Petrology of Serra Geral (Paran�) continental flood basalts, southern Brazil: crustal...

Contrib Mineral Petrol (1985) 91:54-65 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1985

Petrology of Serra Geral (Paranfi) continental flood basalts, southern Brazil: crustal contamination, source material, and South Atlantic magmatism R.V. Fodor 1, Cricket Corwin 1, and Ari Roisenberg 2 1 Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA z Instituto de Geociencias, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil

Abstract. The Serra Geral (Paranfi) continental flood basalt (SG-CFB) province in Brazil is associated with the Jurassic- Cretaceous breakup of Gondwanaland and the transition of continental to oceanic magmatism during the opening of the South Atlantic Ocean. A suite of 24 samples representative of the SG-CFB in Rio Grande do Sul, southern Bra- zil, shows a compositional continuum from basalt (50-53 wt% SiO2, Mg # 60-45), to basaltic andesite, to andesite, rhyodacite, and rhyolite (73 wt% SiO2). Certain compositional aspects of the mafic rocks (e.g., TiO2, K20, CaO, Zr/Nb, Zr/Y, Ti/Zr) resemble those of basaltic dikes and flows associated with the opening of the North Atlantic Ocean.

Fractionation trends are apparent in MgO variation diagrams and calculations show that basalt-basaltic andesite continuums are largely due to removal of plagioclase and clinopyroxene. These mafic rocks can be categorized as (i) having higher or lower incompatible-element contents (e.g. K20 0.6-1.5 wt%; Rb 12-43 ppm; Ba 125-240 ppm) due to incorporation of Brazilian Archean crust or rhyolitic magma by basalt from a particular source material or to partial-melting differences of that source, and (ii) as having higher or lower TiO2, Sr, Ba, and P contents due to source heterogeneities. Crustal components are "obvious" in certain basaltic samples, as where K20 > 1 wt%, SiO2 > 51%, and TiOz ~ 1%, but are insignificant in others (e.g., compositions close to those of South Atlantic basin basalts). Cal- culations indicate origins for intermediate and silicic rocks by removal of pl, cpx, and Ti-magnetite from basaltic an- desire, but crust and magma-mixing affecting basaltic-andesite fractionates were likely also involved. Where contamination is insignificant, Zr, Nb, and Y abundances indicate T-type MORB source material like that for certain S. At- lantic Ocean basalts. Source material was essentially a 9:1 hybrid of N-type and P-type MORB components. N-type MORB lithosphere followed SG-CFB because decompression due to rifting crust enabled partial melting of upper- most (depleted) mantle.

Introduction

The Serra Geral continental flood basalts of southern Brazil (Paranfi basin) are tectonically significant in their associa- tion with the Jurassic-Cretaceous breakup of Gondwana- land. They mark the geochemical transition from continen-

tal to oceanic lithosphere and are well situated for evaluation of basaltic magmas produced in continental versus oceanic lithosphere and their source materials. Our primary interest in Serra Geral (SG) basaltic rocks is in how their compositions depict this transition, and in how their mantle source-areas relate to those of South Atlantic basin basalts that originated in approximately the same global region (i.e. Tristan hot spot). We therefore selected 24 samples from the southern extremity of this 106 km 2 continental flood basalt (CFB) province for a multi-discipline petro- logic examination and for presentation of results and dis- cussions in companion publications. This paper emphasizes petrography and whole-rock major- and trace-elements to determine the origins of SG-lavas, the nature of their source regions, the role of crustal contamination in their histories, and how they compare with South Atlantic basin basalts. The companion paper (Hughes et al. 1985 in prep.) stresses source-material modeling through rare-earth elements and isotopes.

Sample localities and general descriptions

The SG continental flood basalts occupy the Paranfi basin, an ovate-shaped structural feature trending NE-SW through southern Brazil. Potassium-argon dating places the age range at 106-149 m.y. and the peak of volcanism at about 120-123 m.y. ago (Amaral et al. 1966). K - A r ages of five samples of this study are 111 to 129 m.y. The study area is in the state of Rio Grande do Sul, located between latitudes 27 ~ to 30~ and longitudes 50 ~ to 53~ and com- prises ~35,000km 2 (Fig. l). Figure 1 illustrates the numbers and localities of the 24 samples studied. Different symbols denote three geographic sectors that correlate well with certain geochemical aspects of the mafic rocks.

Petrographically, principal textures are intergranular, diktytaxitic to intersertal, hyaloophitic, and vitrophyric. Nearly half the samples are phyric, containing phenocrysts or microphenocrysts of plagioclase, high-Ca clinopyroxene, olivine (iddingsite), and opaque oxides in various combinations that never exceed about eight volume percent of the rock. Groundmass mineralogy is the same except that olivine and/or pigeonite may be present. Modal mineralogy separates the basaltic rocks into those with phenocryst or groundmass olivine (RS26, RS37, RS60, RS68, RS73, EV01, IO2), and those without olivine (RS16, RS20, RS69, RS71, RS80, G215, G402, MO3, MRO3). In addition, MRO3 and G215 contain interstitial quartz, and G215 has amphibole. The diktytaxitic-intersertal areas contain light-brown glass, some altered to smectite, or incipient alkali feldspar that probably devitrified from glass.

28 c

29 ~

30 ~

5 4 ~ 55 ~ 52 ~ 51 ~ 50 ~

@J~ BreziJ

SAR;NOI~ RS 75 �9 RS80

LAGO~ VERMELH~ Pos~sodo CARIZr~Og 3 RS 68 G 402 ~ RS 20 soco~ RS 16 S. JOAOU~M

RS 60 �9 �9 RS 37 ~ V

�9 G 6,~ ""'% " ~ ~ i G 215 v BOM JESOS 9

SOLEDADE7 �9 RS 3 0 o RS 50 t

�9 RS 26 R I0 GRANDE /

DO SUL M 0 2 8 M OI c~,x~,s DO suL R S 6 5 o o M 0 5 ~' (BRAZIL)

ESTRELAV MR 0.3 o ~ -

; ~ T O MAm~ 102 0 EV OI o ~ , ~ . o ~

PORTO ALEGRE

) 8 o

2 9 ~

5 0 ~

Fig. 1. Map showing sample localities in the state of Rio Grande do Sul, southern Brazil. Localities are divided into three geographic sectors, denoted by symbols, which correspond to geochemical aspects of the samples (e.g. Fig. 2)

I g e o g r o p h i c s e c t o r s

I~ .,,/I , ~ II " ' I 1 ~ r s~o~ �9 i ~ o II o , , I I1:,1~

I , ,o ,, ,os,o. L , - , t : - - ' I i i I . ~ . . ~ ........... o ~ , j - , ? 7 ~ ? ',~ I ~ F ~

t - r - , , ................... "o" "< ............ ~ o ~ "o

~ , ' , oO. ~ ~ , ....... . . ~ . - ~ ~ , o ......... ~ ~ ~ - ~ o ~ f . . . . . . . . . . . �9

" Columbia Rive~ ......... 0

C I I I I I I I 4 I I I | i 50 52 54 56 50 52 5~4

J 56

55

o

o . , l ' :o, ~ o

%%1, v ;os'% v ~ 5 [ ' ~ , ,.o : . ,#o~

................................................ "~ R , v e r . . [] " Co/umbio 2

h5

hO

0,5'

..... ":: ~ 14 n �9 : o

�9 �9 �9 ........

