Trace-Element Modeling and Source Constraints for Tholeiitic and Cale-alkaline Basalts from a...
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PLEASE SCROLL DOWN FOR ARTICLE This article was downloaded by: [TÜBİTAK EKUAL] On: 3 May 2011 Access details: Access Details: [subscription number 772815469] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37- 41 Mortimer Street, London W1T 3JH, UK International Geology Review Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t902953900 Trace-Element Modeling and Source Constraints for Tholeiitic and Cale- alkaline Basalts from a Depleted Asthenospheric Mantle Source, Mt. Erciyes Stratovolcano, Turkey B. Kurkcuoglu a ; E. Sen a ; A. Temel a ; E. Aydar a ; A. Gourgaud b a Department of Geological Engineering, Hacettepe University, Beytepe, Ankara, Turkey b Blaise- Pascal University, Clermont-Ferrand, Cedex, France Online publication date: 06 July 2010 To cite this Article Kurkcuoglu, B. , Sen, E. , Temel, A. , Aydar, E. and Gourgaud, A.(2001) 'Trace-Element Modeling and Source Constraints for Tholeiitic and Cale-alkaline Basalts from a Depleted Asthenospheric Mantle Source, Mt. Erciyes Stratovolcano, Turkey', International Geology Review, 43: 6, 508 — 522 To link to this Article: DOI: 10.1080/00206810109465029 URL: http://dx.doi.org/10.1080/00206810109465029 Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
Transcript of Trace-Element Modeling and Source Constraints for Tholeiitic and Cale-alkaline Basalts from a...
PLEASE SCROLL DOWN FOR ARTICLE
This article was downloaded by: [TÜBİTAK EKUAL]On: 3 May 2011Access details: Access Details: [subscription number 772815469]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
International Geology ReviewPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t902953900
Trace-Element Modeling and Source Constraints for Tholeiitic and Cale-alkaline Basalts from a Depleted Asthenospheric Mantle Source, Mt.Erciyes Stratovolcano, TurkeyB. Kurkcuoglua; E. Sena; A. Temela; E. Aydara; A. Gourgaudb
a Department of Geological Engineering, Hacettepe University, Beytepe, Ankara, Turkey b Blaise-Pascal University, Clermont-Ferrand, Cedex, France
Online publication date: 06 July 2010
To cite this Article Kurkcuoglu, B. , Sen, E. , Temel, A. , Aydar, E. and Gourgaud, A.(2001) 'Trace-Element Modeling andSource Constraints for Tholeiitic and Cale-alkaline Basalts from a Depleted Asthenospheric Mantle Source, Mt. ErciyesStratovolcano, Turkey', International Geology Review, 43: 6, 508 — 522To link to this Article: DOI: 10.1080/00206810109465029URL: http://dx.doi.org/10.1080/00206810109465029
Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf
This article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.
International Geology Review, Vol. 43, 2001, p. 508-522. Copyright © 2001 by V. H. Winston & Son, Inc. All rights reserved.
Trace-Element Modeling and Source Constraints for Tholeiitic and Cale-alkaline Basalts from a Depleted Asthenospheric Mantle
Source, Mt. Erciyes Stratovolcano, Turkey B. KÜRKCÜOGLU,1 E. SEN, A. TEMEL, E. AYDAR,
Department of Geological Engineering, Hacettepe University, 06532 Beytepe, Ankara, Turkey
AND A. GOURGAUD
Blaise-Pascal University, UMR-CNRS 6524, 5 rue Kessler, 63038 Clermont-Ferrand, Cedex, France
Abstract
The Mt. Erciyes stratovolcano was built up in an intraplate tectonic environment as a consequence of Eurasian and Afro-Arabian continental collision. However, the volcanic products generally exhibit a calc-alkaline character; minor amounts of tholeiitic basalts are also present. Tholeiitic basalts show high Fe2O3, MgO, CaO, low K2O, and depleted Ba, Nb, and especially Rb (2.3-5.97 ppm) contents, low 87Sr/86Sr (0.703344-0.703964), and high 143Nd/144Nd (0.512920-0.512780) iso-topic ratios. These compositional features show that they were derived from a depleted asthenospheric mantle source, possibly a MORB-like source component. In contrast, calc-alkaline basaltic rocks exhibit relatively high large-ion-lithophile and high-field-strength elements, high 87Sr/86Sr (0.704591-0.70507) and low 143Nd/144Nd (0.51272-0.512394) isotopic ratios.
