Geochemistry of coexisting alkaline and calc-alkaline volcanic rocks from northern Azerbaijan (N.W. Iran)

Journal o f Volcanology and Geothermal Research, 11 (1981) 253--275 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

253

GEOCHEMISTRY OF COEXISTING ALKALINE AND CALC-ALKALINE VOLCANIC ROCKS FROM NORTHERN AZERBAIJAN (N.W. IRAN)

R. R I O U ' , C. DUPUY 2 and J. DOSTAL 3

' M ineralogisches lnstitu t, Universit~t des Saarlandes, 6600 Saarbriicken ( F.R. G. ) ~Centre Gdologique et Gdophysique, U.S.T.L., 34060 Montpellier Cedex (France) 3Department o f Geology, Saint Mary's University, Halifax, N.S. (Canada) (Received October 25, 1980; revised and accepted February 22, 1981)

ABSTRACT Riou, R., Dupuy, C. and Dostal, J., 1981. Geochemistry of coexisting alkaline and calcalkaline volcanic rocks from northern Azerbaijan (N.W. Iran). J. Volcanol. Geotherm. Res., 11: 253--275. The Eocene volcanic rocks of northern Azerbaijan (N.W. Iran) are composed of four distinct associations: strongly undersaturated, alkaline, shoshonitic, and calc-alkaline. All four series axe closely associated in space and time. They show many features c o m m o n with rocks of orogenic zones such as low TiO~, MgO, Nb and high K20 , Ba, Sr, and light REE abundances. The rocks within each series are predominantly related by fractional crystallization. The contents of major and trace elements in the rocks are consistent with their derivation from an enriched upper mantle by variable degrees of partial melting. It is suggested that this enrichment process is related to a subduction zone accompanying the closure of the South Tethyan Sea during the Cretaceous, while melting occurred along deep-seated faults reactivated during Early Tertiary times. INTRODUCTION

Azerbaijan geotectonically lies between the Euroasian and Arabian plates (Dewey et al., 1973; McKenzie, 1972) within an intracontinental orogenic zone. Its evolution during the Early Tertiary is characterized by an unusual magmatic association (Riou, 1979): large amounts of alkaline lavas and intrusives ranging from silica-saturated to strongly undersaturated were emplaced in close association with calc-alkaline rocks. Alkaline magmatism of shoshonitic character associated with island arcs or continental margins (Lef~vre, 1973; MacKenzie and Chappell, 1972) is well documented. However, only a limited a m o u n t of data is available on the high-K undersaturated lavas of orogenic zones (Johnson et al., 1976; Heming, 1979). Furthermore, the geochemical features of these lavas and their close association with calc-alkaline andesites are difficult to reconcile with general models of magma generation in orogenic zones. The purpose of this paper is to present some geochemical data on the various Eocene lavas encountered in northern Azerbaijan, to evaluate their rela-

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255 tionships and to discuss briefly the petrogenetic and tectonic implications of the data. GEOLOGICAL NOTES The studied area is located in the northern part of Eastern Azerbaijan (N.W. Iran) close to the U.S.S.R. border and the Caspian Sea (Fig. 1). Geologically, it lies at the northwestern end of the Central Iranian belt, a volcanic mountain range which runs across central Iran in a NW-SE direction for a distance of a b o u t 1800 km. At its northwestern end, the belt merges with another volcanic mountain chain, the Alborz range, which runs eastwards towards Afghanistan. The studied region includes part of a large volcanic plateau which covers an area of more than 15 000 km 2 roughly between the U.S.S.R. border and the Caspian Sea (Fig. 1). The morphology of the region is dominated by "horst and graben" structures composed of uplifted blocks of Eocene/Oligocene igneous rocks and Neogene sedimentary basins, probably related Plio-Quaternary movements, and by two huge Plio-Quaternary stratovolcanoes, Sabalan and Sahand. Volcanic and subvolcanic activities in the region span from Jurassic to Pleistocene (Didon and Gemain, 1976; Alberti et al., 1976, 1979; Riou, 1979). They culminated during the Eocene and Early Oligocene producing the volcanic plateau. In the Oligocene, after the culmination of the volcanism, numerous alkaline intrusions were also emplaced, marking the end of the alkaline magmatic event. After relatively minor and geographically limited Miocene volcanism (Lescuyer and Riou, 1976), the continental volcanism intensified in the Plio-Pleistocene resulting in the formation of the two large stratovolcanoes (Dostal and Zerbi, 1978) and a volcanic chain in the Varzaran area, c o m p o s e d of calc-alkaline and alkaline lavas. The studied volcanic rocks which are part of the Eocene to Early Oligocene volcanic cycle (Alberti et al., 1976, 1979) were emplaced unconformably upon the folded Upper Cretaceous sequence composed of clastic sediments and submarine volcanic rocks. The volcanic plateau is composed of lava flows and volcanic breccias, the total thickness of which ranges from a few hundred metres to more than 2000 metres. Although submarine volcanics are locally present, particularly in the older part of the Eocene sequence, the volcanic rocks are mainly subaerial; in contrast, all Alborz volcanic series are typically submarine. The geology and tectonic evolution of the area were recently outIined by Didon and Gemain (1976) and Lescuyer and Riou (1976).