I~1[1~ �9 I~ "6 ~ 12

/ / N, 4#an#c held / I 0

= i I I I I I 8 5 0 5 2 5 4 5 6

wi. % S i O 2

�9 field~ n �9 o ........ O . . . / �9 �9

o )

| I I I I I 50 52 54

wl, % SiO 2

I 5 6

Fig. 2. SiO 2 variation diagrams for a suite of the Serra Geral continental flood basalts, southern Brazil. Insets include data for rocks > 60 wt.% SiO2. Geographic sectors refer to sample localities in Fig. 1. Reference fields: N. Atlantic from Weigand and Ragland (1970) and Bertrand et al. 1982; Columbia River is for Picture Gorge and Lower Yakima units, from Waters (1961), Wright et al. (1973), and McDougall (1976). Hawaii line separates Hawaiian tholeiitic and alkalic basalts (Macdonald and Katsura 1964)

Tab

le 1

. W

ho

le-r

ock

co

mp

osi

tio

ns

for

24 m

afic

, in

term

edia

te,

and

si

lici

c ro

cks

of

the

Ser

ra G

eral

pro

vin

ce R

io

Gra

nd

e d

o

Sul

sta

te,

sou

ther

n B

razi

l.

Sec

tors

ref

er t

o

geo

gra

ph

ic

~,

loca

liti

es i

n F

ig.

1. C

olu

mn

s ar

ran

ged

in

ord

er o

f d

ecre

asin

g M

g #

fo

r ea

ch s

ecto

r

Sec

tor

I S

ecto

r II

S

ecto

r II

I

RS

20

G-4

02

RS

76

RS

26

G-6

48

RS

60

RS

16

RS

37

RS

71

G-2

15

RS

80

RS

69

RS

70

RS

30

EV

O1

MO

2

MO

1 IO

2

MO

3

RS

65

MR

O3

R

S50

R

S73

R

S68

SiO

z 51

.50

51.6

5 51

.58

51.7

7 51

.79

51.9

6 53

.71

52.2

7 53

.04

54.1

5 53

.11

53.6

4 53

.22

66.5

5 51

.64

52.8

5 52

.94

53.7

4 54

.35

62.6

9 55

.54

73.0

1 50

.13

49.8

7 T

iO2

1.13

1.

13

1.40

1.

32

1.46

1

.41

1

.41

1.

50

1.72

1.

68

1.90

1

.71

1.

87

1.06

1.

02

1.08

1.

08

1.35

1.

50

1.32

1.

84

0.63

1,

89

2.12

A

1203

13

.78

14.2

9 14

.18

14.4

8 14

.35

14.2

6 14

.28

14.1

8 13

.72

13.1

2 1

3.2

1

13.3

4 12

.94

13.6

4 16

.54

14.7

9 16

.51

15.0

8 14

.23

13.5

8 13

.21

12.5

9 14

.35

13.5

7 F

eO"

11.5

7 11

.64

12.0

8 12

.18

12.6

1 12

.48

12.2

0 13

.33

13.3

0 13

.79

15.1

0 14

.04

14.8

4 5.

84

9.45

9.

84

9.17

10

.71

12.7

0 8.

35

13.6

9 4.

11

12.4

3 13

.39

Mn

O

0.21

0.

20

0.20

0.

20

0.20

0.

20

0.19

0.

22

0.20

0.

19

0.22

0.

21

0.18

0.

09

0.15

0.

14

0.19

0.

17

0.20

0.

13

0.19

0.

05

0.19

0.

21

Mg

O

6.94

6.

81

6.17

6.

18

5.87

5.

82

5.49

5.

16

4.82

4.

33

4.58

4.

22

3.90

1.

45

7.90

7.

59

6.82

5.

10

4.25

2.

42

3.25

0.

49

6.75

6.

37

CaO

10

.95

11.3

2 10

.30

10.0

9 10

.02

10.3

9 9.

71

10.1

0 9.

52

8.57

9.

11

8.45

8.

11

2.79

10

.74

9.70

9.

98

9.06

9.

23

4.57

6.

85

1.09

11

.09

10.8

4 N

azO

2.

16

2.34

2.

28

2.44

2.

17

2.48

2.

46

2.94

2.

79

2.83

2.

47

2.86

2.

89

3.09

1.

86

1.68

1.

83

2.14

2.

13

2.38

2.

62

2.17

1.

93

1.8

1

K2

0

0.63

0.

63

1.09

0.

76

1.12

0.

84

0.62

0.

99

0.66

1

.31

0.

81

1.5

1

1.52

3.

90

1.09

1.

55

1.37

1.

83

0.93

3.

48

2.38

5.

51

0,90

0.

74

P20

5 0.

14

0.14

0.

18

0.18

0.

19

0.18

0.

19

0,19

0.

22

0.23

0.

23

0.23

0,

22

0.25

0.

18

0.17

0.

19

0.21

0.

21

0.23

0.

19

0.11

0.

24

0.25

Tot

al

FeO

F

ezO

3 L

.O.I

. b

Mg

#

Rb

S

r B

a Z

r N

b

Y

La

Cr

Ni

Cu

Z

n

Zr/

Nb

Z

r/Y

Y

/Nb

Ti/

Zr

99.0

1 10

0.15

99

.46

99.6

0 99

.78

100.

02 1

00.2

6 10

0.88

99

.99

100.

20

100.

74 1

00.2

1 99

.69

98.6

6 10

0.57

99

.38

100.

08

99.3

9 99

.73

99.1

5 99

.76

99.7

6 99

.90

99.1

7

7.60

7.

45

7.60

6.

64

5.96

7.

70

7.08

6.

93

7.48

7.

94

8.20

4.

45

4.92

1.

87

4.41

4.

65

4.97

6.

15

7.38

5.

31

5.22

7.

10

6.48

5.

29

7.95

10

.64

11.0

1 4.

41

0.81

1.

02

1.19

1.

40

1.62

1.

03

1.92

0.

85

1.34

0.

78

1.00

1.

03

1.04

1.

23

51.7

51

.1

47.7

47

.5

45.4

45

.4

44.5

40

.8

39.3

35

.9

35.1

34

.9

31.9

30

.7

6.02

5.

49

4.60

6.

92

8.28

4.

04

6.79

0.

80

3.81

4.

83

5.07

4.

21

4.91

4.

78

7.66

3.

67

0.90

1.

59

2.60

1.

28

2.43

0.

87

0.95

0.

76

6.50

5.

07

6.58

9.

24

1.13

2.

14

59.8

57

.9

57.0

45

.9

37.4

34

.1

29.7

17

.5

49.2

45

.9

12

12

26

18

25

17

21

29

26

35

15

40

40

158

26

43

32

48

15

120

68

244

16

13

152

177

173

206

174

177

201

194

185

172

151

158

161

163

238

200

206

185

217

162

170

89

327

389

127

139

224

241

228

212

265

280

285

342

223

354

268

594

238

224

257

360

362

510

502

554

273

303

95

98

123

122

128

132

141

126

146

143

140

144

138

253

127

129

138

167

167

201

159

295

132

148

7 8

9 12

9

10

14

11

13

13

11

10

12

34

14

10

11

19

14

21

14

34

12

13

25

26

30

28

40

32

30

31

34

35

34

43

40

68

27

30

29

36

31

54

44

93

26

26

8 8

10

18

23

17

17

13

16

15

13

19

19

44

13

12

18

19

30

35

30

53

11

13

50

60

85

60

85

100

50

22

60

50

30

85

55

45

200

330

271

145

40

30

35

20

155

100

64

63

66

60

61

58

52

43

43

41

41

47

45

6 99

13

0 10

4 86

36

16

16

6

82

62

143

142

174

148

128

146

132

131

171

131

191

163

130

131

51

112

122

150

72

82

144

28

155

202

80

83

76

92

98

92

114

90

97

97

118

122

127

97

84

68

90

102

119

102

115

145

99

98

13.6

12

.3

13.7

10

.2

14.2

1

3.2

1

0.1

11

.5

11.2

11

.0

12.7

14

.4

11.5

7.