The bulk-rock chemistry of the tholeiitic basalts reflects the chemical composition of the extracted source component. Furthermore, trace-element concentrations may be calculated from an accepted mantle source component (starting composition) for different degrees of partial melting. These calculations also provide a sensitive approach to the origin of tholeiitic basalts. Modeled trace-element compositions of tholeiitic basalts are calculated from a primitive mantle composition. Calculated trace-element compositions imply that tholeiitic basalts are derived by minor fractional melting (1-1.5 %), in the absence of assimilation or deep-crustal melting. The calc-alkaline basalts were subsequently produced from initially tholeiitic basalts by the way of an AFC (assimilation-fractional crystallization) process, with a crustal assimilation of 10-15 %.
The geochemical data, partial melting, and AFC modeling all indicate that basaltic products have a complex evolutionary history involving partial melting from a MORB-like mantle source. The assimilation and fractional crystallization processes are considered as providing an example for the chemical evolution of basaltic products, from tholeiitic to calc-alkaline, in an intraplate environment.
Introduct ion
THE NEOTECTONIC EVOLUTION of the Anatol ian block is linked to continental collision between the Afro-Arabian and Eurasian plates, during the early and middle Miocene (Sengör, 1980; Sengör and Yil-maz, 1981; Dewey et al., 1986). As a result of this convergence, the Anatolian block has moved to the west along two strike-slip faults, the Northern and Eastern Anatolian faults, and has been deformed (McKenzie, 1972; Sengör, 1980; Jackson and McK-enzie, 1984; Dewey et al., 1986; Chorowicz et al.,
1994; Deniel et a l , 1998) (Fig. 1). Anatolian volca-nism is an outstanding example of arc volcanism related to continental collision (Deniel et al., 1998). In both western and eastern Anatolia, calc-alkaline lavas were p roduced when the compres s iona l regime led to crustal thickening in the late Oli-gocene—Early Miocene in the west and in the Pliocene in the east (Yilmaz, 1990; Deniel et al., 1998). In central Anatolia, plate tectonic reconstructions for the last 13 m.y. (Lyberis et al., 1992) indicate that the subducted African slab did not reach central Anatolia and thus cannot directly account for the present day calc-alkaline volcanism in this area (Aydar et al., 1995).
1Corresponding author; fax: + 90 (312) 299 20 34; e-mail: [email protected]
0020-6814/01/531/508-15 $10.00 5 0 8
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MT. ERCIYES STRATOVOLCANO 509
FIG. 1. Neotectonic map of the Anatolian block. Major faults and the study area are placed on a Digital Elevation Model (DEM) of Anatolia, modified from Köse (2000). Abbreviations: AEZ = Aegean extension zone; NAF = North Anatolian fault; EAF = East Anatolian fault; EF = Ecemis fault; TGF = Tuz Gölü fault. Black arrows indicate the direction of plate motion; black half arrows show the relative direction of motion on the faults.
FIG. 2. Digital Elevation Model (DEM) of Mt. Erciyes stratovolcano and locations of some major faults. The DEM was made by digitizing the elevation contours of topographic maps at a scale of 1:25,000, artificially illuminated from the north. The vertical exaggeration is x 1.5. Abbreviations: A.T = Abas Tepe; S.T. = Siharslan Tepe; K.D = Kefeli Dag; KT1 and KT2 = Karniyarik Tepe (all basaltic vents).
Central Anatolia exhibits the well-known Cappa-docian ignimbrites, stratovolcanoes (such as Mounts Kecikalesi, Melendiz, Erdas, Kara, Karaca, Hasan, and Erciyes), and hundreds of monogenetic vents (domes, scoria cones, maars). The most ancient vol-canism of the area is represented by Mt. Kecikalesi, dated as 13 Ma by Besang et al. (1977), which
exhibits mineralogical and geochemical tholeiitic affinities (Aydar and Gourgaud, 1998).