Fig. 1. G e o l o g i c a l m a p o f t h e A h a r r e g i o n s h o w i n g t h e d i s t r i b u t i o n o f t h e P a l e o c e n e r o c k s . 1 = s e d i m e n t ; 2 = a n a l c i m i t e , p h o n o l i t e ; 3 = basalt, b a s a l t i c a n d e s i t e ; 4 = a n k a r a m i t e , alkali basalt; 5 = p o r p h y r i t i c latite; 6 = latite, a n d e s i t e , 7 = t r a c h y t e . L o c a t i o n o f t h e a n a l y z e d s a m p l e s : • = u n d e r s a t u r a t e d series; * = a l k a l i n e series; ~ = c a l c - a l k a l i n e series; o = s h o s h o n i t i c series. A , B , a n d C r e p r e s e n t t h e n o r t h e r n , c e n t r a l a n d s o u t h e r n z o n e s , respectively.

256 PETROGRAPHY

AND MINERALOGY

According to Riou (1979), the volcanic suites of northern Azerbaijan can be divided into three groups: (1) Feldspathoid-rich, strongly undersaturated lavas (SSA) which include analcimites and phonolites. These rocks occur mainly in the northern zone (area A in Fig. 1). (2) Alkaline lavas (SA) composed of ankaramites, alkali basalts and porphyritic latites (trachybasalts and trachyandesites). (3) Calc-alkaline rocks, which can be further subdivided into high-K calcalkaline lavas (CA1) comprising basalts, basaltic andesites and andesites and shoshonitic rocks (CA2) which include K-rich latites and trachytes. All three groups of rocks are closely associated in the studied region and in fact, in the central and southern zones (areas B and C, respectively, Fig. 1), they are typically intercalated. Although there is no obvious stratigraphic zonal relationship among the three rock types, the proportion of CA1 rocks increases from north to south. The SSA rocks are very abundant in the northern zone, where they are exposed over an area of more than 2000 km a . However, they are relatively rare in the central and southern zones. Shoshonitic rocks (CA2) are frequent in the central area whereas the alkaline rocks (SA) are abundant in all three zones. A summary of the main petrographical and mineralogical data of the four series is given in Fig. 2. GEOCHEMISTRY Fifty-one samples from Eocene volcanic series were analyzed for major and several minor trace elements (Tables 1 and 2). Major elements Rb, Sr, Ba, Nb and Zr were determined by X-ray fluorescence while Ni, Cr, V, Cu and Zn were analyzed by atomic absorption. Rare earth elements, Th, Hf and Sc, were determined by instrumental neutron activation. The precision and accuracy of the methods were given by Dupuy et al. (1979). The analyzed samples were collected throughout an area of about 15000 km 2 (Fig. 1).

Major elements Group SSA. The feldspathoid-rich series have SiO~ ranging from 46 to 54%, medium to high A1203 (from 15% in analcimites to 17--21% in phonolites) and variable b u t high contents of K20 (1--7%) accompanied by a K20/Na20 ratio > 1 in the phonolites. Other characteristic features of the SSA rocks include low MgO (<5%), very low TiO2 (< 0.8%) and the absence of an iron enrichment trend. All these rocks are nepheline-normative (10--24% norm. Ne) although its content decreases towards the more differentiated rocks (Table 1). There are only limited amounts of data on the feldspathoid-rich lavas low in Ti. They include basanites, tephrites and phonolites of Papua New Guinea

257 (Johnson et al., 1976; Heming, 1979), tephrites of the Eolian islands (Keller, 1974; Barberi et al., 1973), analcimites from the Highwood Mountains, Montana, and from the Dunedin Complex, New Zealand (Wilkinson, 1968). Although some of these rocks differ by the type of feldspathoids present, they all have a major element composition similar to the analcime-rich lavas of northern Azerbaijan. Group SA. The rocks of the alkaline series can be subdivided into ankaramites, alkali olivine basalts (SiO2 < 52%) and porphyritic latites (52--56% SiO2). According to the K contents, the last group can be also classified as hawaiites and mugearites (nomenclature of MacDonald and Katsura, 1964) or trachybasalts and trachyandesites (classification of Le Maitre, 1962). However, due to their distinctly porphyritic nature, the name porphyritic latite is used. With the exception of ankaramites, all the SA lavas have A1203 contents between 17 and 20%. They display the steep slope of the K20/SiO2 correlation. The more differentiated rocks have high K20 content and a K~O/Na20 ratio averaging 1. The abundances of TiO2 are slightly higher than those of the SSA series but are below 1.3%. The rocks of this series show a small iron enrichment trend. Compared to the SSA lavas, basalts are SiO2-saturated or slightly undersaturated and the degree of undersaturation increases towards the more differentiated rocks. However, the contents of normative nepheline remain below 5% (Table 1). In comparison with K-rich alkali basalts from oceanic islands (e.g., Gough Island, Le Maftre, 1962; Tristan da Cunha, Baker et al., 1964; Marquesas Islands, Bishop and Wooley, 1973; Polynesian Islands, Maury et al., 1978) and with leucite-bearing oceanic basalts (Tristan da Cunha, Baker et al., 1964; Cape Verde Islands, Part, 1950), the SA rocks have significantly lower TiO2 and MgO contents, a smaller variation range for SiO2, a steeper K20/SiO2 correlation and a higher K20/Na20 ratio. The SA series from Azerbaijan differs from the shoshonitic rock association, as defined by Morrison (1980), by the presence of normative nepheline in the majority of lavas, the absence of high-SiO2 rocks and the low MgO contents (< 5% MgO as compared to 6--8% MgO in shoshonitic rocks). On the other hand, there are similarities between the two series. They include a small range of variation in SiO2, a K20/Na20 ratio around 1 and relatively low contents of TiO2, although the SA rocks are significantly lower in TiO2 than shoshonitic rocks from active continental margins (Lef~vre, 1973). Calc-alkaline rocks (CA 1 ). According to SiO2 and K20 contents (Taylor, 1969; Peccerillo and Taylor, 1976), the rocks of the calc-alkaline series have been further subdivided into basalts (< 53% SiO2), high-K, low-Si andesites (53--58% SiO2) and high-K andesites (58--63% SiO2). All the CA1 lavas are quartz- and hypersthene-normative. They have high CaO and Al~O3 and low TiO2 and MgO content and a low MgO/MgO + FeOtot ratio. On the K~O vs.

