::~i!::!::~::~i!i!i~::i!!i!!i::~::i::!ii::i::iiii!ii::i::i~i::!::i~i::ii~::iii::~::i::iiii~ii::i~i::i:: zx Price Point Felsic :::::::::::::::::::::::::::

~..~ii!ii~!i!iiiiiiiiiiiii!iii;iiii!iiii!iiiiiii;iiiiiiiiii!iiiii!iiiii ii ;~ ;!~!E!; E!i!iiiiiiiiiiiiii!!iiiiiiii;Ei::iiiiiii!~i;i::i::

4 6 0 8 6 0 1 2 0 0 1 6 0 0

Age Fig. 15. ENa versus crystallization age for 550-580 Ma volcanic and plutonic rocks from Cape Breton Island. Also shown is the isotopic evolution envelope for Paleozoic Avalonian felsic rocks from Nova Scotia (Murphy et al., 1995), the field of Avalonian plutonic rocks from New Brunswick and Newfoundland (Fryer et al., 1992; Whalen et al., 1994) and 550-580 Ma granitoid plutons of Cape Breton Island. (Barr and Hegner, 1992). Grenville evolution envelope (dotted field) from Dickin and McNutt (1989), Marcantonio et al. (1990), Dickin et al. ( 1991 ), Daly and McLelland ( 1991 ) and Ban and Hegner (1992).

borne out where both argon and U-Pb ages are available for the same pluton.

Saunders et al. (1980) and Saunders and Tarney (1982) noted that K and Rb show strong transverse variations across arcs in plutonic rocks. Average K20 contents in the 550-580 Ma plutons are plotted against SiO2 in Fig. 13. From southeast to northwest across Cape Breton Island, the K 2 0 values increase for any given SiO2 content. Similar differences between the southeastern and northwestern parts of Cape Breton Island are also displayed by Rb and Rb/Ba, K/Ba, La/ Sin, La/Yb and Th/Yb ratios.

Brown et al. (1984) have devised a plot (Rb/Zr versus Nb) for intrusive rocks to demonstrate arc 'maturity' in time and space. Like many key geochemical indicators, Rb/Zr and Nb increase in Cape Breton Island towards the northwest (Fig. 14). The rocks of central and northwestern Cape Breton Island are clearly more mature than the rocks of the Capelin Cove pluton

and the Coastal Volcanic Belt (Fig. 14). The Cheti- camp ptuton falls among the most mature arc rocks. These trends are comparable to those observed above modern volcanic arcs and may be interpreted in terms of magma generation above a NW-dipping subduction zone that was active from 580 to 550 Ma.

An end envelope was derived from the volcanic rocks in the Antigonish Highlands, an undisputed part of the Avalon Composite Terrane (Murphy et al., 1995). This envelope generally lies above the Grenvillian end envelope, although there is a slight overlap (Fig. 15). Most of the end data for 550-580 Ma igneous suites in Cape Breton Island presented in this paper plot within the Antigonish envelope. The two Capelin Cove tonalites that lie above the envelope were modified by the lower crustal contamination. Published ~550 Ma end data for Cape Breton Island and southern New Brunswick generally fall within the Antigonish envelope (Barr and Hegner, 1992; Whalen et ai., 1994); however, a few


I m l

/ '~ IV l F-, I-I. I ~ . l / ' ~

Fig. 16. Avalon Composite Terrane plotted on late Neoproterozoic palinspastic maps: (A) modified from Dalziel ( 1992); (B) modified from McKerrow et al. (1992).

110 J. Dostal et al. /Precambr ian Research 76 (1996) 93-113

points fall slightly below the line, indicating a down- ward revision of the position of the lower limiting line to include all published data for the Avalon Composite Terrane (Fig. 15). The generally increasing K20 versus SiO2 and Rb/Zr versus Nb values, as one passes across Cape Breton Island from southeast to northwest, are reflected in the decrease of Er~j in the same direction (Fig. 15). Note that the gap in the eNj values coincides with the 30 km sampling gap between the Coastal block and central Cape Breton Island. The decrease in end values supports the interpretation of magma production above a NW-dipping subduction zone beneath an increasingly mature magmatic arc.