4 3.

8 3.

8 4.

1 4.

4 3.

2 4.

1 4.

7 4.

1 4.

3 4.

1 4.

1 3.

3 3.

5 3.

7 3.

6 3.

3 3.

3 2.

3 4.

4 3.

2 2.

1 2.

8 2.

6 2.

7 3.

1 4.

3 3.

3 2.

0 71

.3

69.1

68

.3

64.9

68

.4

64.0

59

.9

71.4

70

.6

70.4

81

.4

71.2

81

.2

25.1

K

/Rb

43

6 43

6 34

8 35

1 37

2 41

0 24

5 28

3 21

1 31

1 44

8 31

3 31

5 20

5 K

/Sr

34

30

52

31

53

39

26

42

30

63

45

79

78

199

Ba/

Rb

10

.6

11.6

8.

6 13

.4

9.1

12.5

12

.6

9.7

11.0

9.

8 14

.9

8.9

6.7

3.8

9.1

12.9

12

.5

8.8

11.9

9.

6 11

.4

8.7

11.0

11

.3

4.7

4.3

4.8

4.6

5.4

3.7

3.6

3.2

5.1

5.7

1.9

3.0

2.6

1.9

2.2

2.6

3.1

2.7

2.2

2.0

48.2

50

.2

46.9

48

.5

53.8

39

.4

69.4

12

.8

85.8

85

.9

348

299

355

316

515

241

291

187

467

473

38

64

55

82

36

178

116

514

23

16

9.2

5.2

8.0

7.5

24.1

4.

3 7.

4 2.

3 1

7.1

23

.3

a T

ota

l F

e as

FeO

b

Los

s o

n i

gnit

ion

incl

udes

all

vol

atil

es l

ost

at 1

,000

~ C

; sa

mp

les

igni

ted

afte

r d

ryin

g a

t 11

0 ~

C

2

o,I 0 1 I-

,o_, I-

o

0 15

o

2 0 o4

z

o4

I geogrophic sectors I I Columblb River 0 II ..."..... j/d

I ~ Q,...~ I

o~O~l~P~ "

/ ~-~ --N. A tlonlic 0 L , / field

I i i I i I I i

4 e FeO 1 2 16

"... '".. ":,Columbia River ...... o...~......

o " "$,..= "..

�9 _ _

�9 \ ~ �9 CO 0

IV, Atlant/c

! ! I 30 Mg-#- 45 60

O ( ~ - - -

O �9 O

o =

0 i n - - ~ - - I 15 3o M g # 45 8o

Fig. 3. Variation diagrams to illustrate the degree of magmatic evolution represented by TiO2, FeO, MgO, and alkalis in the Serra Geral suite. Reference fields as in Fig. 2; dashed lines separate andesite, rhyodacite, and rhyolite samples from the mass of basalts and basaltic andesites. Geographic sectors refer to sample localities in Fig. J. Mg:t# is molecular MgO x t00 / (MgO + FeO)

Three intermediate and silicic rocks (RS30, RS65, RS50) have vitrophyric textures with microphenocrysts of plagioclase and clinopyroxene in glassy to partly devitrified groundmasses. Patches of anhedral quartz are in the groundmass of two (RS50 and RS65), whereas the third contains quartz within microglomerocrysts of plagioclase and clinopyroxene (RS30).

Analytical techniques

Compositions were determined by a Phillips 1410 X-ray fluores- cence spectrometer. For major elements, we prepared glass disks according to Schroeder et al. (1980) and used reference samples provided by the US, Canadian, and South African geologic surveys for analytical calibration curves (all correlation coefficients >0.99). We determined Ni, Cr, Ba, Rb, Sr, La, Zn, Cu, Y, Zr, and Nb on pressed-powder pellets (correlation coefficients 0.95 0.99), fer- rous iron by titration, and H20 by loss on ignition (LOI) at 1,000 ~ C; H 2 0 + values take into account 85% Fe oxidation during ignition.

Whole-rock compositions

Chemical compositions of the 24 Serra Geral samples are listed in Table 1 and illustrated in Figs. 2-6. The figures also show fields for basaltic suites appropriate for comparison - the Columbia River Plateau basalts, basaltic rocks associated with the opening (rifting) of the North Atlantic Ocean (North American and North African diabasic dikes

57

4~ 20 o 0

0 m@ m �9 l ~ . 0 o mWm 4 . ~

0

I 0 O . o'":

60 O

O

o~";, ~ 4 , ; ,

4 0 o

o o

20 I m I o l l l I O �9 , D~= oo

0 i l i , I , ,

2 4 6 8

.mi:::"

4 (.... �9 !..- o

o (..) i ~ o o o

o . 1 . I , L ~ o O / n i i , i i , ,

2 4 6 8

wt, % MgO I geogrophic sectorsl

ii I

m III

400 m,..i.

',..,.'~i �9

~ooIZro . o .~ .w,..~ , . ,.. oO 2 4 6 8

ZOO ~l

o I00

o �9 o o

~l l l - - - 0 , ,on, . . , n F ~ w o 0 | I /

2 6 8 4 o

4 0 0 f ~ ' : o o o

ZO0~?": �9149 0 0 .~/ �9 o o ,,,p,,18 %

0,3

0 . 2

0,1

60(;1

400

200

2 4 6 8

i ~ 0 �9 l lm lo O

0 m i l e 0 O0

j (~.: 1

i I

"~ �9 o o

B a ~ o �9 �9 El

�9 toO1 ~ o o

L i l i =

wt, g MgO Fig. 4. MgO diagrams for incompatible elements in the Serra Geral suite, southern Brazil. Geographic sectors refer to sample localities in Fig. 1. Dotted fields are for representative Brazilian Archean crust (Sighinolfi et al. 1981)

and basaltic flows), and normal mid-ocean ridge basalts (N-MORB) from the Atlantic Ocean.

The SiO2 variation diagrams (Fig. 2) show the large range in rock types, from basalts and basaltic andesites (50-55 wt% SiO2), to andesite (63 wt% SiO2), to rhyodacite (66 wt% SiO2), to rhyolite (73 wt% SiO2). In terms of alkali contents, Serra Geral mafic rocks fall into fields for basalts from both the Columbia River (Picture Gorge and Lower Yakima members) and North Atlantic margin rift- zones, and plot below the Hawaiian alkalic-tholeiitic line. Most samples are higher in CaO than those of Columbia River, but compare with basalts of North Atlantic margin rift-zones. In general, TiO2 and FeO increase with SiO2 except where SiO2 > 60 wt%. Two samples (RS73, RS68) are apart from the others by their high TiO2 at relatively low SiO2. This relationship combined with the F e O - S i O z plot define three compositional fields that correspond quite well to geographic locations of the samples (three sectors in Fig. 1). That is, northern-most samples (sector I) are high in FeO compared to southern-most samples (sector II), and the western-most (sector III) are high in TiO2. Comparatively, FeO is more evolved than for either the North Atlantic margin or Columbia River basalts.