The most voluminous Plio-Quaternary vent of the region is the Mt. Erciyes stratovolcano (Figs. 1 and 2), which was built up in an intraplate tectonic environment as a consequence of post-collisional extension (Aydar et al., 1995; Notsu et al., 1995;
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510 KÜRKCÜOGLU ET AL
FIG. 3. Total alkaline versus silica diagram (Le Bas et al., 1986) of Erciyes basaltic products. The dashed line separates alkaline and subalkaline fields, after Miyashiro (1978).
Kürkcüoglu et al., 1998; Kürkcüoglu, 2000). During its formation, Mt. Erciyes mostly produced andesitic lavas, but also some basaltic products. Although Mt. Erciyes is dominantly calc-alkaline, some tholeiitic basalts occur as the initial products.
Tholeiitic basalts might be considered as the primary products of depleted asthenospheric sources (Kempton et a l , 1991; Fitton et al., 1991; Menzies et al., 1991), with an associated lack of crustal contribution. The main topic of this study is to investigate the source constraints for the tholeiitic basalts, and to examine the tholeiitic-calc-alkaline relations in the specific tectonic context. We used trace- and rare-earth-element data and Sr-Nd isotopic ratios to model the genesis (source composition) of the Mt. Erciyes basaltic magma.
Geological Background
Mt. Erciyes, the largest stratovolcano of Central Anatolia, (3917 m elevation) is located approximately 15 km south of the city of Kayseri (Fig. 1). The volcano mainly exhibits lava flows that are related to central and adventive monogenetic vents (scoria cones and domes) and their associated pyro-clastic products (Fig. 2). Volcanic activity developed in two different stages (Sen, 1997; Sen et al., 1999). The first stage began with basaltic activity (tholeiitic basalt), followed by differentiated sequences (basaltic andesite, andesite, dacite). At the end of this stage, an extensive ignimbritic eruption occurred (Sen et al., 1999; Sen, 1997), which has been dated by Innocenti et al. (1975) by K/Ar as 2.7 ± 0.1, 2.8 ± 0.1, and 3.0 ± 0.1 Ma. Stratigraphi-
cally, the tholeiitic basalts are the first products of the stratovolcano. The second stage exhibits more differentiated calc-alkaline basaltic andesites, andesites, dacites, and rhyolites. There are no calc-alkaline basalts sensu stricto. The stratigraphy of Mt. Erciyes is detailed in Sen (1997) and Kürkcüoglu et al. (1998).
Mt. Erciyes volcanism developed in a complex tectonic environment. Previous geodynamic models suggested a subduction event (Pasquare, 1968; Innocenti et al., 1975; Ayranci, 1991) or intraplate magmatism (Aydar et al., 1995; Notsu et al., 1995; Kürkcüoglu et al., 1998). The origin of the Mt. Erciyes volcanism is a matter of debate. Kürkcüoglu et al. (1998) proposed different mantle sources for the derivation of basaltic magmas. Tholeiites are derived from depleted asthenospheric (MORB-like) mantle sources, whereas other basaltic (including alkaline and calc-alkaline) magmas may be related to an enriched mantle component. A new trace element modeling of partial melting and AFC processes is our main contribution to the debate on the genesis of the Mt. Erciyes basaltic magmas.
Geochemistry
Analytical techniques Major- and trace-element compositions were
determined using a Philips PW 1480 X-Ray Fluorescence spectrometer, from fused glass tablets and pressed powdered samples at the Geological Engineering Department of Hacettepe University, Turkey. A detailed explanation of the preparation of samples is provided in Alici et al. (1998). The
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MT.
ERC
IYES
STR
ATO
VOLC
ANO
511
TABL
E 1.
Maj
or (%
) and
Tra
ce-E
lem
ent (
ppm
) Dat
a an
d Sr
and
Nd
Isot
opic
Rat
ios o
f Tho
leiit
ic a
nd C
alc-
alka
line
Bas
altic
Pro
duct
s of M
t. Er
ciye
s
SiO
2
Al 2O
3
Fe2O
3
MnO
M
gO
CaO
Na 2
O
K2O
TiO
2
P 2O
5
LOI
Total
Nb
Zr
Y
Sr
Rb
Ga Ni
Co
Cr
V
Ba
87Sr
/86Sr
14
3Nd/
144N
d
____
__
Thol
eiiti
c ba
salts
__
____
C88
-79
48.4
17.6
1
9.15
0.15
8.