Apatite

Magnetite

~naleime

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L

Fig. 2. Summary of the main petrographical and mineralogical data of the Eocene Series from Azerbaijan. {After Riou and Ohnenstetter, 1981.)

Glass

Apatite

Magnetite

K-Feldspar

Plagioclase

Clinopyroxene

Magnetite

K-Feldspar

Plagioclase

Biotite

Amphibole

Orthopyroxene

Clinopyroxene

Olivine

BASALT

CA 1 SERIES

Or52

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An35

End7 Wo39

SHOSHONITE

CA 2 SERIES

FSl4

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18.88

8.02

19.32

7.67

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54.3

52.2

[Mg] Q

49.11 0.69 15.04 9.99 0.16 5.38 8.26 5.34 2.83 0.57 3.90

45.97 0.81 14.81 11.64 0.21 5.77 11.44 4.76 1.19 0.48 3.83

SiO2(wt.%) TiO 2 A1203 Fe~O 3 T MnO MgO CaO Na20 K20 P,O s L.O.I.

2.96

10.71

42.5

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3.79

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159

24.44

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52.72 0.39 18.65 2.51 0.20 0.56 4.78 7.57 5.03 0.15 7.19

106

4.66

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53.55 0.46 21.41 3.81 0.20 1.90 3.05 4.71 6.78 0.24 4.84

105

1.52

14.89

38.8

53.62 0.59 20.20 5.44 0.22 1.55 5.54 5.69 4.91 0.28 2.86

182

11.47

1.03

67.3

49.00 0.76 12.15 10.19 0.15 9.31 13.92 1.64 1.65 0.21 1.99

2.74 19.03

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49.05 0.78 12.51 9.58 0.15 11.73 10.10 2.11 1.81 0.24 2.28

232a

104

264

265

241

A l k a l i n e series

U n d e r s a t u r a t e d series

Major e l e m e n t c o m p o s i t i o n o f r e p r e s e n t a t i v e E o c e n e v o l c a n i c r o c k s f r o m e a s t e r n A z e r b a i j a n

TABLE 1

11.44

0.38

56.7

49.05 1.10 15.76 11.58 0.17 6.54 11.07 2.79 1.89 0.25 0.87

164

7.09

5.48

48.1

50.71 0.75 17.37 8.25 0.17 3.47 7.08 1.58 7.23 0.37 3.52

121

5.47

3.76

45.0

5 4 . 0 5~ 0.76 20.45 6.20 0.12 2.26 5.60 4.19 4.81 0.63 1.70

232b

4.42

6.16

43.4

54.28 0.85 19.42 6.17 0.10 2.14 4.75 5.26 4.26 0.63 2.89

163

bO O~ 0

227

213

204

206

12.39

2.44

48.4

53.23 1.02 17.75 9.45 0.13 3.94 8.38 3.12 2.09 0.26 1.38

13.16

1.84

47.7

53.56 0.90 18.55 9.18 0.18 3.35 7.21 3.70 2.01 0.29 1.33

10.21

4.93

47.1

54.38 0.81 17.73 8.13 0.17 3.25 7.90 3.13 2.29 0.34 1.48

3.15

7.07

36.8

58.24 0.65 17.10 6.35 0.12 1.66 6.50 3.92 3.25 0.25 2.28

3.28

7.65

39.3

61.08 0.62 17.79 5.49 0.09 1.56 5.40 4.20 3.93 0.23 1.16

2.10

7.28

32.3

61.14 0.50 19.03 4.50 0.10 0.99 5.16 5.24 2.77 0.23 0.90

4.80 4.61

51.7

52.57 1.10 17.62 9.40 0.23 3.54 7.38 3.50 3.20 0.45 1.55

2.68

29.3

60.84 0.47 18.06 4.54 0.14 0.83 5.03 5.07 4.75 0.25 2.26

230

265, 241 = analcimite; 264, 106, 105, 182 = p h o n o l i t e ; 159 = p o r p h y r i t i c latite; 104, 232a = a n k a r a m i t e ; 164 = alkali basalt; 232b, 163 = p o r p h y r i t i c latite; 121 = t r a c h y t e ; 86, 83 = basalt; 211, 80, 227 = basaltic a n d e s i t e ; 213, 204, 206 = a n d e s i t e ; 111 = basalt; 230 = latite. [Mg] : M g / ( M g + F e ) ( a t o m ) ; F%O3 w = t o t a l Fe as Fe203.