The polarity of the 550-580 Ma subduction is the same as that deduced for the ~ 630 to 600 Ma period based upon the calc-alkaline volcanic arc geochemistry of the East Bay Hills, Coxheath and Sporting Mountain igneous suites in southeastern Cape Breton Island, compared to the back-arc igneous suites present in the Antigonish Highlands (Keppie and Dostal, 1991 ; Barr, 1993). This indicates a prolonged period of subduction with a northwest-polarity between ~ 630 and 550 Ma. The apparent 20 Ma gap in magmatic activity in Nova Scotia between 600 and 580 Ma may not result from intermittent subduction. Elsewhere, accretion of aseismic ridges, changes in the age of the subducted oceanic crust or differential shortening of previously weakened hot continental crust have caused temporary cessation of magmatism (cf. Pilger, 1981; Wortel, 1984; Isaaks, 1988).

Neoproterozoic paleomagnetic data indicate that the Avalon Composite Terrane was located adjacent to Gondwana (Johnson and Van der Voo, 1986). How- ever, paleomagnetic data do not adequately constrain the specific location, and two alternative hypotheses have been proposed: northwest Africa and northwest South America. A more extensive discussion of this problem is given in Keppie et al. (1995). Based upon lithostratigraphic comparisons, O'Brien et al. (1983) favored placing the Avalon Composite Terrane adjacent to NW Africa. However, detrital zircon popula- tions in the Neoproterozoic deposits of the Avalon Composite Terrane contain concordant zircons with ages of 610-630 Ma, 1000 Ma, 1160-1200 Ma, 1520- 1550 Ma, 1835 Ma 1950-2000 Ma and 2600 Ma (Kep- pie and Krogh, 1990). Zircons between 1.0 and 1.6 Ga cannot be derived from rocks presently exposed in NW Africa, and Keppie and Krogh (1990) have suggested

a source in NW South America. This reconstruction is consistent with end data which suggests that a similar Neoproterozoic magma source lay beneath the Avalon Composite Terrane and South America, which was distinct from that beneath NW Africa (Nance and Mur- phy, 1994). Early Cambrian palinspastic reconstructions suggest that the Avalon Composite Terrane was bordered by two continental-rise deposits in the Meguma and Gander terranes, which may be accommodated by assuming that the Meguma-Ava- Ion-Gander formed a peninsula off northwestern South America. There is currently considerable debate about the relative positions of the major continents in the latest Proterozoic due in large part to the paucity of reliable paleomagnetic data (Van der Voo and Meert, 1991). Examples include those of Daiziel (1992), McKerrow et al. (1992) and Keppie et al. (1995). Plotting the Avalon Composite Terrane on these latest Precambrian reconstructions of Dalziel (1992) and McKerrow et al. (1992) shows the Avalon Composite Terrane as a magmatic arc off northwestern South America on the periphery of the latest Proterozoic supercontinent or Gondwana (Fig. 16). This subduction took place during the terminal stages of amalgamation of Gondwana. It is inferred that subduction and Avalonian magmatic activity ended, not because of continent-continent collision, but due to plate reorgan- isation near the beginning of the Cambrian. Such a tectonic setting allowed the magmatic record of convergence to be well preserved in the Avalon Composite Werrane.

Acknowledgements

The study was supported by the Nova Scotia Depart- ment of Natural Resources and the Natural Sciences and Engineering Research Council of Canada. Thanks to Keith Bell for spike and J. Blenkinsop for lab super- vision and support. Reviews by A.M. Goodwin, A. Cadman and J.V. Owen greatly improved the paper.

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550-580 Ma magmatism in Cape Breton Island (Nova Scotia,Canada): the product of NW-dipping subduction during the final stage of amalgamation of Gondwana

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