Geochemistry of Late Proterozoic basaltic rocks from southeastern New Brunswick, Canada

Precambrian Research, 47 (1990) 83-98

83

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Geochemistry of Late Proterozoic basaltic rocks from southeastern New Brunswick, Canada J. DOSTAL a and S.R. McCUTCHEON 2 1Department of Geology, Saint Mary's University, Halifax, Nova Scotia, B3H 3C3 (Canada) 2New Brunswick Department of Natural Resources and Energy, P.O. Box 50, Bathurst, New Brunswick, E2A 3Z1 (Canada) (Received May 1, 1989; revised and accepted October 9, 1989)

Abstract Dostal, J. and McCutcheon, S.R., 1990. Geochemistry of Late Proterozoic basaltic rocks from southeastern New Brunswick, Canada. Precambrian Res., 47: 83-98. Late Proterozoic volcanic rocks of the Coldbrook Group ( ~ 630-600 Ma) from the Avalon Zone of the Appalachians in southeastern New Brunswick (Canada) are predominantly bimodal and occur in three belts/fault blocks (Eastern, Central and Western). The basaltic rocks were emplaced in a convergent plate margin setting and range from incompatible element-depleted island arc tholeiites in the Eastern belt to calc-alkalic types of the Western belt. The distinct compositional zonation of the basaltic rocks resembles the across-arc variation observed in recent volcanic arc systerns. The trends include an increase of the LILE and HFSE abundances and the Zr/Si02, Th/Si02, La/Yb, Th/Nb, T h / H f and Zr/Y ratios from the Eastern belt in the southeast through the Central belt to the Western belt in the northwest. The zonation of the Coldbrook Group is attributed to a northwest-dipping subduction zone during Late Proterozoic time. It is suggested that some of the Late Proterozoic basins in the Avalon Zone of the Northern Appalachians reached a stage of oceanization.

Introduction The Caledonia Highlands in southeastern New Brunswick are part of the Avalon Zone of the Canadian Appalachians. The Avalon Zone (Williams, 1978) stretches along the southeastern margin of the Appalachians, extending from eastern Newfoundland, through northern Nova Scotia, southern New Brunswick and ultimately to south Carolina. It is thought to be a composite terrane formed by amalgamation of distinct "suspect" terranes during the latest Precambrian Cadomian orogeny (Williams and Hatcher, 1982). The terrane is characterized by the occurrences of abundant Late Proterozoic ( ~ 600 Ma) volcanic-sedimentary sequences and early Paleozoic platformal sediments with 0301-9268/90/$03.50

Acado-Baltic fauna. The Late Proterozoic volcanic sequences, which are inferred to overlie Helikian or older metasediments and gneisses (Olszewski and Gaudette, 1982), have been variously interpreted as continental rift (e.g., Strong et al., 1978), ensialic volcanic arc (e.g., Rast et al., 1976) or intra-cratonic deposits (e.g., Giles and Ruitenberg, 1977; O'Brien et al., 1983). In order to better understand the origi n and tectonic setting of these Late Proterozoic volcanic rocks, we undertook a geochemical study of the rocks of the Coldbrook Group from the Caledonia Highlands. The purpose of this study is to present major and trace element data on the basaltic rocks and to discuss the geochemical constraints on their petrogenesis and tectonic setting.

© 1990 Elsevier Science Publishers B.V.

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Geological setting The geology of the Coldbrook Group was described by Giles and Ruitenberg (1977), Ruitenberg et al. (1979), and McLeod (1986, 1987). The Coldbrook Group overlies stromatolite-bearing platformal shallow marine carbonates (Ashburn Formation) of Neohelikian age (Hofmann, 1974), and turbiditic sandstones, siltstones and cherts (Martinon Formation). These rocks were deformed and metamorphosed about 800 Ma ago (Olszewski and Gaudette, 1982). The Coldbrook Group is a bimodal volcanic sequence with minor intercalated sedimentary rocks. It includes mafic and felsic pyroclastic rocks and lavas (Venugopal et al., 1975 ) that are associated with and intruded by a variety of Hadrynian plutonic rocks ranging in composition from gabbro, through diorite, tonalite and granodiorite to granite (Ruitenberg et al., 1979; Currie et al., 1981; McLeod, 1986). Coldbrook rocks are unconformably overlain by Lower Cambrian to Lower Ordovician fossiliferous sedimentary rocks (Hayes and Howell, 1937; Alcock, 1938) and locally by Eocambrian sedimentary and volcanic rocks (Currie, 1987; Nance, 1987; Barr and White,

1988). The Coldbrook Group constitutes the largest part of the Avalon Zone in southern New Brunswick (Fig. 1 ). The group is divisible into three major belts (Western, Central and Eastern belts - Giles and Ruitenberg, 1977; Ruitenberg et al., 1979), each characterized by a distinct rock assemblage and structural style. These volcanic belts are largely separated by two belts of intrusive rocks so that the contact relationships between the adjacent belts are not clear. A facies relationship between the Central and Eastern belts was suggested by Giles and Ruitenberg (1977) but more recently McLeod (1986) proposed that there is a tectonic boundary between the two. In fact, it is possible that all three volcanic belts were tectonically juxtaposed (McCutcheon and Ruitenberg, 1987) and the Precambrian domain underwent shorten-

J. DOSTAL AND S.R. MCCUTCHEON

ing of unknown extent, probably prior to Carboniferous time. The volcanic rocks of the Coldbrook Group were interpreted as representing either an ensialic arc (Rast et al, 1976) or an intra-cratonic rift (Giles and Ruitenberg, 1977). The Central volcanic belt (CB) is typified by terrestrial felsic and mafic volcanics with minor intercalated shallow subaqueous and subaerial sedimentary rocks (Ruitenberg et al, 1979). The section is at least 8 km thick (Giles and Ruitenberg, 1977). The most abundant volcanic rocks are lithic tuffs (including ignimbrites) and flows with a predominance of felsic types. The rocks of this belt are only weakly deformed, generally lack cleavage perhaps due to the buttressing effect of the Western and Central intrusive belts and contain subgreenschist to greenschist facies metamorphic assemblages. According to McLeod (1986) and Barr and White (1988), the Eastern volcanic belt (EB) is composed to two sequences. The first, which occurs predominantly in the northwestern part of the belt, was deposited mainly in subaerial environments. The second sequence crops out primarily in the southeastern part of the belt, particularly south of the NE-SW-trending Cradle Brook thrust. This sequence contains a subaqueous suite dominated by massive and pillowed basaltic lava flows, pillow breccias, mafic and felsic tuffaceous rocks, siltstones (in places pyritic) and fine-grained sandstones (McLeod, 1986). Minor polymictic conglomerates, arkosic sandstones, limestones and cherts are also present. The total thickness of the sequence is about 10 km (Giles and Ruitenberg, 1977). Barr and White (1988) noted that the northwestern sequence is younger (Eocambrian) than the southeastern one of Late Proterozoic age. The rocks were deformed and metamorphosed mainly under chlorite zone conditions although locally biotite has been observed (McLeod, 1986). The Western volcanic belt (WB) is divided into two parts by the younger NE-SW-trend-