14.61

18.65

42.1

2.20

51.5

[Mg]

50.65 0.80 18.22 8.63 0.12 5.08 10.17 2.53 1.11 0.16 4.08

3.72

48.72 1.09 16.41 10.94 0.15 5.37 9.86 2.24 0.48 0.23 4.01

SiO2(wt.%) TiO 2 A1203 Fe203 w MnO MgO CaO Na20 K20 P=O. L.O.I.

Q Ne Hy Ol

80

111

211

86

83

S h o s h o n i t i c series

Calc-alkaline series

TABLE 1 (continued)

bD b.z

Rb(ppm} Sr Ba La Ce Sm Eu Tb Yb Lu Th U Zr Hf Nb Sc V Cr Ni Cu Zn

45 43 90 1555 557 2026 981 1421 3132 28.7 27.4 37.5 54.5 54.1 77.8 6.07 5.88 8.66 1.69 1.55 2.37 0.86 0.75 1.02 1.73 1.52 2.26 0.28 0.26 0.37 7.5 9.5 9.8 1.7 2.2 2.7 160 120 240 2.4 2.4 3.9 9 8 16 24.1 26.4 12.0 380 260 292 5 37 8 14 17 9 147 143 90 95 77 99

106

105

182

134 69 145 70 23 968 706 2349 1864 471 871 2190 2143 1402 1032 82.2 35.1 39.3 48.4 10.9 165.5 68 73.1 98.2 24.3 9.8 5.32 7.15 8.79 4.1 2.77 1.55 2.08 2.49 1.16 1.18 0.75 0.94 1.11 0.54 2.61 1.60 2.36 2.84 1.43 0.47 0.29 0.40 0.48 0.23 25.9 11.8 11.5 15.6 2.3 nd 3.0 3.2 5.1 1.1 345 158 248 266 87 6.4 2.9 3.5 4.4 1.9 32 14 20 21 6 11.5 1.2 3 4.1 49.9 135 88 155 148 320 10 3 2 3 300 9 4 4 5 44 91 52 34 68 120 70 63 86 104 69

159 46 527 365 8.7 19.8 3.3 0.94 0.36 1.24 0.22 1.9 nd 82 1.24 6 35.4 235 810 263 112 100

232a

104

264

265

241

Alkaline series

U n d e r s a t u r a t e d series

Trace e l e m e n t c o m p o s i t i o n o f r e p r e s e n t a t i v e E o c e n e volcanic r o c k s f r o m e a s t e r n A z e r b m l a n

TABLE 2

121

232b

52 150 91 497 1668 857 346 980 1079 16.7 18.3 31.5 34.7 68.4 68.7 4.4 5.02 4.9 1.31 1.55 1.58 0.64 0.72 0.71 1.81 2.12 2.42 0.30 0.37 0.41 3.3 4.8 9.3 nd nd nd 120 172 199 2.37 2.4 3.7 10 11 14 32.3 19.0 10.4 265 235 150 62 18 12 20 12 13 84 188 209 88 87 71

164

112 643 930 32.3 66.7 5.2 1.60 0.72 2.24 0.45 8.8 nd 179 3.6 14 11.0 176 22 16 158 75

163

206

62 595 476 16.5 33.5 3.63 1.12 0.43 1.76 0.26 3.9 nd 123 4.0 8 24 223 28 9 35 70

42 482 692 13.9 30.5 4.35 1.18 0.61 2.58 0.43 4.5 nd 132 1.5 10 24.5 225 7 9 100 109

65 811 648 25.2 48.2 3.89 1.22 0.50 1.64 0.26 4.8 nd 158 2.3 13 13.9 164 9 10 44 73

70 720 942 21.7 42.7 3.77 1.08 0.47 1.60 0.26 6.2 nd 167 3.0 11 11.5 154 10 10 157 66

91 696 1052 20.8 43.5 3.48 1.02 0.40 1.51 0.24 7.3 nd 184 3.5 11 10.3 142 9 9 50 68

94 953 992 20.6 39.3 2.76 0.92 0.39 1.23 0.20 7.4 nd 166 2.9 10 7 102 8 8 76 63

98 664 755 49.8 104.7 7.8 2.09 1.12 2.57 0.43 20.9 nd 270 6.5 21 25 218 29 14 95 90

170 874 1173 46.6 88.2 4.6 1.33 0.61 2.04 0.48 21.6 nd 292 5.4 28 4.5 80 6 5 28 69

230

16 446 295 8.3 17.0 2.64 0.79 0.35 1.45 0.24 2.7 nd 94 2.4 6 25.6 240 8 25 88 69

204

12 509 239 12.0 26.0 4,17 1.17 0.64 2.49 0.42 2.7 nd 135 2.3 8 33 293 55 24 86 82

213

*Major e l e m e n t c o m p o s i t i o n and o t h e r d a t a o n t h e s a m p l e s are given in Table 1.