39
11.4
1 3.
36
0.18
1.3
7
0.09
10
0.11
6.3
123.
6 22
.4
398.
5 2.
3
16.5
106.
2
35.8
201.
2
165.
2
52.7
0.70
3344
0.51
292
ERC
96-
116
49.2
1 17
.17
11.2
6 0.
177
6.61
9.
13
3.75
0.71
1.7
7
0.36
0.
04
100.
2
12.3
214.
8
29.7
45
7.6
5.6
23.7
86
.89
51.5
11
8.8
168.
7
282.
6
0.70
3964
0.51
278
ERC
98-
1
49.2
1
17.0
9
11.1
4
0.17
2 7.
07
9.42
3.47
0.
57
1.7
0.41
-0
.65
99.6
7.02
166.
5
23.2
36
5.6
5.97
16.8
96
48.6
n.a
162
160.
5
____
__
Cal
c-al
kalin
e ba
salts
____
__
ERC
96-
75
54.4
5 18
.26
6.82
0.10
2 5.
44
9.07
2.89
1.01
0.
95
0.17
0.
3 99
.47
6.09
98.7
14
.2
335.
4 24
.97
16.1
47
39.5
n.a
135
199.
8
0.70
468
0.51
268
ERC
96-
138
54.5
7
17.7
6
7.16
0.
115
5.37
8.12
3.45
1.
18
1.04
0.32
0.
38
99.4
5
17
181.
5
22.8
45
7.4
22.2
19.5
69
.9
39.7
117.
8
125.
6
284.
8
ERC
96-
141
54.3
5
17.2
8 7.
44
0.12
3 5.
81
8.61
3.
39
1.17
1.11
0.45
0.
63
100.
37
12.7
7 17
5.4
18.3
447.
6 24
.72
16.1
82
33.6
n.a
106
299.
2
ERC
91-
3 54.2
17.6
6
7.23
0.
11
5.67
8.43
3.
51
1.04
0.99
0.32
0.
55
99.7
1
19
169.
5 22
460.
4
23.1
17
.8
7.67
26
.1
207.
8
123.
6
239.
6
0.70
4666
0.51
2678
C88
-83
55.4
17.0
5 6.
9
0.11
5.
85
8.25
3.
64
1.27
0.94
0.21
99
.62
15.8
17
3.1
24.6
309.
1 50
.1
19
25.1
51
.1
36.7
77.2
319.
9
C88
- ER
C92
- ER
C91
-87
2
29
55.7
18.1
6.8
0.11
5.
59
8.4
3.78
1.28
0.
94
0.18
10
0.88
21.8
160.
3
23.3
48
9.6
29.1
18.6
77.3
24.4
116.
1
126.
7
305.
3
0.70
507
0.51
239
56.6
3
17.4
1
7.25
0.12
4.
01
7.03
4.09
1.28
1.08
0.
38
0.35
99.6
3
22.9
216.
5
29.3
47
4.5
28.8
20.6
27.8
21
78.6
114.
5
362
0.70
4661
0.51
2674
56.4
6
17.3
7
7.57
0.
13
3.62
6.62
4.16
1.3
2
1.12
0.49
0.
41
99.2
7
18.7
215.
8 29
421.
7
30
20
35.8
20.7
78.2
119.
6
312.
3
0.70
4591
0.51
2714
ERC
95-
64
51.5
3 16
.92
9.09
0.14
3 5.
97
8.81
3.
72
0.94
1.55
0.
47
0.65
99
.79
14.6
22
8.2
30.1
528.
2 13
.5
22
97.4
42.9
178.
1 13
7.4
238.
1
0.70
4633
0.51
272
ERC
95-
66
53.1
6
16.4
9 8.
4
0.13
7 6.
54
8.4
3.39
1.16
1.3
0.36
0.
14
99.4
8
11.8
19
5.6
25.8
48
2.2
21.5
20.8
106.
2
45.5
223.
1
159.
4
238
ERC
95-
121
54.6
3 17
.6
6.54
0.10
6 4.
55
8.02
3.74
1.28
1.
02
0.32
0.
61
98.4
2
15.9
17
7.4
22.5
47
6.7
32.5
20
.1
43.7
35
.4
74.1
122.