85

G E O C H E M I S T R Y OF L A T E P R O T E R O Z O I C BASALTIC R O C K S F R O M N E W B R U N S W I C K

C~'50'

64" 30' "---I 46"00'

NEW

<

BRUNSWICK

F~I

/

JOHN

Carboniferous ~.-,G~-;~',;,rroll y

Precomtman

intrmiom

45"00'

45*00 ~

66-50'

64.30'

Fig. 1. Simplified geological map of the Coldbrook Group showing three volcanic belts separated by Precambrian intrusions. Numbers refer to areas where samples were collected.

ing Kingston dike swarm and associated mylonite zone. The stratigraphyand total thickness of the belt are only poorly known. The southern segment contains mainly terrestrial felsicvolcanics,whereas the northern part consists of subequal amounts of mafic and felsic rocks with minor interbedded sedimentary rocks (McCutcheon and Ruitenberg, 1987). The mafic lava flows are massive or amygdaloidal,a few meters thick and are intercalatedwith pyroclastics. The belt is intensely deformed. The rocks were metamorphosed under greenschist faciesconditions and contain predominantly biotite zone metamorphic assemblages. Reliable radlometric ages of the Coldbrook Group are available mainly on the intrusive plutonicbodies:625 + 15 M a ( U - Pb on zircon, Watters, 1987), 597 +_18 M a (Rb-Sr whole rock isochron, Barr, 1987), 530_+16 M a (Rb-Sr whole rock isochron, Mcleod, 1986), 598_+ 27 M a (K-Ar on hornblende, Barr, 1987), 550 ± 1 Ma, 625 -+5 M a and 615 -+1 M a (U-Pb on zir-

con, Barr and and White, 1988). The age of the Late Proterozoic volcanic rocks from the Caledonia Highlands is probably in the range of 600630 Ma whereas the overlying Eocambrian volcanic sequences have an age between about 550 and 565 Ma (Barr and White, 1988). Similar Late Proterozoic volcanic rocks of the Avalon zone in Newfoundland have ages ranging between 630 and 570 Ma (Dallmeyer et al., 1983; Krogh et al., 1988) and the correlative Fourchu Group volcanic and related plutonic rocks from southeastern Cape Breton Island yielded ages ranging between 608 and 631 Ma using a 4°Ar39Ar plateau on hornblende from the plutons (Keppie et al., 1990a).

S a m p l i n g and analytical notes Samples were collected from each of the three volcanic belts. Preference was given to mafic rather than felsic rock units, even though in the Western and Central belts, the latter are equal

86

to or more voluminous than the former. Nearly all EB samples came from south of the Cradle Brook thrust. In the Western and Eastern belts, most rocks are from representative sections which were sampled in detail. On the other hand, the CB samples were collected from scattered localities because of the lack of a good section. The rocks in the CB belt are well preserved but the outcrops are not good enough to establish the thickness of the flow units or their internal variability. Sixty-five maflc rocks devoid of amygdules, secondary veins and Fe-oxide staining were selected for the analyses. These rocks were the most massive and petrographically freshestlooking samples from the collected suite. From this 65 sample set, 46 rocks have loss-on-ignition ( L O I ) < 6 % and SIO2<56% (LOI-free). Major and some trace elements (Zr, Y, Nb, Cr, Ni, V, Cu, Zn, Ba, St, Rb, Ga and Pb) were determined by X-ray fluorescence. The analyses of the other trace elements including rare earth elements (REE), Co, Hf, Ta, Th, and Sc were performed by instrumental neutron activation on twenty-eight of these samples. The precision and accuracy of the analytical methods were described in Dostal et al. (1986). In general, the precision is better than 5% for major element and 3-10% for trace elements. Representative analyses for rocks from each belt are presented in Table II and a complete set of the data can be obtained from the authors. The analyses of clinopyroxene were performed by an electron microprobe.

Petrography The mafic volcanic rocks of the Coldbrook Group have recrystallized to greenschist or subgreenschist-facies mineral assemblages. Relict subophitic and porphyritic textures are present in some thin sections. A few samples also contain relict clinopyroxene phenocrysts. Most phenocrysts are zoned. Clinopyroxenes are mainly low-A1 and low-Ti augites which show a

J. DOSTALANDS.R.MCCUTCHEON TABLE I Average composition of clinopyroxene relics in the basalts of the Coldbrook Group Belt

Si02 (%) Ti02 Al20a FeO MnO MgO CaO NauO K20

Eastern (n=21)

Central (n=95)

Western (n=45)

52.22 0.54 3.35 7.70 0.24 15.57 20.18 0.19 0.03

50.74 (0.65) 0.86 (0.18) 3.10 (0.61) 10.26(1.06) 0.35 (0.09) 15.18(0.68) 19.17(0.62) 0.35 (0.06) 0.04 (0.01)

51.86 (0.70) 0.71 (0.15) 2.46 (0.29) 8.26 (0.55) 0.25 (0.05) 16.03{0.72) 20.14 (0.52) 0.27 (0.02) 0.02 (0.01)

(0.73) (0.10) (0.75) (2.01) {0.08) {0.83) (1.60) (0.04) (0.02)

Si A1Iv

1.927 0.073

1.883 0.117

1.913 0.087

A1TM Ti Mn Fe2+ Mg Ca Na

0.072 0.015 0.008 0.238 0.856 0.798 0.013

0.019 0.024 0.011 0.318 0.840 0.763 0.025

0.020 0.020 0.008 0.255 0.882 0.796 0.019

Values in brackets: s.d. (standard deviation ); n = number of analyses.

restrictedcompositional range (Table 1 ).Nonquadrilateral components such as Ti, AI, Fe 3+ and N a constitute < 6 % of the total cations.In terms of Si02-A1203 in clinopyroxene (cf. LeBas, 1962), the host rocks are subalkaline. The W B lavas contain clinopyroxene with the lowest abundances of A1203 ( ~ 2.5% ). Compared to the Eastern belt, C B pyroxenes are higher in Ti, Fe, M n and N a (Table I). The mafic rocks are composed chiefly of actinolite, chlorite,epidote and sodic plagioclasewith minor amounts of quartz, Fe-Ti oxides, sphene and carbonates. Secondary veins and amygdules filledwith quartz, calciteand chloriteoccur in several rocks.