Rb(ppm) Sr Ba La Ce Sm Eu Tb Yb Lu Th U Zr Hf Nb Sc V Cr Ni Cu Zn

227

111

80

83

86

211

S h o s h o n i t i c series

Calc-alkaline series

TABLE 2 (continued)

¢o

264

Na20 15

K20

m

1o-

io

/o

I

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~/ i / / /oy / ~/'/ /07 .~,./

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50

60

I 70

SiO 2 %

Fig. 3. ( N % O + K 2 0 ) v e r s u s S i O 2 . T h e h e a v y l i n e d e l i n e a t e s t h e b o u n d a r y b e t w e e n a l k a l i n e and subalkaline fields, after Irvine and Baragar (1971). • = undersaturated series; * = a l k a l i n e s e r i e s ; ~ = c a l c - a l k a l i n e s e r i e s ; o = s h o s h o n i t i c series.

SiO2 and (Na20 + K20) vs. SiO2 diagrams, they display a linear trend of positive correlation with the slope distinctly lower than the SSA and SA series (Fig. 3). The CA1 series also includes rocks with higher SiO2 contents than the latter series (Fig. 3, Table 1).

Shoshonitic series (CA2). This group includes rocks with shoshonitic character. For a given SiO2 content, they are higher in K20 and (Na20 + K20) abundances than the equivalent rocks of the CA1 series (Fig. 3). Shoshonitic rocks have quartz and hypersthene in their norms and have a K20/Na20 ratio equal to 1 (Table 1). Although there are some significant geochemical differences among the four volcanic rock series, particularly in the abundances of alkalies and in degree of silica saturation, all these rocks have some c o m m o n features. The similarities include a low Fe/(Fe+Mg) ratio, low TiO2, high A1203 and K20 contents and porphyritic character. These features are typical of calc-alkaline and shoshonitic suites of orogenic zones.

265

Trace elements Lithophile elements. A subdivision of the rocks based u p o n the mineralogy and the major element composition is consistent with and is further corroborated by the abundances of lithophile elements. The rocks of the SSA and CA2 series have higher abundances of lithophile elements than those of the other two suites. On an average, shoshonitic and SSA series are higher in Nb and Zr than the SA and CA1 series (Fig. 4) and for a given SiO2 content, the SA group is higher in Zr, Nb, La, Rb and Ba than the CA1 rocks (Fig. 5). Regarding the calc-alkaline series, basalts have abundances of lithophile elements similar to basaltic lavas from the island arc of the Lesser Antilles (Brown et al., 1977). On the other hand, high contents of Rb, Ba and Sr in andesites resemble continental margin andesites (Dupuy and Lef~vre, 1974; D u p u y and Dostal, 1978; Thorpe et al., 1976). Shoshonites from Azerbaijan are richer in lithophile elements than equivalent rocks of the typical island arcs (MacKenzie and Chappell, 1972). Their concentrations of Ba, and Sr, though, are comparable to shoshonites from the continental margin of Peru (Dupuy and Lef~vre, 1974) whereas the very high Rb content is similar to shoshonitic lavas from Argentina (HSrmann et al., 1973) or Yellowstone Park (Nicholls and Carmichael, 1969). Except for high Rb contents, the abundances of lithophile elements in lavas of the alkaline series are similar to those of the alkali olivine basalt association (Frey et al., 1978). The undersaturated lavas (SSA) resemble nephelinites (Frey et al., 1978) or high-K undersaturated rocks of New Guinea (Heming, 1979) in the concentrations of Sr, Zr and Nb. However, the SSA series are enriched in Ba and Rb. Compared to the tephrites and basanites from New Guinea (Heming, 1979), analcimites and phonolites from Azerbaijan are 2 times higher in Rb and 4 times higher in Ba. La p p m

100

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0

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~

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500

Fig. 4. L a v e r s u s Zr. T h e s y m b o l s a r e t h e s a m e as in Fig. 3.

Zr ppm

266 N b ppm

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ol

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.

t[f* ~ 0 I 50

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Si 0 2 %

F i g . 5. N b v e r s u s SiO~. T h e s y m b o l s a r e t h e s a m e as i n F i g . 3.

Although all four series have variable contents of several lithophile elements, their Zr/Nb, Zr/La, Ba/La and Ba/Rb ratios fall within the range of values for volcanic suites of orogenic zones. However, due to the enrichment of Ba and Rb, shoshonites and undersaturated lavas have lower K/Rb (240 and 400, respectively) and K/Ba (~ 25) ratios than the comparable rocks from orogenic belts (MacKenzie and Chappell, 1972; Heming, 1979}. Elemental ratios in all four series involving relatively immobile trace elements such as La/Yb, Nb/Zr and La/Zr vary in relatively narrow ranges and increase systematically with the increasing enrichment of lithophile elements. On the other hand, the ratios involving alkali and alkali-earth elements (e.g., Ba/La, Ba/Rb, Ba/Nb) are highly variable and do not show systematic trends.

Rare earth elements (REE). (1) Undersaturated series: Analcimites and phonolites (Fig.6A) have very similar REE patterns which show distinct light REE (LREE) enrichment and fractionation of heavy REE (HREE). With the exception of one analcimecumulative phonolite (sample R106), their La/Yb ratios are rather constant (19--20). Compared to oceanic alkali basalts (Kay and Gast, 1973), the undersaturated rocks from Azerbaijan have high total abundances of REE. They also differ from undersaturated potassic rocks of the East African rift zones (Gerasimovskiy et al., 1972). However, the REE patterns and abundances are closely comparable to those of shoshonitic rocks from Peru (Klerkx et al., 1974) or Stromboli (Dupuy et al., 1981). (2) Calc-alkaline series: Basalts and andesites have typical calc-alkaline REE

267

A

100

"'''~

B -.