Figure 3 expresses compositional variation in terms of

58

~ f

ZOO f C I � 9

0

200

0

0 O G

Z 4 6 Q

0 0 II ~ �9 II II I 0

0 I00 ~ . / o

o o

o 0 n i o i o i i i

2 4 6 8 150 "o

0 I~l �9

, o . .o. - o~

0 I I a l i . 2 4 6 e

I /VI . , o I 0 0 .~ l , ~ 0 0 0

5 0 .~ i l i i IO~I p

o o 0 0 n I I I i n I

2 4. 6 8

% M g O

Fig. 5. MgO diagram for transition metals in the Serra Geral suite, southern Brazil. Symbols as in Figs. 1, 2, and 4. Dotted field for Zn in representative Brazilian Archean crust (Sighinolfi et al. 1981)

FeO, Mg 4+, and alkalis. The general trends are increasing TiO2 with increasing FeO* and decreasing Mg4+ for the main mass of mafic samples. The TiO2 - Mg # plot presents most SG basalts as an extension of North Atlantic margin basalts. Highest Mg:~ is <60, low compared to some MORB that have Mg4+ >70 (e.g. Frey et al. 1974). The most primitive sample here, therefore, represents evolved magma. The KzO/Na20 plot displays the comparatively high K20 of sector II samples (low FeO samples) and a large KzO/NazO range due to K enrichment over Na in the intermediate and silicic samples.

Figures 4 and 5 illustrate that in nearly every case the transition from mafic to silicic rocks is increasing incompatible and decreasing compatible elements. Of particular interest with regard to mafic rocks is that the incompatible elements K, Rb, Ba, La, P, Zr, and Y, and the compatible elements Ni and Cr, are consistently high in the most mafic samples of the study (EVO1, MO1, MO2; Mg4+ 57-60; Ni > 95 ppm; Cr > 200 ppm; TiO2 1-1.1 wt. % ; relict olivine phenocrysts). Other distinctions are that the two high-TiOz samples (RS73, RS68) have higher St, Ba, and P205. Among transition metals, Zn largely behaves as an incompatible element.

Incompatible-element concentrations show, in general, positive correlations with one another (Fig. 6). Rather systematic variations of weathering-susceptible elements, such as K and Rb, indicate that alteration did not significantly affect original concentrations. (In addition, our microprobe study of these rocks (Corwin et al. 1985) shows that higher whole-rock K20 is attended by higher Or in plagioclase,

indicating that K20 contents are entirely primary, and not secondary.) The basaltic rocks display similarity with North Atlantic margin rocks in Zr, Nb, Rb, and Y abundances and ratios of these elements but are removed from N- MORB. Ratios (e.g. Zr/Y, Zr/Nb, Ti/Zr, and K/Rb) are basically within small ranges, although those of the intermediate and silicic rocks may or may not be similar to those of the mafic rocks. For example, the Zr/Y = 4 line passes through the highly evolved rocks as well as the main mass of basaltic rocks, but K/Rb=250 and Ba/Rb= 10 lines do not (these elements become compatible with increasing SiO2). With respect to ocean floor basalt, Zr/Y of 3-5 and Zr/Nb of 10-14 are comparable to South Atlantic T-type MORB (le Roex etal. 1983; Humphris and Thompson 1983).

Discussion

Establishing origins for these Serra Geral rocks, the nature of their source material, the influence of continental crust, and the role that Serra Geral magmatism had in the tectonic history of the South Atlantic Ocean requires recognizing that: (1) melts represented by the most mafic rocks are not in equilibrium with Mg-rich mantle material (Mg 4+ s < 60, low Ni); (2) the compositional range of the suite is large and therefore requires evaluation of crystal fractionation, magma mixing, and crustal contamination; (3) mafic members of this suite have geochemical characteristics similar to other basaltic rocks from the Mesozoic Atlantic-rift environment (Figs. 2, 3, 6); and (4) petrography, trace-element plots, and microprobe analyses (Corwin et al. 1985) indicate that weathering did not affect original elemental concentrations.

Fractional crystallization and contamination

To evaluate fractional crystallization we applied compositions of likely SG "parental" basalts to mixing (Bryan et al. 1969) and Rayleigh calculations. We assumed that comparable liquids could have precipitated appropriate phenocrysts to yield residual liquids resembling compositions of evolved samples. Our intent was to investigate trends and not to prove whether or not these rocks are of the same ilk. Compositions of plagioclase and F e - T i oxide used in the tests are from our microprobe study of these rocks. Olivine compositions are from data for co-existing clinopyroxene and olivine in similar rock types (Hawaii; Fodor et al. 1977a), taking into account liquid-olivine KD~0.30.

Tables 2 and 3 illustrate major- and trace-element relationships between potential parent-daughter pairs. While major-element correlation is good, trace-element correlation is not systematic. This is likely due to assimilation- fractional crystallization (AFC) processes that occurred, which are better indicated in independent isotope studies of SG rocks (Fodor et al. 1985; Mantovani et al. 1985).

The thrust of crystal fractionation modeling of"parental" liquids represented by some of the most mafic samples (RS20, RS76, EV01) is: (i) evolved mafic rocks are products of largely plagioclase and clinopyroxene fractionation, and lesser olivine, in amounts ranging from 7-60% of "parental" liquids (Table 2; Fig. 7); and (ii) in terms of KzO (K being a good representative of all incompatible elements), there were several paths of magma evolution (four depicted

59

12o ~ o ~ /

'~ N b .2~

/ i S / ; ~ . - - ' ~ ,

I / ~ J ~ . - . . . . . . . . . . . . . . . . . . ~ M o ~ e 01..~---- i ' ~ / i . . . . . . . U " - - ' ~ ' " I I

I O 0 2 0 0

geographic s e c t o r s

�9 I o 11 o III

3 0 0

2 0 0

I 0 0

0

o

~ 75.'~ ~,A , . t / o

- I I I I i 2

Z r T i O 2

t t 80 '~ o 200 q.9 O

r " ~ I I '~176 ~ , ~ . ~ o ~

. . . . . . . -W':~? . . . . . . . ~ o j

Y . ' S - : ; , ~ Z " H D - , ~ , ~,,o.,,c ~ .oO - - . / . . / . - " L ~ 5 - , . . m / o " " ~ / ~ - - - , ~ " ..." . ~ . r ~ 5 . . - r

/ " . . . . . / . . . . ?><~, Ano.t/~ , - " �9 / . . . . ' ~ " . . . ' 5 , ~ -

" ' ,~o ' ~o o ~ , ,, , ~ , Z r

6Or ~ j

X , o "~ 2o L ~ . ~ o - " " z~ / u_-~0-L ~176 ,oo

0 ~ I 0 I00 2 0 0 3 0 0 4 0 0

Z r

K 2 0

R b

f l l , .

2 0 0 4 0 0

Bo

o

o B o / R p ~ I O

I I 6O0

Fig. 6. Covariation diagrams for incompatible elements in the Serra Geral suite. Insets are used to show andesite, rhyodacite, and rhyolite where not included in main plot. N. Atlantic and Columbia reference fields as in Figure 2; MORB for N-type from Bryan eta]. (1981) and le Roex et al. (1983). Geographic sectors refer to sample localities in Fig. 1

in Fig. 7b), two from an RS20 parent, and two from RS76 and EVO1 parents (Table 3). But while there may be different magma batches, many could have come from the same source-material but acquired varying characteristics through crustal contamination. Or, in the case of the relationship between RS20- and RS76-type magmas (Fig. 7b), they may only represent different degrees of partial melting, as Fig. 8 illustrates.

To summarize, there are Serra Geral subgroups defined by whether or not parent liquids had higher or lower contents of K, Rb (and most other incompatible elements), and FeO. Fractionation trends in Fig. 7 illustrate this point and several subgroups. Only sector III rocks (samples RS73, RS68) seem to have a source-area distinctive from that of the main mass of these SG rocks as indicated by higher TiO2, P205, Ba, and Sr signatures (Figs. 2, 4), that do not relate to sector I and II rocks by contamination or partial melting (Fig. 8).