3
281.
1
ERC
96-
139
54.3
7
18.5
4
7.62
0.13
4.
97
7.19
3.
66
1.23
1.13
0.
47
0.43
99
.73
13.5
2
209.
8 21
.3
392.
6 21
.44
16.2
54
35.9
n.a
96
359.
9
ERC
96-
140
55.0
8
18.5
5
7.63
0.12
9 4.
7 6.
42
3.46
1.22
1.14
0.41
1.5
2
100.
26
ERC
96-
208
54.7
1
17.6
1
6.68
0.
106
4.6
8.1
3.5
1.2
1.04
0.32
1.1
98.9
6
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512 KÜRKCÜOGLU ET AL.
TABLE 2. Rare-Earth and Th, U, Pb, Hf, and Ta Compositions of Representative Samples of Basaltic Products of Mt. Erciyes, in ppm
La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu Th Pb U Hf Ta
______ Tholeiilic ______
C88-79
5.65 15.1 1.95
10.7 2.87 1.2 3.84 3.83 2.44 2.28 0.34 0.3 5 0.1 2.8 0.2
ERC96-116
16.4 36.9
n.a 18.4 4.45 1.41 4.93 4.7 2.69 2.82 0.41 1.9 4 0.4 4.6 0.9
ERC98-1
13.3 29.5 3.79
17.8 4.7 1.57 5.64 4.83 2.86 2.48 0.3 1.5 5 0.3 4.1 0.8
______Calc-alkaline______
ERC96-75
15.6 31.6
n.a 13.8 3.25 0.91 3.38 3.25 1.62 1.59 0.24 4.1
10 1.2 2.9 0.8
ERC96-141 ERC96-139
22.6 47.6 4.93
21.1 4.8 1.33 5.56 4.04 2.21 1.89 0.25 4.5 3 1 4.3 1.2
26.9 50.7 6
24.7 5.4 1.47 5.23 4.35 2.51 2.21 0.27 4.4 6 0.9 5.1 1.3
ERC95-64
19.7 41
n.a 19.4 4.32 1.3 4.56 4.11 2.39 2.43 0.33
ERC91-3
23.9 47.7 4.31
20.7 3.62 1.2 3.84 3.7 2.22 2.12 0.31
ERC91-29
27.9 55.9
5.08 24.3 4.75 1.4 4.57 4.59 2.57 2.58 0.4
spectrometer was calibrated using international standards (USGS and GEOSTANDARDS) to ensure accurate data.
Furthermore, three samples are from Aydar et al. (1995). REE analyses were performed using a JY7011 Inductively Coupled Plasma Emission (ICP) spectrometer at Blaise Pascal University. Sr and Nd isotopic compositions were determined in a fully automated V.G. isomass 54E and Thompson-CSF TSN 206SA mass spectrometers at Blaise Pascal University and the Geological Engineering Department of Hacettepe University, respectively. U, Pb, Th, Hf, and Ta concentrations were determined, using an inductively coupled plasma mass spectrometer (ICP-MS) at ACME Analytical Laboratories Ltd., Canada. Analytical errors are typically ± 2% for major elements and less than ± 10% for trace elements. The 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0.1194. The 143Nd/144Nd ratios were normalized to 146Nd/144Nd = 0.7219. Over the periods of measurement, replicate analyses of standards gave, for the National Bureau of Standards (NBS) 987, 87Sr/86Sr = 0.701244 ± (2σ) and, for La Jolla 1 4 3Nd/1 4 4Nd = 0.51184 ± 3 (2σ).
Major and trace element geochemistry According to the total alkali versus silica dia
gram (Le Bas et al., 1986), the basaltic Mt. Erciyes volcanics exhibit a tholeiitic to calc-alkaline character (Fig. 3). Major-element compositions of basalts are listed in Table 1. MgO contents range from 8.4 to 6.6 wt% in tholeiites and 6.5 to 4.5 wt% in calc-alkaline basalts. Oxides versus SiO2 diagrams show that MgO, Fe2O3, and TiO2 correlate linearly and negatively, whereas Na2O and especially K2O correlate linearly and positively with increasing SiO2. Other oxides, Al2O3, CaO, and P2O5, do not show good correlation trends. The geochemical evolution of the Mt. Erciyes volcanics was detailed in Kürkcüoglu et al. (1998).