GEOCHEMISTRY OF LATE PROTEROZOIC BASALTIC ROCKS FROM NEW BRUNSWICK

Geochemistry Alteration

The volcanic rocks of the Coldbrook Group were affected to a variable degree by secondary

87

alteration/metamorphic processes. The predominantly hydrous alteration might have modified the chemical composition of some rocks, particularly those with high LOI. In such rocks, mobile elements, including K, show considerable variation. Thus, the analyses of the strongly altered samples ( > 6% LOI) were

TABLE IIA

Major and trace element compositions of representative basaltic rocks of Coldbrook Group Western belt WB5 Si02 (%) TiO2 A12Oz Fe20z MnO

Central belt

WB31 WB29 WB30 WB10 WB39 WB4

Na20 K20 P20b LOI

51.46 1.18 15.67 9.39 0.17 7.99 3.61 5.26 0.55 0.29 4.10

52.42 1.19 15.48 9.22 0.14 7.22 6.51 3.47 0.94 0.20 3.00

51.20 1.08 15.88 9.69 0.15 7.44 6.80 3.99 0.77 0.18 2.80

52.34 1.12 16.16 9.08 0.17 6.60 6.72 3.74 0.96 0.18 2.70

Total

99.67

99.79

99.98

99.77 100.48

(Mg)

0.65

0.63

0.63

0.62

0.59

0.59

0.58

21 318 159 221 253 69 17 75 25 125 9 6 16 25.7 48.7 0.56 3.06 6.46 15.1 34.1 18.6 4.63 1.21 0.75 2.62 0.42

15 140 104 212 261 65 27 73 26 109 7 10 13 24.9 49.7 0.57 3.13 5.90 13.8 29.6 15.7 4.33 1.00 0.71 2.34 0.37

20 334 155 203 270 65 38 74 24 115 7 14 13 25.5 49.5 0.57 2.90 5.50 14.5 31.7 16.9 4.37 1.33 0.75 2.39 0.40

5 491 142 246 21 17 13 101 28 125 9 5 17 26.7 41.7 0.50 3.15 4.18 15.6 36.5 18.4 4.61 1.31 0.71 2.55 0.42

6 285 129 224 185 38 8 76 33 158 9 11 18 29.5 51.3 0.75 3.92 6.79 18.3 44.3 22.7 5.48 2.04 0.88 3.22 0.51

13 198 209 251 46 24 32 97 28 151 8 10 15 28.1 37.4 0.54 3.66 5.46 20.2 51.1 24.9 5.93 1.56 0.87 3.01 0.45

MgO CaO

Rb(ppm) Sr

Ba V Cr Ni Cu Zn Y Zr Nb Pb Ga Sc Co Ta Hf Th La Ce Nd Sm Eu Tb Yb Lu

7 118 119 301 246 45 86 31 186 9 13 16 32.3 43.0 0.54 4.57 6.57 18.8 47.1 23.7 5.71 1.22 0.74 2.78 0.46

51.30 1.03 18.40 9.15 0.16 6.02 6.21 3.76 0.78 0.17 3.50

50.04 1.26 16.30 9.84 0.15 6.33 9.22 2.81 0.51 0.20 2.60

(Mg) = M g / M g + F e 2+ with Fe~+/Fe ~+ taken as 0.15.

51.33 1.08 17.41 8.60 0.14 5.41 5.23 5.31 1.06 0.25 4.30

WB15 CB10

CB15

CB12

CB13

CB16

CB7

CB29

CB28

52.67 1.28 16.84 8.06 0.14 4.83 8.60 3.00 1.42 0.31 3.20

46.85 1.23 16.64 11.62 0.21 8.29 6.45 4.06 0.24 0.26 4.10

47.27 1.03 15.87 11.83 0.17 8.20 9.37 2.44 0.39 0.14 2.60

47.12 1.13 16.39 11.98 0.21 7.98 7.09 2.76 0.66 0.18 3.40

49.42 1.06 15.48 11.43 0.20 7.56 7.20 3.41 0.59 0.15 3.10

50.91 0.96 14.92 11.21 0.24 7.01 5.56 4.85 0.33 0.13 3.40

51.68 1.03 15.14 11.27 0.20 6.55 5.64 5.08 0.05 0.15 2.90

49.78 0.97 15.35 11.61 0.19 6.72 9.52 2.17 0.52 0.13 1.90

48.66 1.08 15.84 11.94 0.21 6.88 8.60 2.79 0.85 0.16 2.10

99.26 100.12 100.35

99.95

99.31

98.90

99.54

99.52

99.69

98.86

99.11

0.57

0.61

0.60

0.59

0.59

0.58

0.56

0.56

0.56

27 480 231 209 52 21 23 64 27 175 10 9 20 23.8 38.1 0.63 4.99 7.22 22.7 53.8 27.5 5.85 1.59 0.76 2.52 0.38

1 242 155 284 272 74 11 99 28 108 7 14 14 31.8 54.6 0.37 2.44 1.03 10.9 27.1 16.2 3.78 1.29 0.81 2.77 0.45

7 140 98 247 231 108 18 91 25 58 5 9 15 34.9 74.6 0.34 1.72 0.70 4.51 12.1

9 205 314 246 181 95 14 113 28 72 5 5 18 31.7 71.1 0.33 2.17 0.88 6.95 17.7 11.5 3.32 1.08 0.71 2.95 0.50

6 203 448 234 120 78 70 110 28 74 7 17 15 30.1 66.2 0.44 1.82 1.54 9.05 21.3 12.8 3.15 1.06 0.74 3.01 0.47

5 95 43 132 15 310 281 190 169 67 67 48 87 146 96 29 33 76 89 7 8 12 14 14 15 38.4 39.3 64.2 62.9 0.44 0.55 1.95 2.97 2.14 3.66 8.81 13.0 20.9 30.1 12.5 17.6 3.13 3.94 1.05 0.94 0.73 0.91 3.09 3.65 0.50 0.61

12 151 172 234 180 78 11 99 24 59 4 11 14 35.1 68.6 0.29 1.73 0.76 4.51 12.2

20 194 234 238 158 79 16 92 27 78 5 3 16 33.8 69.3 0.30 2.11 0.85 6.44 17.1

2.46 0.86 0.59 2.56 0.46

3.07 1.08 0.72 3.02 0.51

2.70 0.93 0.61 2.56 0.45

88

J. DOSTALAND S.R. MCCUTCHEON

T A B L E IIB Major a n d trace element composition of representative basaltic rocks of Coldbrook Group

Eastern belt EB11 Si02 (%) Ti02

EB15

EB12

EB2

EB18

EB25

EB19

EB6

Fe203 MnO MgO CaO Na20 K20 P2Os LOI

47.71 0.62 12.82 10.74 0.17 12.85 8.39 2.11 0.33 0.04 3.90

53.16 0.70 15.09 7.64 0.14 7.95 7.80 4.78 0.03 0.06 2.30

50.27 0.76 14.51 10.43 0.18 9.89 5.79 4.04 0.04 0.06 4.40

53.38 0.62 14.81 8.17 0.28 7.21 8.43 4.86 0.05 0.05 2.10

48.83 0.83 15.10 10.16 0.22 8.43 8.67 3.35 0.03 0.06 4.10

49.77 0.83 15.07 11.78 0.35 6.84 8.32 2.21 0.03 0.06 5.30

55.68 1.27 13.96 13.23 0.14 5.54 2.49 3.17 0.01 0.08 5.40

51.11 1.73 14.54 14.10 0.20 5.55 4.55 4.46 0.07 0.12 3.60

Total

99.68

99.65

100.37

99.96

99.78

100.56

100.97

100.03

(Mg)