The overall categorization for these SG rocks, then, is two major groups that have different source-regions. They conform to the low-Ti and the high-Ti groups observed

by Bellieni et al. (1984) and Mantovani et al. (1985) else- where in the Serra Geral province. Those studies, however, do not take issue with the various subgroups of the low-Ti group that formed by contamination and/or partial-melting differences. There is some mingling of subgroups across Rio Grande do Sul, such as where MO3 has a composition more compatible with sector I rocks (Fig. 1).

Other perspectives of contamination are in Fig. 9. Mafic samples EVOI, MOI, and MO2 have low TiO2 values, and therefore are not highly fractionated, yet contain substantially higher K and Rb than N-, T-, and P-type basaltic rocks of the South Atlantic free of continental crust. How- ever, samples RS20 and RS73 and possible derivatives therefrom are compatible with oceanic basalts in these re- spects. There are also similarities between some SG and South Atlantic basalts at the whole-rock level (Table 4).

Although different percentages of melting of a particular mantle material can account for K, Rb, La, and Ba differences in coexisting basalts (Fig. 8), this is not a ready explanation for the high abundances of these elements in EVO1, MOI, and MO2 because P, Ti, and Sr in these rocks

60

Table 2. Least-squares approximations for selected parent-daughter relationships between basaltic samples of the Serra Geral province, southern Brazil. Observed (obs) is actual "parent" composition; calculated (calc) is least-squares determination of "parent" based on "daughter" and phases subtracted from parent liquid. R2=sum of squares of residuals. Res is fraction of residual (daughter) liquid

Parent RS20 RS76 EVO1

Daughter liquid: RS60 RS37 RS71 RS80 G648 RS70 IO2 MRO3 obs calc calc calc calc obs cale calc obs calc calc

SiO2 51.50 51.42 51.31 51.48 51.33 51.58 51.53 51.62 51.64 51.70 51.61 TiO2 1.13 1.24 1.20 1.29 1.28 1.40 1.39 1.32 1.02 0.89 0.88 AlzO3 13.78 13.74 13.84 13.77 13.87 14.18 14.25 14.16 16.54 16.51 16.56 FeO* 11.57 11.64 11.79 11.49 11.84 12.08 12.21 12.02 9.45 9.29 9.55 MgO 6.94 6.84 6.96 6.96 6.91 6.17 6.20 6.17 7.90 7.93 7.87 CaO 10.95 11.15 11.26 10.89 11.22 10.30 10.34 10.23 10.74 10.64 10.79 NazO 2.16 2.19 2.39 2.22 1.90 2.28 2.08 2.28 1.86 1.80 1.85 K20 0.63 0.71 0.75 0.48 0.53 1.09 1.05 1.03 1.09 1.15 1.06

~Y~R 2 0.08 0.24 0.31 0.18 0.07 0.02 0.07 0.04

res 83.2 73.5 69.3 61.2 93.2 65.6 60.7 41.8 plg 4.8 9.2 11.7 16.3 2.3 16.0 21.3 31.9 cpx 11.3 15.4 15.9 20.2 4.1 15.5 11.4 18.7 olv -- 1.1 2.1 1.3 -- 2.2 6.4 7.9

pig Anv8 used cpx FslTWos/used olv Fo77 used for EVO/; FO74 used for RS20, RS76

16 0

C~ 15

o~ 14

o/#" 0,Y ~C.~ ~_~-"~ ~ _ ~ ' ~

I I i I I I 7 8 9 I0 it 12

wt, % CoO

a

%1~ 7 v I Evo~

RS

= s I I I I I 6 9 I0 I I 12 1:3 14 15

wt, % FeO

Fig. 7. Compositional trends attributed to subtraction of plagioclase (pl), clinopyroxene (cpx), and olivine (ol) for Serra Geral basalt and basaltic andesite. Arrows indicate directions liquid compositions move due to crystal subtraction. Several magqna groups and subgroups are depicted with "parental" liquids represented by compositions RS20, RS76, and EVOI. Symbols as in Figs. 1 and 2

should therefore also be enriched (based on bulk Kos of 0.01-0.1 for mantle assemblages considered). Consequently, the best explanat ion for EVOI , M O I , and MO2 (sector II, Fig. 1) composi t ions relative to RS20-type magmas (sector I, Fig. 1) is that they represent basal t that substantially interacted with incompatible-element enriched material by assimilation or magma mixing. Al though unusual source

composi t ions (e.g., mantle enriched in crustal components ~i la Hoffman and White 1982) cannot be ruled out as an explanat ion for these basalts, the simpler, direct approach of crustal contaminat ion has appeal in light of knowledge about Brazilian crustal rocks. As Oliveira (1982), Oliveira et al. (1982), Leonardos and Fyfe (1974), and Sighinolfi et al. (1981) show, basement rock of eastern and south- eastern Brazil is comprised of migmatites, gneisses, granulites, and charnokites. Many of these rock types are enriched in the elements seemingly high in samples EVO1, M O I , MO2, such as K, Rb, and Ba (see fields in Figs. 4 and 5).

To evaluate assimilation quantitatively, we calculated mix-products made from melts o f " uncon tamina ted" basal t and Brazilian crust to account for the least fractionated, seemingly contaminated SG-basal t , EVO1. To produce such a low-TiO2, high-SiO2 basal t as EVO1, start ing basal t had to meet composi t ional requirements of relatively low TiO2 and SiO2. Table 5 shows that an 89:11 mix o f a Wal- vis ridge basal t and Brazilian Archean granulite can produce a composi t ion resembling EVO1. MgO in the product is too low but another choice of start ing basal t with slightly higher Mg at the expense of A1, Ca, and N a would make a product that matches better. Fall ing out of these tests was the observat ion that the start ing basal t had to have substantially more K 2 0 than N - M O R B (which is typically <0.1 wt%). The reason is that to achieve a produc t with K 2 0 > 1%, as in EVO1, a mix-product with endmember basal t having ~ 0.1% KzO would have SiO 2 >~ 52%, even when the choice of crustal endmember was low in SiO2 ( ~ 6 5 wt .%) relative to a high K 2 0 ( > 5 wt%).

Crustal contaminat ion could have been a factor in many SG " p a r e n t a l " basalts, but is " o b v i o u s " only in certain samples (as in EVOI) . It was p robab ly insignificant in RS20-type magmas, and an RS76 magma (Fig. 7) may relate to RS20 either by part ia l melting or by " m o d e r a t e "

61

Table 3. Calculated and observed trace-element abundances in selected rocks of the Serra Geral province, southern Brazil. Relationships based on major-element models in Table 2. Obs is amount (ppm) observed in daughter; calc is amount calculated by Rayleigh fractionation. Distribution coefficients from le Roex and Erlank (1982) except for Ni and Cr, from Schilling et al. (1978)

Parent RS20 RS76 EVOI

Daughter liquid: RS37 RS80 RS70 IO2 MRO3

obs calc obs calc obs calc obs calc obs calc

Rb 29 16 15 19 40 39 48 43 68 62 Sr 194 156 151 148 161 162 185 206 170 184 Ba 280 170 223 203 268 337 360 388 502 568 Zr 126 123 140 144 138 178 167 201 159 287 Nb 11 9 11 11 12 13 19 23 14 33 Y 31 29 34 32 40 38 36 38 44 48 La 13 11 13 12 19 15 19 20 30 29 Ni 43 60 41 63 45 60 86 64 16 55 Cr 22 16 30 11 55 28 145 102 35 49

I00

I--, 50 Z

_1

0

er"

5 W d a .

<

�9 RS 76 ..-..

_ . . " . _ .. ""~ , r , . .~a ._ __

. "'",...L~". �9 RS 73 ..,. �9 .~..i

~' ~ ' e _ . . . . ", / ,~ ' " e . . _

I I I I I I I I i i i I I i I i i !