Basalts sensu stricto are plotted over the alkaline subalkaline division line of Miyashiro (1978), and are interpreted as tholeiitic basalts according to their trace-element compositions (Fig 3). Trace-element data are listed in Tables 1 and 2. Some basalts exhibit highly depleted Nb, Ba, La, Ce, U, Ta, and especially Rb, and are considered as tholeiitic basalts, whereas others exhibit calc-alkaline characteristics.
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MT. ERCIYES STRATOVOLCANO 513
Correlation diagrams between incompatible trace elements and SiO2 show that some trace elements exhibit unusual behavior. For example, there is no clear correlation between Ba, Nb, and Sr, and silica. Moreover, Zr content decreases with increasing SiO2, whereas Rb increases. Such variations cannot be explained by a single fractional crystallization process.
Chondrite-normalized trace-element patterns (Fig. 4) show that tholeiitic basalts have low concentrations of large-ion lithophile (LIL; Rb, Sr, K) and some high-field-strength (HFS) elements (such as Th, La, and Ce) compared to calc-alkaline basalts (Fig. 4A). We suggest that tholeiitic and calc-alkaline basalts may be derived either from different mantle sources or by different geochemical processes from the same source. Furthermore, the Mt. Erciyes tholeiitic basalts are similar to the Serviletta tholeiitic basalts of the Taos volcanic area of north-central New Mexico and the Rio Grande Rift tholei-ites (Fig. 4B). The calc-alkaline basalts of Mt. Erciyes exhibit trace-element patterns that are similar to the Lake Mead tholeiitic basalts of Nevada (Fig. 4C) and the Rio Grande Rift tholeiites. Incompatible-element contents and chondrite-normalized trace-element patterns indicate that the tholeiitic basalts probably were derived from asthenospheric mantle sources.
Isotope geochemistry 87Sr/86Sr and 143Nd/144Nd isotopic ratios of
basalts are listed in Table 1. 87Sr/86Sr and 143Nd/ 144Nd isotopic compositions of tholeiitic and calc-alkaline basalts range between 0.703344-0.703964 and 0.512920-0.512780, and 0.704591-0.705070 and 0.512720-0.512394, respectively. The 87Sr/ 86Sr-143Nd/144Nd diagram (Fig. 5) shows that tholeiitic basalts plot within the mantle array, close to the MORB area and within the Rio Grande rift axis field. Although they partially overlap with the OIB field, the LIL and HFS elements do not reflect an OIB signature.
The low Sr and high Nd isotopic ratios (Table 1) and depleted LIL and HFS elements show that tholeiitic basalts may be derived from depleted asthenospheric sources. Moreover, the La/Nb versus 87Sr/86Sr diagram also demonstrates that the tholeiitic basalts were derived from a depleted asthenospheric mantle (Fig. 6), possibly from a MORB-like mantle source.
Calc-alkaline basalts also plot in the mantle array, close to the Río Grande Rift axis field. One
FIG. 4. Chondrite-normalized (Thompson, 1982) incompatible trace-element patterns for representative basaltic samples of the Erciyes volcanics. References are from McMillan and Dungan (1986), Duncker et al. (1991), and Feuerbach et al. (1993).
sample plots in the Rio Grande flank field. Relatively high LIL- and HFS-element contents and Sr isotopic compositions, together with low Nd isotopic ratios, indicate at least two alternative origins: (1) these basalts may be derived from different mantle sources; or (2) the mantle component may have undergone various geochemical processes.
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514 KÜRKCÜOGLU ET AL.
FIG. 5. 87Sr/86Sr-143Nd/144Nd variation diagrams for representative basaltic products of Mt. Erciyes. Data are from Chen and Frey (1985), Chen et al., (1990), Cohen and O'Nions (1982), Chaffey et al. (1989), and Gibson et al. (1993). Mantle array and bulk earth (B.E) from Faure (1986).
FIG. 6. 87Sr/86Sr-La/Nb diagrams of tholeiitic and ealc-alkaline basaltic samples. Data from Chen and Frey (1985), Altherr et al. (1988), Kempton et al. (1991), Gibson et al. (1993), and Ormerod et al. (1991). Symbols are the same as in Figure 3.