I00

z~z

~ ~o (2= C

I00

La Ce

Sm Eu

Tb

Yb Lu

Fig. 6. R E E abundances n o r m a l i z e d t o a e h o n d r i t i c average ( F r e y et al., 1968). A. under-

saturated series (SSA); B. alkaline (SA) and shoshonitic (CA=--sample 111) series; C. calc-alkaline series (CA t).

patterns with LREE enrichment (Fig. 6C). According to the La/Yb ratios, the analyzed calc-alkaline rocks can be subdivided into two groups: (a) basalts and basaltic andesites with La/Yb ratios < 6 and unfractionated HREE patterns, features similar to calc-alkaline lavas of island arcs (Jake~ and Gill, 1970); (b) andesites with La/Yb ratios > 10 and slightly fractionated HREE are comparable to calc-alkaline lavas from continental margins (Dostal et al., 1976, 1977a,b; Thorpe et al., 1976).

268 (3) Alkaline series: The REE abundances of the alkaline lavas have features similar to both the undersaturated and the calc-alkaline series (Fig. 6B). The basalts have moderate La/Yb ratios (~ 9) and a LREE enrichment whereas trachyandesites are strongly enriched in LREE with an La/Yb ratio of about 15. (4) Shoshonitic series: The REE patterns of two analyzed shoshonites are closely comparable to the more differentiated lavas of the alkaline series although the former has higher total REE contents. Transition elements. The SAA, SA and CA series have typically low Sc, Ti, Cr and Ni and moderate to high V contents which are within the range of abundances in lavas from orogenic zones (Andriambololona, 1978; Coulon, 1977). The high V/Ni ratios further corroborate the similarities with andesitic suites (Taylor et al., 1969). Feldspathoid-rich lavas differ from the alkaline and calc-alkaline rocks by high Cu contents and a Cu/Zn ratio > 1. These features have also been reported from undersaturated lavas of New Guinea (Heming, 1979) and seem to be characteristic of feldspathoid-rich rocks from orogenic zones. In all the studied series, the abundances of Cr, Ni, Zn and Sc decrease with increasing differentiation. The distinct decrease of Sc when plotted against an index of differentiation such as the Mg/(Mg+Fe) ratio, indicates p y r o x e n e fractionation (Frey et al., 1974}. The variation of Cr is consistent with the early crystallization of Cr-rich diopside followed by Cr-poor salite in the more differentiated rocks. In calc-alkaline and undersaturated rocks, Ti and V decrease simultaneously with differentiation whereas the alkaline series show initial increase in the abundances of these elements followed by their decrease in more evolved rocks. These variations of Ti and V are related to the fractionation of magnetite which crystallizes early in the calc-alkaline and undersaturated series while in the alkaline series it appears slightly later (Riou, 1979). Compared to intraplate alkaline series, the Azerbaijan lavas are distinctly lower in Ti, Cr, Ni and Zn but higher in Cr. With the exception of Cu in the undersaturated series, the abundances and variations of minor and trace transition elements are similar to those of high-A1 basalts from orogenic zones.

PETROGENESIS There are t w o important aspects to the petrogenesis of the Paleogene volcanic rocks from northern Azerbaljan. The first aspect is the relationship among the rocks of a single series and the second includes the relationship among the different series and their origin. Relationship within a single rock series

Petrography (phenocrysts of olivine, clinopyroxene, plagioclase, etc.) and geochemical data (increase of lithophile elements and decrease of some transi-

269 tion elements with increasing degree of differentiation) suggest that fractional crystallization played a role in the evolution of the individual series. However, this process alone cannot account for all the variations observed within the single series. For example, in the calc-alkaline series, the derivation of REE abundances in andesite 204 from spatially associated basalt 83 would require solidification of more than 50% of magma involving crystallization of clinopyroxene and/or hornblende. The lack of the latter mineral in basalts makes this hypothesis rather improbable. However, even such a high degree of solidification would not explain the high concentrations of Rb and Ba in the andesite. Likewise, the major element compositions of these two rocks, particularly their relatively small differences in Ca and A1 are not readily consistent with fractional crystallization. Similar difficulties are encountered with the other rock series from northern Azerbaijan and have also been reported from various orogenic volcanic belts {Kay, 1977; Gill, 1978). It seems that some other mechanism, probably crustal contamination (Dupuy et al., 1979), has influenced the composition of the rocks during differentiation. In fact, crustal contamination may account for the distinct enrichment of several lithophile elements {e.g., Rb, Ba) in the more differentiated rocks and for their irregular variations. Relationship among the various series and their origin In order to evaluate the origin of calc-alkaline, undersaturated and alkaline series and their mutual relationship, only the most primitive and least differentiated rocks of each series are considered. Calc-alkaline basalts. Numerous recent geochemical and petrological studies have suggested that the volcanic rocks of orogenic areas were produced by partial melting of the upper mantle peridotite overlying the Benioff zone (e.g. Ringwood, 1974; Kay, 1977; Dostal et al., 1977a,b; Lopez-Escobar et al., 1977). Assuming that the upper mantle parent had chondritic abundances of REE, model calculations of partial melting show that a spinel lherzolite source cannot explain the La/Yb ratio (~ 5) of the studied calc-alkaline basalts. The high La/Yb ratio requires a garnet-bearing residuum after melting. However, the presence of garnet would also lead to fractionation of HREE while the analyzed basalts have flat H R E E patterns. The distribution of REE including distinct LREE enrichment of the calc-alkaline basalts from northern Azerbaijan suggests their derivation from an upper-mantle source already enriched in LILE. Such a process which could also account for high contents of Ba and Rb, has been frequently invoked for volcanic rocks of orogenic areas related to the subduction zones (Ringwood, 1974; Kay, 1977; Hawkesworth et al., 1979). Alkali and undersaturated basalts. The model calculationa (Fig. 7) show that tne REE contents of the most primitive basalts (samples 164 and 241) m a y be generated by a low degree of partial melting of upper mantle garnet lherzo-