R b B a Th U K To Nb La Ce Sr Nd P H f Z r Sm Ti Tb Y

0 04

4 0

Fig. 8. Mantle normalization for RS20 and RS76 to illustrate that ,-, different percentages of partial melting or crustal contamination ce 20 could account for their overall compositional differences. RS73 does not relate (e.g. note St, Ba) and therefore had another source- area. Normalization values from Wood (1979). REE, Ta, Hf, Th, 0 and U for samples from Hughes et al. 1985, submitted paper

contaminat ion of an RS20-type magma. In any case, there appears to be a relationship between geographic location (sectors I, [I, III) and gross differences in contaminat ion signatures.

To evaluate viability of fractionation origins for intermediate and silicic rocks ( > 60 wt% SiO2), we applied calculations to relate them to a basaltic-andesite liquid (MRO3). Table 6 shows results for RS65, RS30, and RS50 representing liquids after 32-55% crystallization of pl cpx > rot. Comparisons of calculated and observed abundances support fractionation relationships between high- SiO2 and marie samples only in a general sense. It appears that parent liquids higher in TiO 2 and knowledge of the compositions of any assimilated crustal component are nec- essary to achieve good parent-daughter matches. In any case, the calculations do not invalidate origins by crystal removal. If basaltic andesite is actually parental for RS65, RS30, and RS50, and basaltic andes�9 in turn is a fraction- ate of a basaltic liquid, rhyolite RSh0 then represents only 20% of liquid. Moreover, because SG basalts are fraction- ares of more primitive basalt, RS50 represents only ~ 10% of primitive basaltic liquid.

Igeogroph/r sectors

ii I

C] III

0

M o2zo �9 �9 0

EVOI H i ~ ~ ~ S, .tlonticfield / [3 / .ll/_ ~ �9 �9 /

/ , i RS20 / /

L ----I . . . . . . i ~ I

I 2 0

0 II�9

0

~_.~ ~ 0 , ~ O field /

,z RS20 ~

n Y i 0 2 z 3

O M02

" - - , 0 mum

0 I EV 0 / I �9 0 C}.,.RS75 S, Aflonf/r / CM / ~ m [] field / /

v "==~ m = ..- '"

/ / ~RS2O ~ /

0 I ~ - ~ - i ~ I a 0,1 0,2 0.3 0 ,4 0,5

P 2 0 5

Fig. 9. Variation diagrams to illustrate K and Rb enrichment in certain Serra Geral basalts relative to South Atlantic Ocean basalts (Bouvet and Walvis areas; le Roex etal. 1983; Humphris and Thompson 1983), therefore indicating contamination by continental crust (weathering is not a factor here). Geographic sectors refer to sample localities in Fig. ]

Crustal origin for rhyolites ?

Figure 4 illustrates that rhyolite RS50 has a composition equivalent to Brazilian Archean crust. Are SG-rhyolites, then, crustal melts or differentiates of basaltic magmas? Zinc abundances help answer this question. Zinc behaves

62

Table 4. Whole-rock compositions illustrating similarity between Serra Geral sample RS73 and a Walvis ridge sample

Table 5. Walvis ridge basalt mixed with Brazilian Archean granulite to produce an EVOl-like magma

Serra Geral Walvis ridge A B 89 : 11 Serra Geral DSDP Leg 74 Walvis ridge Brazil mix of EVOI

RS73 Site 527" basalt" granulite b A + B

SiO2 50.13 50.10 SiO2 49.82 66,69 51.68 51.64 TiO2 1.89 1.89 TiOz 1.22 0.27 1.12 1.02 A1203 14.35 15.33 A1203 17.16 14.97 16.92 16.54 FeO* 12.43 12.06 FeO* 9.60 2.85 8.86 9.45 MgO 6.75 6.40 MgO 6.75 0.30 6.04 7.90 CaO 11.09 10.38 CaO 12.55 1.77 11.36 10.74 NazO 1.93 2.67 Na20 2.26 3.50 2.40 1.86 KzO 0.90 0.59 K20 0.41 6.51 1.08 1.09 P205 0.24 0.25 P205 0.13 0.12 0.13 0.18

Rb 16 11 Rb 10 189 29 26 Sr 327 159 Sr 255 95 237 238 Ba 273 197 Ba 188 782 253 238 Zr 132 125 Zr 89 464 130 127 Y 26 39 Y 27 215 48 27 Nb 12 17 Nb 12 21 13 14 La 11 16 Zn 54 54 54 84 Ni 82 45 Ni 52 30 50 99

a From Humphris and Thompson (1983) From Thompson and Humphris (1984; Table 6) b From Sighinolfi et al. (1981 ; Table 2, sample JN33)

Table 6. Calculated and observed major- and trace-element abundances relating intermediate and silicic rocks as fractionates of basaltic andesite MRO3. Abbreviations and distribution coefficients as in Table 2 and 3

Parent: MRO3

Daughter liquid: RS65 RS30 RS50 RS65 RS30 RS50 obs calc calc calc

obs calc obs calc obs calc

SiO2 55.45 55.53 55.54 55.53 Rb 120 102 158 122 244 150 TiOz 1.84 2.46 2.59 2.55 Sr 162 179 163 177 89 168 AlzO3 13.21 13.35 13.29 13.28 Ba 510 757 594 898 554 1104 FeO* 13.69 13.54 13.51 13.52 Zr 201 233 253 274 295 333 MgO 3.25 3.49 3.48 3.30 Nb 21 17 34 18 34 21 CaO 6.85 6.72 6.72 6.91 Y 54 57 68 64 93 73 Na20 2.62 2.12 2.44 1.95 La 35 44 44 52 53 62 K20 2.38 2.46 2.31 2.62 Ni 16 1 6 1 6 1

Cr 30 9 45 6 20 4 Z'R 2 0.76 0.71 1.06

res 68.9 57.0 45.4 plg 12.0 17.1 23.3 cpx 13.5 16.7 19.3 nat 8.6 10.8 12.5

as an incompatible element in the SG-suite (Fig. 5) and is greater in SG evolved rocks than in compositions re- ported for Brazilian Archean crust. This supports a frac- t ionation origin for the rhyolite. Moreover, if rhyolite represents molten crust, the consistent plotting of RS50 trace- element abundances along apparent fractionation-trends (Figs. 4, 5) would be coincidental when considering the wide ranges that most trace elements have in Brazilian crust (e.g., a crustal melt could have Y = 2 0 ppm as well as Y = 100 ppm; Fig. 4). Yet the origin of SG rhyolite cannot be as straightforward as crystal fractionation; strontium and oxygen isotope compositions of SG evolved rocks indicate crustal contributions (Halpern et al. 1974; Cordani et al. 1980; Fodor et al. 1985; Mantovani et al. 1985). On the

other hand, high K 2 0 / N a 2 0 in SG rhyolite and intermediate rocks does not a priori manifest crustal contributions. This feature is also present in certain areas of the South Atlantic free of continental crust, such as Tristan da Cunha and Gough islands, a Walvis ridge seamount, and the Rio Grande rise (Baker et al. 1964; le Roex 1985; Fodor et al. 1977b, c).