0.72

0.70

0.67

0.66

0.64

0.56

0.48

0.46

3 65 72 258 702 149 42 72 12 27 1 7 12 36.5 63.6 0.07 0.61 0.13 1.22 3.79

< 1 85 21 252 729 61 229 75 17 35 1.5 3 11 35.9 57.4 0.09 0.75 0.12 1.57 5.15 4.00 1.34 0.51 0.36 1.55 0.27

< 1 85 22 302 516 101 39 103 15 33 1.5 19 14 35.9 57.1 0.08 1.22 0.47 1.43 3.79 3.98 1.44 0.34 0.35 1.57 0.27

1 188 73 259 566 49 2 156 16 38 3 6 13 43.5 51.3 0.13 0.86 0.43 2.62 7.30

1 210 28 322 505 66 21 101 19 41 1.5 10 15 40.1 41.6 0.10 0.86 0.22 1.82 5.85

1 185 106 538 60 15 26 124 32 89 4 11 17 39.9 45.8 0.22 2.24 0.71 4.64 13.1

1.44 0.57 0.38 1.44 0.25

1.52 0.59 0.40 1.72 0.31

< 1 49 10 533 19 20 11 107 24 51 3 9 17 35.8 46.1 0.13 1.36 0.48 2.34 7.23 6.00 2.04 0.75 0.52 2.37 0.41

A1203

Rb (ppm) Sr Ba V Cr Ni Cu Zn Y Zr Nb Pb Ga Sc Co Ta Hf Th La Ce Nd Sm Eu Tb Yb Lu

1.03 0.40 0.27 1.22 0.21

eliminated and the following discussion is based mainly on elements such as Zr, Nb, Ti and REE which are generally considered to be immobile during low-grade metamorphism and alteration (Condie, 1982).

< 1 176 13 293 303 87 1 101 21 48 2.5 14 37.5 54.1 0.15 1.15 0.31 3.08 8.46 1.81 0.72 0.44 1.82 0.30

3.10 1.16 0.76 3.12 0.56

Major elements The mafic volcanic rocks of the Coldbrook Group display a relatively wide range of SiO2 {46-56% on a LOI-free basis) although most of

GEOCHEMISTRY OF LATE PROTEROZOIC BASALTICROCKS FROM NEW BRUNSWICK 16

12-

TABLE III

/

j i

/

.J

Average composition of Late Proterozoic basaltic rocks [Si02 < 56% and (Mg) > 0.55 ] of Coldbrook Group

" ' ~ ) "-....~

Belt

4-

FeOt/MgO Fig. 2. FeOt Vs FeOt/MgO diagram of Miyashiro (1974) for the basaltic rocks of the Coldbrook Group. Field of CB basalts: dashed-dotted line; field of EB basalts: dashed line; field of WB basalts: solid-line. Dividing {dashed) line between tholeiitic ( T H ) and calc -alkalic (CA) field after Miyashiro {1974).

them have basaltic composition (Table II). According to the relations of immobile incompatible elements such as Zr/TiO2 vs Si02 (Winchester and Floyd, 1977), the mafic rocks of all three belts have subalkaline affinities. They are olivine- or subordinately quartz-normative and have high contents of A1 and Ca but low concentrations of Ti and P like recent volcanic arc basalts. The (Mg)values ( M g / M g + F e 2+ with Fea+/Fe 2+ =0.15) of the mafic rocks vary from 0.72 to 0.46 although the majority of the samples have (Mg) between 0.65 and 0.55. There are compositional differences among the three belts. The WB mafic volcanic rocks are calc-alkalic on the basis of the FeOt-FeOt/ MgO and SiO2-FeOt/MgO criteria of Miyashiro (1974) and do not exhibit an Fe-enrichment trend (Fig. 2). The mafic rocks of the Central and Eastern belts are higher in Fe, Mg and Mn and lower in K and are tholeiitic in terms of the SiO2-FeOdMgO and FeOt-FeOt/MgO relationships (Fig. 2). Their concentrations of Fe and Ti increase slightly with increasing differentiation, a feature typical of tholeiitic suites. The CB mafic rocks are higher in A1, K, Ti and P but lower in Mg than the EB rocks of comparable SiO2 and (Mg) values (Tables II and

III).

89

SiO 2 (%)

Eastern

Central

Western

(n=6)

(n=10)

(n=9)

MnO MgO CaO Na20 K20 P2Ob

52.44 0.76 15.12 10.21 0.23 9.21 8.20 3.69 0.09 0.06

(Mg)

0.66

Ti02 Al~03

Fe203

V (ppm) Cr Ni Sc Co Zn Cu Sr Rb

(1.90) (0.11) (0.90) (1.75) (0.08) (2.35) (1.12) (1.23) (0.13) (0.01)

281 554 85 38 54 101 56 135 1

(29) (155) (36) (3) (7) (30) (87) (63) (0.8)

Ba

38

(27)

Pb Ga Y Nb

9 13 17 1.8 0.10 37 0.91 0.28 1.96 5.72 1.43 0.52 0.37 1.55 0.27

Ta Zr Hf

Th La Ce Sm Eu Tb Yb Lu

(6) (1) (3) (0.7) (0.03) (7) (0.23) (0.15) (0.73) (1.89) (0.25) (0.14) (0.06) (0.21) (0.04)

50.36 1.17 16.25 12.47 0.21 7.72 7.72 3.48 0.43 0.18

(1.78) (0.18) (0.63) (1.00) (0.02) (0.76) (1.76) (1.11) (0.24) (0.05)

53.68 1.18 16.96 9.33 0.16 6.73 6.74 4.07 0.94 0.22

0.58

0.61

273 190 81 35 66 111 37 159 7 196 10 15 28 6 0.36 79 2.14 1.26 7.85 19.7 3.27 1.08 0.76 3.09 0.51

(44) 229 (41) 180 (13) 46 (4) 27 (6) 44 (23) 79 (27) 23 (59) 283 (6) 15 (125) 163 (4) 9 (1) 16 (3) 27 (1.2) 8 (0.09) 0.58 (15) 140 (0.39) 3.57 (0.98) 6.05 (2.68) 17.0 (5.75) 39.8 (0.47) 5.01 (0.15) 1.39 (0.11) 0.77 (0.43) 2.64 (0.06) 0.42

(1.21) (0.10) (1.03) (0.66) (0.01) (1.04) (1.71) (0.93) (0.30) (0.06)

( 33 ) (109) (22) (3) (5) (13) (10) (138) (8) (46)