270

A

10

I

I

~

~

I

I

I

I

I

I

t

I

I

I

~

B

L L

I

La Ce

I

I

I

I

L

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t

I

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I

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]

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Fig. 7. R E E a b u n d a n c e s of (A) s a m p l e 241 a n d (B) s a m p l e 164 c o m p a r e d t o liquid prod u c e d by e q u i l i b r i u m partial m e l t i n g of a g a r n e t lherzolite source. A. P a r e n t = 55 Ol, 25 Opx, 10 Cpx, 10 Grt; m e l t w i t h F ( t h e degree o f partial m e l t i n g ) = 2%: 20 O1, 20 Opx, 30 Cpx, 30 Grt. B. P a r e n t = 55 O1, 25 Opx, 12 Cpx, 8 Grt; m e l t w i t h F = 6%: 20 O1, 20 Opx, 30 Cpx, 30 Grt. P a r t i t i o n c o e f f i c i e n t s o f S c h n e t z l e r a n d P h i l p o t t s ( 1 9 7 0 ) for olivine a n d p y r o x e n e s and o f S h i m i z u and K u s h i r o ( 1 9 7 5 ) for g a r n e t were used. Comp o s i t i o n o f s o u r c e r o c k is 4 t i m e s c h o n d r i t i c a b u n d a n c e s .

lite having four times chondritic abundances of REE. However, such a mechanism cannot account for the abundances of several other incompatible elements. The calculated concentrations of elements with high valency (> 3+) such as Ti, Zr, and Nb are higher than those observed in the rocks, whereas for t h e elements with low valency (< 3+) such as Ba, Sr and Rb, the observed abundances are higher than the calculated ones. These differences cannot be explained by a simple process such as fractionation of the Ti-rich phase (rutile, ilmenite; McCallum and Charette, 1978) but require a more complex mechanism. However, the chemical similarities among the primitive rocks of the different series including the rather similar relative fractionation of the most incompatible trace elements (Fig. 8) suggest a c o m m o n process for the genesis of all volcanic suites from northern Azerbaijan. The genetic relationship among these rocks is further indicated by their close association in space and time. A petrogenetic model for volcanic rocks from northern Azerbaijan should thus account for: (a) the high contents of incompatible elements and their increasing relative enrichment with the decrease of their valency (a problem c o m m o n for most volcanic rocks of orogenic zones); (b) the increase in the

271

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Z

o

"1"

v

o o

10

rc

I

I

Ba Th Nb

I

i

i

I

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Ce

Sr

I

i

I

i

Zr

Sm

Eu

i

Ti

|

I

i

I

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Yb

Lu

V

Fig. 8. Chondrite-normalized lithophile elements and R E E abundances in three basic lavas representative of the undersaturated (264) alkaline (85) and calc-alkaline (83) series.

abundances of many "more incompatible" elements from calc-alkaline to alkaline and undersaturated rocks; (c) systematic variations of the ratios involving trace elements of variable degrees of incompatibility, e.g., Nb/Zr, La/Yb (they vary systematically from CA1 through SA to SSA series). The model which probably best explains most observed features is the one which suggests that parental magmas were derived by partial melting of the uppermantle peridotite enriched in incompatible elements by hydrous fluids released from the descending oceanic lithosphere (Best, 1975; Hawkesworth et al., 1979; Mysen, 1979). This model has been recently successfully applied to rocks of Stromboli (Dupuy et al., 1981). Unfortunately, such a mechanism cannot be, at present, fully quantitatively tested for all incompatible elements since the crystal-vapor partition coefficients are available only for a few REE (Mysen, 1979). However, the limited model calculations (Mysen, 1979; Dupuy et al., 1981) suggest that metasomatism of the upper mantle leads to an increase of the total REE contents and relative enrichment of LREE. This mechanism would probably also enrich elements with low valency (< 3+) more than REE, which in turn would be more enriched than elements with high valency (> 3+). The differences in the composition of the parental magmas of the northern Azerbaijan volcanic series may be due to the variable degrees of partial melting (related to the different depths of the upper mantle anatexis) and/or to the heterogeneous upper mantle source.