Magma mixing

Did rhyolitic magmas (of whatever origin) mix with basalt magmas to produce "con tamina ted" basalts and intermediate rocks? The N i - - Z r relationship in Fig. 10 shows that this was possible. While most SG samples fall along an

120

0 f roc t io= I ~ I

I00 200 500

Z r Fig. 10. Fractionation path for a hypothetical parental basalt for the Serra Geral suite (Ni 160, Zr 70 ppm), using Ni bulk KD'S of 3, 2.3, and 1.9 for crystallization of ol:pl:cpx at 1:1:1, 1:2:2, 1:3:2 in three increments of 25% crystallization. Samples far from path, such as EVOI and MO2, could be mix products of basalt and rhyolite (rhyolite represented by RS50). Symbols same as in Figs. 1 and 2

800 . I

8roz// granuffles

6 0 0

/ Rs 5 0

K/Sr ~ I R 400

s 30 200

1 o RS 6 5 ~ 0 ~ R 03

0 II10 �9 ~ �9 �9 �9 �9 �9

I I I 0 5 I0 15

Bo/Rb

Fig. 11. Hyperbolic mixing-curve illustrating that magmas having compositions of andesite RS65 and rhyodacite RS30 could be mix- products of magmas represented by basaltic andesite MRO3 and rhyolite RSh0; or magma representing certain Brazilian Archean granulites (Sighinolfi et al. 1981) could be the silicic endmember. Symbols same as in Figs. 1 and 2

expected N i - Z r fractionation curve from a hypothetical basalt of Ni 160 and Zr 70 ppm, samples E v a 1 , MO1, and MO2 plot in a field compatible with binary mixing. On the other hand, basaltic andesites have more FeO* and TiO2 than basalts (Table 1; Fig. 3) and are therefore not mix-products of basalt and high-SiO2 (low TiO2) material. But intermediate rocks RS65 and RS30 can be mix-products of basaltic andesite and rhyolite because they have lower TiO2 and FeO* than basalt and fall on a hyperbolic mixing curve for certain crustal elements in Fig. 11.

Source-material and tectonic implications

The site of origin for these SG-CFB samples was probably located near the region now occupied by Tristan da Cunha (Neill 1976; Duncan 1981). Two South Atlantic volcanic features that also had origins at or near that hot spot are the Rio Grande rise and the Walvis ridge (Humphris and Thompson 1983; Thompson et al. 1983). Key places to ex-

63

6

4

6

4

@ _ Serro Gero/ ~ o basa/fic rocks

+

| @

I I I I I I I I IO 20 30 40

Z r / N b

~ ) o o r e f e r e n c e d a t a

OoO o ""

o,,, , , . . .+ [o , , /" ~,~',.,

Walvis ridge I, Rio Gronde rise

\ o ",z ~ , M . . . . . . 8~ ~ oa �9 o,,,,,? \ .~ . . . . . t_

_5< o 4, "\ ,9 "~2 ~ "

I I I I I ,o 20 3'o '

Z r / N b

Fig. 12. Hyperbolic mixing-curves for certain sector I and sector III Serra Geral basalts, and plume-related (T, A, B) (P-type) and depleted (N) (N-type) mantle materials. T, A, and B respectively refer to Zr/Nb and Zr/Y compositions representing Tristan, Ascen- sion, and Bouvet hotspots (Thompson et al. 1983; Harris 1983; le Roex and Erlank 1982; and le Roex et al. 1983). N1 is from Wood (1979), Zr 11.4, Nb 0.31, and Y 4.1 ppm, and N2 represents Zr 40, Nb 1, and Y 20 ppm. Reference data: FAMOUS from le Roex et al. (1981) ; N. Atlantic from Bertrand et al. (1982), SWIR is Southwest Indian ridge from le Roex et al. (1983); Walvis ridge from Humphris and Thompson (1983); and Rio Grande rise from Thompson et al. (1983). Numbers at tick marks refer to fraction of N-type in the mix; visual reference crosses at 15-5

amine, then, for clues to SG-CFB source compositions are basaltic rocks from that area of the South Atlantic sea floor, where magmas did not ascend through continental crust.

In general, Walvis and Rio Grande basalts differ from these Serra Germ basalts by higher TiO2 and lower FeO/ MgO and K20 (Thompson etal. 1983; Humphris and Thompson 1983), but some geochemical features are similar enough (shown in a previous section) to SG basalts to indicate that sources were similar and that contamination was not significant in every sample of Serra Geral basalt (e.g. RS20, RS73). Therefore, certain trace-element ratios in certain Serra Geral samples should reflect source compositions.

The ratios Zr/Nb, Zr/Y, and Y/Nb in South Atlantic basaltic rocks have been applied to mixing curves for inter- preting source compositions in terms of N - M O R B and P- M O R B components (le Roex et al. 1983; Humphris and Thompson 1983). The similarity illustrated in Fig. 12 for the SG and S. Atlantic provinces indicates that the SG-CFB sources, too, can be interpreted in terms of mixes between depleted (N-type) and enriched, or plume (P-type), components. The hyperbolic mixing curves using endmembers of P-MORB (Tristan da Cunha and Bouvet islands) and a

64

representative N - M O R B composition illustrate that many SG-CFB samples have compositions representing hybrids of about 90% N-type and 10% P-type materials. (le Roex et al. (1983) present details for the geology to explain such material, namely mantle metasomatism.) Because data points in Fig. 12 do not form a restricted field where all fall along a particular mixing line, heterogeneities in the source-region are likely. Indications of source-region heterogeneities in the overall compositions of these Serra Geral rocks are subtle, however. Most pronounced are the differences in Ti, P, Ba, and Sr contents in RS73 and RS68 (sector III) relative to other samples. These features are superimposed on the overall T -MORB quality suggested here for source material.

This comparison between these SG basalts and other South Atlantic rocks suggests that essentially the same source-region qualities have existed over 100 m.y. More- over, a zone of depleted mantle (essentially free of metasomatism) capable of producing N - M O R B likely existed beneath South America prior to continental rifting to account for N - M O R B crust subsequently created offshore (e.g. Frey et al. 1974). Whether basalt with CFB, N-MORB, or ocean- island characteristics was produced in the South Atlantic region depended on what proportions of depleted and enriched source-material were represented in the melt produced. Here, continental crust played a role because decompression due to rifting would have influenced the depths and zones of mantle melting (and therefore melt compositions) during the separation of Gondwanaland. Assuming that decompression enabled melting in shallower (and therefore cooler) zones above the mantle heat source (the zone least influenced by metasomatism originating from underlying mantle), SG-magmatism "g raded" into N- MORB magmatism that attended the actual opening of the S. Atlantic Ocean.

Conclusions

The southern portion of the Serra Geral CFB province is compositionally varied largely due to crystal fractionation, crustal contamination, and to mantle heterogeneities. Two compositional groups observed in the sampled area relate to source-region variances and are recognized mainly by higher or lower TiOz. Crustal contamination affected some basaltic magma chambers more dramatically than others, creating several subgroups, based largely on incompatible- element and FeO abundances, within the Serra Geral province. There is a general correlation between the " intensi ty" of contamination and geographic location. A caveat, however, is that melting of source-naaterial enriched in crustal components cannot be categorically discounted in favor of assimilation o f crust.

I f intermediate and silicic rocks are essentially fractionates of mafic rocks, as calculations suggest to be possible, the most evolved represent ~ 10% of primitive basaltic liquid. Rhyolites produced by anatexis, however, cannot be ruled out because of the SiO 2- and KzO-rich nature of Brazilian crust. Also, some intermediate rocks could be products of magma mixing.

Based on compositions of other South Atlantic rocks, source-regions having the same general qualities have been productive in South Atlantic since Jurassic time. These sources produced basalts ranging from N-type to T-type to P-type MORB, depending on the degree of metasoma-

tism of the material that partially melted. The SG basalts studied here, as other Atlantic rift-related basalts, are from T-type mantle that melted due to heat and decompression associated with hotspots and continental rifting. N - M O R B that succeeded these rift-related basalts as oceanic lithosphere was from upper, depleted mantle essentially unaf- fected by earlier metasomatism. Pertinent is that both CFB and MORB can originate from mantle beneath a particular global location.