(3) (2) (3) (1) (0.08) (33) (0.79) (0.91) (3.11) (9.36) (0.72) (0.30) (0.06) (0.31) (0.05)

n = number of samples; values in brackets = 1 s.d. (standard deviation); ( M g ) = M g / M g + F e 2+ with Fe3+/Fe 2+=0.15;

Fe2Oa= total Fe expressed as Fe203; major elements recalculated to 100% on LOI-free basis

90

J. DOSTAL AND S.R. MCCUTCHEON

Ti 100

2.0/

\\

5O l

,,

,' \,f °

//

i

PB

0.5 ,

r

/ i

i

i

i

i

i

/

,~#-

/

\ ~^,

/

l i

A-

~

~/

\

,AT

MORB

-X \

CAB

X

J

Sc Ti V Or Mn Fe Co Ni Zn

Fig. 3. MORB-normalized average transition element contents of the basalts [with (Mg) > 0.55 ] from the three belts of the Coldbrook Group. The normalizing values are from Dupuy et al. (1982). EB ( + ), CB ( A ) and WB ( • ) . TABLE IV

Zr "- / 90

v

Y

V

v 50

v

V

V

~ • 3xY 10

Fig. 4. Ternary Ti-Zr-Y diagram of Pearce and Cann (1973) for the basaltic rocks of the Coldbrook Group from EB ( O ) , CB ( • ) and WB (X). Fields: W P B = within-plate basalts; IAT=island arc tholeiites; MORB=ocean-floor basalts; C A B = calc-alkalic basalts.

Elementalratios in basaltic rocks [SiO2 < 56%, (Mg) > 0.55 ] from Coldbrook Group

2 xNb

!

Belt F~stern Zr/SiO2(X104) Th/SiO2(X10 s) Ti/Zr Ti/V Zr/Y Zr/Th Th/Hf Th/Nb Nb/La La/Yb Th/La

Central

0.71 (0.14) 1.57 (0.32) 0.53 (0.28) 2.45 (1.81) 123 (18) 89 (13) 16 (1) 26 (3) 2.22 (0.11) 2.79 (0.43) 167 (82) 90 (52) 0.30 (0.12) 0.57 (0.36) 0.16 (0.08) 0.20 (0.11) 0.93 {0.14) 0.80 (0.14) 1.25 (0.40) 2.53 (0.78) 0.15 {0.09) 0.15 (0.07)

Western 2.62(0.62) 11.3 (1.7) 51 (9) 31 (4) 5.18(1.09) 23 (5) 1.74 (0.35) 0.72(0.11) 0.50 (0.07) 6.45(1.03) 0.36 (0.07)

4

80

50

,

20

Y

Trace elements

Fig. 5. Ternary Nb-Zr-Y diagram of Meschede (1986) for the basaltic rocks of the Coldbrook Group from E B (O), C B (•) and W B (X). Fields: W P A = within-plate alkali basalts; WPT=within-plate tholeiites; VAB=volcanic arc basalts; M O R B = mid-ocean floor basalts.

The compatible trace elements Ni, Cr and Co are variably but mutually intercorrelated and congruently decrease with increasing differentiation within each sequence. The contents of most transition elements in the basalts of all three belts are similar to those of modern volcanic arc basalts. Relative to mid-ocean ridge basalts (MORB), all the Coldbrook basalts are depleted in Ni, and with the exception of EB basalts also in Cr (Fig. 3). However, compared to many modern equivalents (e.g., Basaltic Volcanism Study Project, 1981), the basalts have

higher abundances of Ni and Cr and lower V / Ni and V/Cr ratios for a given (Mg). Gill (1979) concluded that the Archean tholeiites have higher abundances of Ni than comparable modern basalts and Reid et al. (1987) argued that certain Mid-Proterozoic tholeiites resemble the Archean basalts in their Ni contents. The Late Proterozoic tholeiites of New Brunswick are enriched in Ni relative to similar modern analogues of a given (Mg) value although to a lesser degree than the older basalts. In the tholeiitic suites, V exhibits a positive correlation with Ti,

Values in brackets= 1 s.d.(standarddeviation).

GEOCHEMISTRY OF LATE PROTEROZOIC BASALTIC ROCKS FROM N E W BRUNSWICK

Fe, and Zr which all increase with the decrease of (Mg). However, in the calc-alkalic suite of the Western belt, V, like Ti, does not vary noticeably with differentiation although both these elements display a weak positive correlation with (Mg). Compared to the basalts of the Central and Western belts, the EB tholeiites have a lower T i / V ratio (Table IV) which is also lower t h a n that of MORB but close to the ratios of primitive island arc tholeiites (IAT) (Gill, 1981; Ewart, 1982), some komatiites (Smith and Erlank, 1982) and the basalts of greenstone belts (Nesbitt and Sun, 1976). The base metal Zn and Cu do not display any systematic variations with Zr or (Mg) in any of the basalts.The average abundances of Cu are significantly lower than the typical Cu concentrations in the recent analogues (Andriambololona and Dupuy, 1978) suggesting that Cu was removed during low grade metamorphism (Fyfe et al., 1978; Dostal and Dupuy, 1987). High-field-strength-elements (HFSE) including Zr, Nb, P, Ta, and Hf exhibit covariation and their abundances increase with the increase of differentiation. However, in basaltic

91 A

2010-

1.0 ¸

0.5

i

i

i

l

J

,

,

i

J

,

,

i

,

,

•

10-

== o

~.0-

0.5 10-

1.0 ¸ 0.5-

0.1 Fig. 7. MORB-normalized trace element patterns for representativebasaltsof the Coldbrook Group. A. Western belt: W B - 4 = +; WB-10=0; B. Central belt: C B - 2 8 = O ; CB29=X; C. Eastern belt:EB-11 =X; EB-12 = 0. Normalizing values afterPearce (1982, 1983).

A 50-

101-

50-

B

50-

c =

=

105-

La Ce

Nd

Sm Eu

Tb

Yb Lu

Fig. 6. Chondrite-normalized REE abundances in the basaltic rocks of the Coldbrook Group. A. Western belt: WB4 = O; WB-10 = × ; B. Central belt; CB-13 = X CB-15= 0 ; C. Eastern belt: E B - 6 = X ; EB-15 = 0 , EB-18= +.

rocks of a given (Mg), the abundances of these elements and also Th increase from the EB tholeiites through the CB tholeiites to the WB calcalkalic basalts (Table III). The EB tholeiites are distinctly impoverished in most of the incompatible elements including HFSE; their abundance levels are similar to those of IAT (Dupuy et al., 1982; Gill, 1987). The differences in the distribution of HFSE are also shown on discrimination diagrams such as T i Zr-Y (Pearce and Cann, 1973) where the analyses move from the island arc tholeiitic field for the EB basalts to the calc-alkalic field for the WB basalts (Fig. 4) or in the Nb-Zr-Y plot (Meschede, 1986) where the rocks display distinct variation within the volcanic arc field (Fig. 5).