272 CONCLUSIONS The Eocene volcanic rocks from northern Azerbaijan consist of three distinct rock series: strongly undersaturated, alkaline and calc-alkaline. In addition, some of the calc-alkaline rocks have characteristics of shoshonitic association while others are typical calc-alkaline lavas. All rock types are closely associated in time and space and do not show clear geographic zonation. The undersaturated and alkaline series differ from similar suites of a typical "within-plate" tectonic setting by low TiO2, MgO, Nb and trace transition elements. Their abundances resemble lavas of orogenic zones. Within the individual series, geochemical trends such as increase of several lithophile elements and decrease of some transition elements with increasing degree of differentiation suggest that crystal fractionation played an important role during the evolution of the series. However, as in volcanic suites of orogenic zones where continental crust is present, this process alone cannot account for all observed variations. It seems that some other process, probably crustal contamination, affected the rocks during differentiation. Although there are significant differences among the four associations, they have also some c o m m o n characteristics. The similarities include low Mg/ (Mg+Fe) ratios, low TiO2, high A1203, K20, Ba and Sr contents, relative depletion of Nb, Zr and porphyritic character (features which are typical of orogenic lavas), and suggest a genetic relationship among all these rocks. The model calculations of trace elements imply that the parental magmas of these suites were derived from an upper mantle enriched in lithophile elements. Similarities between such a parental material and those frequently invoked for recent orogenic lavas argue for the derivation of the studied rocks from a comparable source, probably an upper mantle peridotite wedge overlying the subduction zone. However, there is no obvious geological evidence for the presence of a subduction zone during the Eocene. Thus, to reconcile the contrasting features and the close association of the different series, it can be suggested that the lavas of the studied area were derived from a previously enriched upper mantle source which underwent partial melting along deepseated fracture zones reactivated during the Early Tertiary. The upper mantle enrichment process could be related to a possible subduction of the South Tethyan oceanic lithosphere under the Iranian plate during the Upper Cretaceous. The differences among the various rock-series may be related to the variable degrees of partial melting. ACKNOWLEDGEMENTS

The first author (R.R.) is grateful to the Deutsche Forschungsgemeinschaft and Prof. G. Lensch for providing facilities for the study. The study was supported by the Centre G~ologique et G~ophysique, Montpellier, and the Natural Sciences and Engineering Research Council of Canada (operating grant A3782).

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275 Maury, R.C., Andriambololona, R. and Dupuy, C., 1978. Evolution compar6e de deux s~ries alcalines du Pacifique central: rSle de la fugacit6 d'oxyg~ne et de la pression d'eau. Bull. Volcanol., 41: 1--32. McKenzie, D.P., 1972. Active tectonics of the Mediterranean region. Geophys. J.R. Astron. Soc., 30: 109--185. Morrison, G.W., 1980. Characteristics and tectonic setting of the shoshonite rock association. Lithos, 13: 97--108. Mysen, B.O., 1979. Trace element partitioning between garnet peridotite minerals and water-rich vapor: experimental data from 5 to 30 kbar. Am. Mineral., 64: 274--287. Nicholls, J. and Carmichael, I.S.E., 1969. A commentary on the absarokite-shoshonitebanakite series of Wyoming, U.S.A. Schweiz. Mineral. Petrol. Mitt., 49: 47--64. Part, G.M., 1950. Volcanic rocks from the Cape Verde Islands. Bull. Br. Mus. Mineral., 1: 27--72. Peccerillo, A. and Taylor, S.R., 1976. Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, northern Turkey. Contrib. Mineral. Petrol., 58: 63--81. Ringwood, A.E., 1974. The petrological evolution of island-arc systems. J. Geol. Soc. London, 130: 183--204. Riou, R., 1979. Petrography of the Eocene alkaline lavas of the Northern Azerbaijan (Iran). Neues Jahrb. Geol. Abh., 9: 532--559. Riou, R. and Ohnenstetter, D., 1981. Mineralogy of co-existing undersaturated alkaline, alkaline, shoshonitic and calc-alkaline series of northern Azerbaijan (N.W. Iran). Contrib. Mineral. Petrol. (submitted). Schnetzler, C.C. and Philpotts, J.A., 1970. Partition coefficients of rare-earth elements between igneous matrix material and rock-forming mineral phenocrysts, 2. Geochim. Cosmochim. Acta, 34: 331--340. Shimizu, N. and Kushiro, I., 1975. The partitioning of rare-earth elements between garnets and liquid at high pressures: preliminary experiments. Geophys. Res. Lett., 2: 413--416. Taylor, S.R., 1969. Trace-element chemistry of andesites and associated calc-alkaline rocks. Ore. Dept. Geol. Miner. Ind. Bull., 65: 43--64. Taylor, S.R., Capp, A.C., Graham, A.L. and Blake, D.H., 1969. Trace element abundances in andesites. II - Saijan Bougainville and Fiji. Contrib. Mineral. Petrol., 54: 65--78. Thorpe, R.S., Potts, P.J. and Francis, P.W., 1976. Rare-earth data and petrogenesis of andesites from the North Chilean Andes. Contrib. Mineral. Petrol., 54: 65--78. Wilkinson, J.G.F., 1968. Analcimes from some potassic igneous rocks and aspects of analcime-rich igneous assemblages. Contrib. Mineral. Petrol., 18: 252--269.