Acknowledgments. This project is supported by the National Science Foundation Grants OCE-8308899 and EAR-8302737, and by a research grant from the Geological Society of America.

References

Amaral G, Cordani UG, Kawashita K, Reynolds JH (1966) Potas- sium-argon dates of basaltic rocks from southern Brazil. Geo- chim Cosmochim Acta 30:159-189

Baker PE, Gass IG, Harris PG, LeMaitre RW (1964) The volcano- logical report of the Royal Society Expedition to Tristan da Phil Trans R Soc London, Ser A 256:439-575

Bellieni G, Brotzu P, Comin-Chiaramonti P, Ernesto M, Melfi AJ, Pacca IG, Piccirillo EM 0984) Flood basalt to rhyolite suites in Southern Parana plateau (Brazil): paleomagnetism, petrogenesis, and geodynamic implications. J Petrol 25 : 579-618

Bertrand H, Dostal J, Dupuy C (1982) Geochemistry of early Me- sozoic tholeiites from Morocco. Earth Planet Sci Lett 58 : 225-239

Bryan WB, Finger LW, Chayes F (1969) Estimating proportions in petrographic mixing equations by least-squares approximations. Science 163:926-927

Bryan WB, Thompson G, Ludden JN (1981) Compositional variation in normal MORB from 22-25~ mid-Atlantic ridge and Kane fracture zone. J Geophys Res 86:11815-11836

Cordani UG, Sartori PLP, Kawashita K (1980) Geoquimica dos itosopos de estroncio e a evolugao da atividade vulcanica na bacia do Parana (Sul do Brasil) durante o Cretaceo. An. Acad Brasil Cienc 52 : 811-818

Corwin C, Fodor RV, Roisenberg A (1985) Silicate-phase compositions in the Serra Geral (Paran~) continental flood-basalt province, southern Brazil. Neues Jahrb (in press)

Duncan RA (1981) Hotspots in the southern oceans - an absolute frame of reference or motion of the Gondwana continents. Tec- tonophysics 74: 29-42

Fodor RV, Keil K, Bunch TE (1977a) Contributions to the Miner- al chemistry of Hawaiian rocks. VI. Olivines in rocks from Haleakala and West Maui volcanoes, Maui, Hawaii. Pacific Science 31:299-308

Fodor RV, Husler JW, Kumar N (1977b) Petrology of volcanic rocks from an aseismic rise: implications for the origin of the Rio Grande rise, South Atlantic Ocean. Earth Planet Sci Lett 35:225-233

Fodor RV, Keil K, Husler JW, McKee EH (1977c) Petrology and K-Ar age of volcanic tuff and ash from the Walvis seamount province, DSDP site 359. Init Rep Deep Sea Drilling Project 39 : 525-536

Fodor RV, Corwin C, Sial AN (1985) Crustal signatures in the Serra Geral flood-basalt province, southern Brazil: O- and Sr- isotope evidence. Geology (in press)

Frey FA, Bryan WB, Thompson G (1974) Atlantic Ocean floor: geochemistry and petrology of basalts from Legs 2 and 3 of the Deep-Sea Drilling Project. J Geophys Res 79:5507 5527

Halpern M, Cordani UF, Berenholc M (1974) Variations in strontium isotopic composition of Parana basin volcanic rocks of Brazil. Rev Brasil Geociencias 4:223-227

Harris C (1983) The petrology of lavas and associated plutonic inclusions of Ascension Island. J Petrol 24:424~470

65

Hoffman AW, White WM (1982) Mantle plumes from ancient oceanic crust. Earth Planet Sci Lett 57:421-436

Humphris SE, Thompson G (1983) Geochemistry of rare earth elements in basalts from the Walvis ridge: implications for its origin and evolution. Earth Planet Sci Lett 66:223-242

Leonardos OH, Fyfe WS (1974) Ultrametamorphism and melting of a continental margin: the Rio de Janeiro region, Brazil. Con- trib Mineral Petrol 46:201-214

le Roex AP (1985) Geochemistry, mineralogy and magmatic evolution of the basaltic and trachytic lavas from Gough Island, South Atlantic. J Petrol 26:149-186

le Roex AP, Erlank AJ (1982) Quantitative evaluation of fractional crystallization in Bouvet Island lavas. J Volcanol Geotherm Res 13:309-338

le Roex AP, Erlank AJ, Needham HD (1981) Geochemical and mineralogical evidence for the occurrence of at least three dis- tinct magma types in the "FAMOUS" region. Contrib Mineral Petrol 77 : 24-37

le Roex AP, Dick HJB, Erlank A J, Reid AM, Frey FA, Hart SR (1983) Geochemistry, mineralogy and petrogenesis of lavas erupted along the southwest Indian ridge between the Bouvet triple junction and 11 degrees east. J Petrol 24: 267-318

MacdonaId GA, Katsura T (1964) Chemical compostition of Haw- aiian lavas. J Petrol 5:82-133

Mantovani MSM, Marques LJ, De Sousa MA, Civetta L, Atolla L, Innocenti F (1985) Trace element and strontium isotope constraints on the origin and evolution of Parana continental flood basalts of Santa Catarina state (southern Brazil). J Petrol 26 : 187-209

McDougall I (1976) Geochemistry and origin of basalt of the Col- umbia River Group, Oregon and Washington. Geol Soc Am Bull 87:777 792

Neill WM (1976) Mesozoic epeirogeny of the South Atlantic margin and the Tristan hot spot. Geology 4:495-498

Oliveira MAF (1982) Bulk geochemistry of the Paraiba do Sul granulitic belt. Rev Bras Geociencias 12:369-374

Oliveira EP, Lima MIC, Carmo UF, Wernicle E (1982) The Ar- chean granulite terrain from east Bahia, Brazil. Rev Bras Geo- ciencias 12:356-368

Schilling JG, Sigurdsson H, Kingsley RH (1978) Skagi and western Neovolcanic zones in Iceland 2. Geochemical variations. J Geo- phys Res 83 : 3983-4002

Schroeder B, Thompson G, Surlanowska M, Ludden JN (1980) Analysis of geologic materials using an automated X-ray fluo- rescence system. X-ray Spectrometry 9:198 205

Sighinolfi GP, Figueredo MCH, Fyfe WS, Krongberg BI, Oliveira MAF (1981) Geochemistry and petrology of the Jequie granulitic complex (Brazil): an Archean basement complex. Contrib Mineral Petrol 78 : 263-271

Thompson G, Humphris SE (1984) Petrology and geochemistry of rocks from the Walvis ridge: Deep Sea Drilling Project leg 74, sites 525, 527 and 528

Thompson G, Humphris SE, Schilling JG (1983) Petrology and chemistry of basaltic rocks from Rio Grande rise, South At- lantic Deep Sea Drilling Project Leg 72, hole 516F. Initial Re- ports of the Deep Sea Drilling project 72:457-466

Waters AC (1961) Stratigraphic and lithologic variations in the Columbia River basalt. Am J Sci 259:583-611

Weigand PW, Ragland PC (1970) Geochemistry of Mesozoic doler- ite dikes from eastern North America. Contrib Mineral Petrol 29:195-214

Wood DA (1979) A variably veined suboceanic upper mantle-ge- netic significance for mid-ocean ridge basalt from geochemical evidence. Geology 7 : 499 503

Wright TL, Grolier JJ, Swanson DA (1973) Chemical variation related to the stratigraphy of the Columbia River basalt. Geol Soc Am Bull 84:371 386

Received September 30, 1985 / Accepted May 21, 1985

Petrology of Serra Geral (Paran�) continental flood basalts, southern Brazil: crustal...

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