92

The chondrite-normalized REE abundance patterns of the WB mafic rocks (Fig. 6A) exhibit light REE (LREE) enrichment and light to moderate fractionation of heavy REE (HREE) with (La/Yb)n~2.8-5.5 and (La/ S m ) n ~ 1.4-2.1 (n = chondrite-normalized). The HREE abundances are less variable than the concentrations of LREE (La~~33-69; Ybn~ll.7-16.1). Similar REE patterns of mafic rocks and their abundances levels are seen in calc-alkalic suites of orogenic zones (e.g., Ewart, 1982; Dostal et al., 1982; Hickey et al., 1984). One of the more fractionated basaltic andesites exhibits a small but distinct negative Eu anomaly suggestive of plagioclase fractionation. However, the Eu anomaly could also be due to alteration (Whitford et al., 1988). The REE patterns of the CB basaltic rocks (Fig. 6B ) range from nearly flat ones with (La/ Yb)~ ~ 1 to patterns which show slight LREE enrichment with (La/Yb)n ~ 1.4-2.4. Some of the latter patterns can be derived from a primitive basaltic magma with flat REE patterns by fractionation of clinopyroxene. Some IAT have similar REE patterns and comparable absolute REE abundances (e.g., Ewart, 1982; Gill, 1987). Their shape is also comparable to the enriched tholeiites of Archean greenstone belts. The EB tholeiites with Lan ranging from 3.7 to 14.1 have lower LREE contents relative to the other suites. Their REE patterns show LREE depletion, or are flat, with (La/Yb)n ranging from 0.55 to 1.1 and resemble those oftholeiites from primitive island arcs, N-type MORB and depleted tholeiites of Archean greenstone belts. Trace element abundances of the basaltic rocks from the three volcanic belts of the Coldbrook Group normalized to N-type MORB are shown in Fig. 7. The patterns of the WB basalts (Fig 7A) resemble those reported by Pearce (1983) from continental margin arcs or some evolved island arc regimes. They are characterized by a distinct enrichment of large-ion-lithophile elements (LILE) including T h and Ce relative to HFSE accompanied by a steep slope between Ce and Ti and negative Nb and Ta

J. DOSTAL AND S.R. MCCUTCHEON

anomalies. All the elements to the left of Ti are enriched in the WB basalts compared to MORB. The patterns of the CB basalts (Fig. 7B) are also enriched in LILE including Th and Ce relative to HSFE but to a lesser degree than the WB calc-alkalic basalts. The abundances of Zr, Hf, Sin, Ti, Y and Yb are low, with values usually similar to or less than those of MORB and with a nearly flat Zr-Sc segment. The patterns of the CB basalts (Fig. 7B), which commonly show small depletion of Nb and Ta, are similar to those of recent tholeiites from island arcs. Mean contents of immobile incompatible elements in the CB basalts (Table III) are intermediate between the values for calc-alkalic and tholeiitic basalts from island arc environments (Jakes and White, 1972). The MORB-normalized patterns of the EB tholeiites (Fig. 7C) which are similar to those of some island arc basalts, particularly tholeiites from primitive arcs (Pearce, 1983), show depletion of immobile incompatible elements (HFSE and REE) with respect not only to the other suites but also to N-type MORB. The only exception is Th, which is enriched relative to the other immobile incompatible elements. However, the magnitude of the Th enrichment is smaller than in the other suites. In these tholeiites, the patterns of HFSE and REE are relatively unfractionated and do not show a significant negative Nb-Ta anomaly.

Petrogenesis The chemical composition of the mafic volcanic rocks of the Late Proterozoic Coldbrook Group is typical of volcanic arc lavas. Such a setting is also consistent with the occurrence of the Late Precambrian plutons of granodiorite to diorite composition (Ruitenberg et al., 1979). The close proximity and similar ages of the volcanic rocks of the three belts suggest that they are related to the same volcanic arc system. The progressive compositional changes of the mafic lavas of the Coldbrook groups across the Caledonia Highlands are similar to across-arc vari-

GEOCHEMISTRY OF LATE PROTEROZOIC BASALTICROCKS FROM NEW BRUNSWICK

ations encountered in m o d e m island arcs (Arculus and Johnson, 1978; Gill, 1981). The E B basaltic rocks closely resemble tholeiites of primitive island arcs. They have the lowest abundance of incompatible elements with Ti/ V < 2 0 and Ti/Zr> 100 (Table IV) and were probably emplaced on relativelythin continental crust or even on oceanic crust. In fact,their Zr/Y and Ta/Yb ratios are characteristic of oceanic arcs (Pearce, 1983). The C B basalts are comparable to island arc tholeiites from mature or ensialic arcs except that the abundances of some LILE are more akin to calc-alkalicsuites. In general, the C B basaltic rocks are transitionalbetween the E B and W B suites in terms of concentrations of immobile incompatible elements and have their Ti/V and Ti/Zr ratios usually in the range of 20-28 and 50-110 respectively.The W B mafic rocks correspond to typicalcalc-alkalicbasaltic types. The have the highest contents of incompatible elements usually with T i / V > 25 and T i / Zr < 60. The increase in the abundances of the incompatible elements in the basaltic rocks from the Eastern belt through the Central belt to the Western belt is accompanied by an increase of ratios such as Zr/SiO2, Th/SiO2, T h / Hf, T h / N b and Zr/Y and a decrease of Z r / T h and N b / L a (Table IV). The basalts from all three belts have lower MgO contents and (Mg) values than those predicted for primary basalt melts indicating that the rocks underwent differentiation. The relatively high contents of Ni and Cr in most basalts suggest that only moderate amounts of mafic minerals had been removed during fractional crystallization. The decrease of Ni and Cr is accompanied by a decrease of Sc and, in the EB and CB basalts, also by an increase of V. This reflects the crystallization of olivine and pyroxene with little or no fractionation of FeTi oxides in the tholeiitic suites, which would rapidly deplete V. The decrease of Ti, V and Fe with increasing differentiation in the WB calcalkalic rocks implies crystallization of Fe-Ti oxides in this suite. The negative Eu anomalies

93

observed in the evolved rocks of both calc-alkalic and tholeiitic series indicate plagioclase fractionation. However, fractional crystallization cannot explain the compositional differences among the basaltic rocks of the three belts. In particular, this process cannot readily account for the difference in ratios involving elements with high and similar degrees of incompatibility (e.g. T h / N b , T h / L a ) as well as in ratios involving elements with different degrees of incompatibility (e.g. T i / V ) in basaltic rocks of comparable (Mg) and SiO2. Similar arguments can be invoked against the derivation of the rocks from the same source by variable degrees of melting. These divergent features are probably mainly due to heterogeneous source regions. Some characteristics of the mantle sources of the three belts can be obtained from the incompatible element ratios in the basalts (Fig. 8). In the T h / Y b vs Ta/Yb diagram of Pearce (1982, 1983), a vector with a slope of unity, parallel to the field of MORB and within-plate basalts, represents processes of mantle enrichment or I /

10-

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,"

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I

/// II

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l B,s,L+s

•

i

•

iI

, "/..~/

,,'/+e'/ l

E,~

+I ooOO.',,'/.+7$C ,, .;,,( .

, ,

W

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/,-<~,,/

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O.Ol

o:1

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Fig. 8. T h / Y b vs T a / Y b plot for t h e Coldbrook Group basalts (after Pearce, 1982, 1983). Vectors represent variations expected for: w = within plate e n r i c h m e n t / d e p l e t i o n ; s = subduction related metasomatism; c = crustal contamination; [ = fractional crystallization. * = primordial mantle (Pearce, 1983 ). Fields of volcanic arc basalts (dashed line ), n o n - a r c / w i t h i n plate basalts (solid lines) a n d N-type M O R B (dashed-dotted line). W B = X; CB = 0 ; E B = O.

94

depletion which are not related to the subduction zone and include even fractional crystallization and partial melting effects. Enrichment of the mantle wedge above a subduction zone is represented by a subvertical vector. The experimental data (Tatsumi et al., 1986) suggest that unlike HFSE, LILE can be transported by aqueous fluids liberated from the subducted oceanic lithosphere. However, the subvertical vectors on these diagrams can be also due to crustal contamination since rocks of the continental crust overlap into the T h / Y b enriched field (Pearce, 1982, 1983). EB Tholeiites

Figure 8 shows that the mantle source of the EB tholeiites was depleted in Ta relative to the primordial mantle prior to a limited subduction-related metasomatic enrichment in Th. An alternative explanation for the depletion of HFSE in IAT including the EB basalts relative to primordial mantle a n d / o r chondrites is the presence of accessory minerals such as sphene, rutile, ilmenite and zircon in the residue after melting and extraction of basalts (e.g., Dixon and Batiza, 1979; Saunders et al., 1980). However, the occurrence of accessory minerals in the upper mantle residue is not consistent with the similarities of element ratios including Ti/Y, Ti/Zr and T b / L u in the EB tholeiites, MORB and chondrites, the lack of depletion of the other HFSE-Zr, Hf and Ti relative to other equally incompatible elements in volcanic arc basalts (Arculus and Powell, 1986) and the experimental studies of Green and Pearson (1986), which ruled out residual Ti-rich mantle phases in the generation of arc basalts. The origin of the HFSE-depleted IAT probably involves a high degree of melting (20-30 %; Nicholls and Ringwood, 1973) of an upper mantle source already depleted in some incompatible elements in a prior melting event (e.g., Green, 1973; Dupuy et al., 1982). The composition of this source was probably similar to that of N-type MORB (Dupuy et al. 1982; Gill,

J. DOSTAL AND S.R. MCCUTCHEON

1987). However, the higher abundances of Th (and probably also of K, Rb, Sr and Ba) and rarely even LREE in some of the basalts relative to N-type MORB require that the source was selectivity enriched in LILE by materials derived from the subducted crust (Green, 1973, 1976; Kay, 1980) or that the basalts were contaminated by continental crust. The similarities of the LILE enrichment to those of IAT from primitive arcs, where continental crust is absent, favour subduction-related enrichment. This is also consistent with the low T h / L a ratio of the EB tholeiites (Table IV) accompanied by low LREE abundances. The ratio which is considered to be sensitive to crustal contamination (Hildreth and Moorbath, 1988) is similar in most EB tholeiites to that of the mantle ( T h / L a ~ 0.12; Sun, 1982). This suggests that the contribution from arc crust was at most very small.

CB and WB Basalts

Figure 8 suggests that the WB and most of the CB basalts were derived from an upper mantle source different from that of EB tholeiites. The subvertical trend for these basalts (Fig. 8) together with their enrichment and distribution of incompatible elements is similar to that of modern arc-related mafic rocks and may have been acquired from a source that underwent variable compositional modification comparable to those suggested for the upper mantle wedge overlying modern subduction zones. Alternatively, the enrichment in the basalts may reflect contamination of basaltic magma by incompatible element-enriched crustal material during the rise and storage of magma or the introduction of continentally derived sediments to the magma source region via subduction (Stern et al., 1984; Hickey et al, 1986). From the distribution of immobile incompatible elements alone it is difficult to constrain the relative role of mantle heterogeneity and crustal

GEOCHEMISTRY OF LATE PROTEROZOIC BASALTIC ROCKS FROM NEW BRUNSWICK

contamination (particularly in lower crust) in basalt genesis. However, they were probably both involved. The high T h / L a ratio particularly in the WB basalts (Table IV) suggests that these rocks were affected by crustal contamination. The involvement of crustal contamination is not surprising considering the abundance of felsic rocks in the volcanic pile (see Ruitenberg et al., 1979). Conclusions The volcanic rocks of the Late Proterozoic Coldbrook Group were formed in a convergent plate margin setting. The EB tholeiites, which are depleted in HFSE and LREE, resemble island arc tholeiites from primitive island arcs. However, the abundance of felsic volcanics in the sequence favours a more evolved arc or backarc. The EB basalts, which also share some geochemical features with depleted tholeiites of the Archean greenstone belts, were probably derived from an upper mantle source similar to that of N-type MORB but by a higher degree of melting. This source was enriched in LILE by components produced by dehydration of a subducted oceanic slab. The enrichment was only minor, resulting mainly in the enrichment of T h and probably also of K, Rb, Sr and Ba. The CB basalts are tholeiites with variable enrichment of LILE including LREE. Their abundances of incompatible elements resemble those of enriched island arc tholeiites and enriched tholeiites of Archean greenstone belts and are intermediate between the IAT and calcalkalic basalts of island arcs. Most of t h e m were derived from a different mantle source than the EB tholeiites and were probably emplaced farther away from the trench. The distribution of incompatible elements, the relative abundances of felsic volcanic rocks and the presence of sedimentary rocks in the Central belt are consistent with an origin in an evolved arc, the rocks were emplaced on a continental crust. The calc-alkalic basalts from the Western belt and their association with abundant felsic volcanic

95

rocks favour an origin in a continental margin setting. The basalts exhibit the largest enrichment of LILE. The basaltic rocks of the Coldbrook Group exhibit distinct zonation expressed by the occurrence of incompatible element-depleted tholeiites in the southeastern part of the Eastern belt and by WB calc-alkalic basalts which are most enriched in incompatible elements in the northwestern section of the Coldbrook Group. Similar transverse variations have been frequently observed in modern volcanic arc systems which overlie subduction zones (Arculus and Johnson, 1978; Gill, 1981 ). The across-arc compositional changes may be related mainly to an upper mantle source which was progressively more enriched in incompatible elements although the basaltic magmas could have become concurrently more contaminated by crust that became progressively thicker to the northwest. The observed zonation of the Coldbrook Group, if not tectonically assembled, {i.e. the belts are in their original sequence) is consistent with a northwestward-dipping subduction zone during Late Proterozoic time. The correlative volcanic sequences of the Fourchu Group in the Avalon Zone of southeastern Cape Breton Island show compositional zonation which is also compatible with a subduction zone dipping to the northwest and a trench located southeast of Cape Breton Island (our unpublished data). The Eocambrian volcanic rocks overlying the Coldbrook Group were emplaced in an extensional setting (Barr and White, 1988), suggesting subsequent rifting of the Late Proterozoic volcanic arc. The occurrence of island arc basalts, particularly incompatible element-depleted EB tholeiites, implies subduction of the oceanic lithosphere and suggests that basins associated with Late Proterozoic magmatic arc systems in the Avalon Zone of the Canadian Appalachians (Keppie et al., 1990b) reached a stage ofoceanization. The oceanic lithosphere may be represented by ophiolites of the Burin Group from the Avalon zone in Newfoundland dated at

96

763+2 Ma (U-Pb on zircon, Krogh et al., 1988).

Acknowledgements The study was supported by the Natural Sciences and Engineering Research Council of Canada (operating grant A3782). Thanks are due to Dr. N. Arndt for critical comments on the manuscript.

References Alcock, F.J., 1938. Geology of the Saint John region, New Brunswick. Geol. Surv. Can. Mem., 216, 65 pp. Andriambololona, R. and Dupuy, C., 1978. Repartition et comportement des elements de transition dans les roches volcaniques. I. Cuivre et zinc. Bull. B.R.G.M., 2: 121138. Arculus, R.J. and Johnson, R.W., 1978. Criticism of generalized models for the magnmtic evolution of arc-trench systems. Earth Planet. Sci. Lett., 39: 116-126. Arculus, R.J. and Powell, R., 1986. Source component mixing in the regions of arc magma generation. J. Geophys. Res., 91: 5913-5926. Barr, S.M., 1987. Field relations, petrology and age of plutonic and associated metavolcanic and metasedimentary rocks, Fundy National Park area, New Brunswick. In: Current Research, Part A. Geol. Surv. Can. Pap., 871A: 263-280. Barr, S.M. and White, C.E., 1988. Petrochemistry of contrasting Late Precambrian volcanic and plutonic associations, Caledonian Highlands, Southern New Brunswick. Marit. Sediments Atl. Geol., 24: 353-372. Basaltic Volcanism Study Project, 1981. Basaltic Volcanism on the Terrestrial Planets. Pergamon, New York, N.Y., 1286 pp. Condie, K.C., 1982. Archean andesites. In: R.S. Thorpe (Editor), Andesites: Orogenic Andesites and Related Rocks. Wiley, New York, N.Y., pp. 575-590. Currie, K.L., 1987. The Avalonian terrane around Saint John, New Brunswick, and its deformed Carboniferous cover. In: Geol. Soc. Am. Centennial Guide - Northeastern Section, pp: 403-408. Currie, K.L., Nance, R.D., Pajari, G.E. and Pickerill, R.K., 1981. Some aspects of the pre-Carboniferous geology of Saint John, New Brunswick. In" Current Research, Part A. Geol. Surv. Can. Pap., 81-1A: 23-30. Dallmeyer, R.D., O'Brien, S.J., O'Drisscoll, C.F. and Hussey, E.M., 1983. Chronology of tectonothermal activity in the western Avalon Zone of the Newfoundland Appalachians. Can. J. Earth Sci., 20: 355-363. Dixon, T.H. and Batiza, R., 1979. Petrology and chemistry of recent lavas in the northern Marianas: implications

J. DOSTALANDS.R.MCCUTCHEON for the origin of island arc basalts. Contrib. Mineral. Petrol., 70: 167-182. Dostal, J. and Dupuy, C., 1987. Gold in Late Proterozoic andesites from Northwestern Africa. Econ. Geol., 82: 762-766. Dostal, J., Dupuy, C. and Coulon, C., 1982. Cainozoic andesitic rocks of Sardinia (Italy). In: R.S. Thorpe (Editor), Andesites: Orogenic Andesites and Related Rocks. Wiley, New York, N.Y., pp. 353-370. Dostal, J., Baragar, W.R.A. and Dupuy, C., 1986. Petrogenesis of the Natkusiak continental basalts, Victoria Island, Northwest Territories, Canada. Can. J. Earth. Sci., 23: 622-632. Dupuy, C., Dostal, J., Marcelot, G., Bougault, H., Joron, J.L. and Treuil, M., 1982. Geochemistry of basalts from central and southern New Hebrides arc: implication for their source rock composition. Earth Planet. Sci. Lett., 60: 207-225. Ewart, A., 1982. The mineralogy and petrology of TertiaryRecent orogenic volcanic rocks with special reference to the andesite-basaltic compositional range. In: R.S. Thorpe (Editor), Andesites: Orogenic Andesites and Related Rocks. Wiley, New York, N.Y., pp. 25-95. Fyfe, W.S., Price, N.J. and Thompson, A.B., 1978. Fluids in the Earth's Crust. Elsevier, New York, N.Y., 383 pp. Giles, P.S. and Ruitenberg, A.A., 1977. Stratigraphy, paleogeography and tectonic setting of the Coldbrook Group in the Caledonia Highlands of southern New Brunswick Can. J. Earth Sci., 14: 1263-1275. Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Springer, Berlin, 390 pp. Gill, J.B., 1987. Early geochemical evolution of an oceanic island arc and backarc: Fiji and the South Fiji basin. J. Geol., 95: 589-615. Gill, R.C.O., 1979. Comparative petrogenesis of Archaean and modern low K tholeiites. A critical review of some geochemical aspects. In: L.H. Ahrens (Editor), Origin and Distribution of the Elements. Proc. 2nd Symp. Paris, UNESCO, May 1977. Pergamon, Oxford pp. 431-447. Green, D.H., 1973. Experimental melting studies on a model upper mantle composition at high pressure under watersaturated and water-undersaturated conditions. Earth Planet. Sci. Lett., 19: 37-53. Green, D.H., 1976. Experimental testing of "equilibrium" partial melting of peridotite under water-saturated, high pressure conditions. Can. Mineral., 14: 255-268. Green, T.H. and Pearson, N.J., 1986. Ti-rich accessory phase saturation in hydrous mafic-felsic compositions at high P,T. Chem. Geol., 54: 185-201. Hayes, A.O. and Howell, B.F., 1937. Geology of Saint John, New Brunswick. Geol. Soc. Am. Spec. Pap. 5, 146 pp. Hickey, R.L., Gerlach, D.C. and Frey, F.A., 1984. Geochemical variations in volcanic rocks from central-south Chile (33-42°S). In: R.S. Harom and B.A. Barreiro (Editors), Andean Magmatism: Chemical and Isotopic Constraints. Shiva, Cheshire, pp. 72-95.

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