Trace-Element Modeling
Mantle sources and relations of tholeiitic and calc-alkaline magmas may be explained by trace-element modeling. Two principal processes are presented below.
Partial melting process A trace-element model has been calculated to
test the partial melting process, according to Rollin-son's (1993) method. Trace-element modeling of basaltic magma from an asthenospheric mantle
source requires the calculation of trace-element content using the fractional melting process. This involves both the major- and trace-element contents as well as the mineralogical composition of the source component. Primitive-mantle chemical compositions from Taylor and McLennan, (1985) are required as a starting composition. The content of any trace element from the source component may be calculated by the partial melting formula (Rollin-son, 1993), expressed below:
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MT. ERCIYES STRATOVOLCANO 515
TABLE 3. CIPW Norms and Mineral-Melt Partition Coefficients (Kd) Used in the Melting Models1
Plj K-Felds Ne
O1 Cpx Opx
Mg I1
Ap
ERC96-116
59.83 4.2
0.18
17.2 12.33
-2.73
3.36 0.85
C88-79
55.69 1.06 2.81
16.06 19.58
-0.12
2.6 -
Primitive mantle1
11.23 0.12
-39.2 4.74
44.47
-0.3 -
Rb Sr
Ba Nb Zr
K Ti Y La Ce Nd Sm Eu Gd Dy Er Yb Th Hf Ta
Plj
0.071 1.83
0.23 0.01 0.012 0.17 0.04
0.03 0.19 0.12 0.045 0.067 0.34 0.063 0.055 0.063 0.067 0.01 0.051 0.02
K-felds
0.32 2.3
3.04 0.04 0.003
0 0 0.017 0.052
0.039 0.026 0.028
1.26 0.023 0.025 0.017 0.028 0.007 0.009
-
Ne
0.44 0.006 0.09 0.011 0.055
0 0 0.01 0.01 0.011 0.013 0.012
0.043 0.013 0 0.014
0.016 0.014 0.008
-
O1
0.0098 0.014 0.0099
0.01 0.001 0.0068 0.02
0.01 0.0067 0.0069 0.0066 0.0066 0.0068 0 0.0096 0.0011 0.0014
0 0.002
-
Cpx
0.031 0.06
0.026 0.005 0.1 0.038 0.4
0.9 0.056 0.15 0.31 0.5 0.51 0.61 0.68 0.65 0.62 0.03 0.263 0.013
Opx
0.022 0.04
0.013 0.15 0.18 0.014
0.1 0.18 0.02 0.02 0.03 0.05 0.05 0.09
0.15 0.13 0.34 0.013 0.01 0.048
Mg
0.11 0.11
0.028 0.7 0.71
0.045 4 0.0039 0.01 0.016 0.026 0.024
0.025 0.018 0.3 0 0.018 0.1 0.16 0.23
I1
0 0
0 2.3 0.28 0
0 0.0045 0.098 0.11 0.14 0.15 0.1 0.14
0 0 0.17
--
2.7
Ap
0 2.1 0.05
0 0 0
0 0 8.6 11.2 14 14.6
9.6 15.8
8.3 22.7 8.1
---
1CIPW norms of primitive mantle composition (Taylor and McLennan, 1985) are calculated from major-element composition. Sources: a for Kd values. Compiled by the authors from Fujimaki et al., 1984; Irving and Frey, 1978; Drake and Weill, 1975; Pearce and Norry, 1979; and Lemarchand et al., 1987.
where CL is the weight concentration of the trace element in the liquid, C0, is the weight concentration of the trace element in the original unmelted solid, F is the melt fraction, and D0 is the whole-
rock bulk partition coefficient of the original solid. Accurate computations can be made by precisely calculated D0 values for whole rocks. The bulk partition coefficient of any whole-rock element may be
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516 KÜRKCÜOGLU ET AL.
TABLE 4. Model of Trace-Eelement Compositions Calculated Using the Fractional Melting Process
F (%)2
Rb
Sr
Ba
K,K2O
Zr
Y
Ti
Nb
La
Ce
Nd
Sm
Eu
Gd
Dy
Er
Yb
Th
Ta
Hf
Calculated whole-rock partition
coefficient D0
0.023
0.23
0.04
0.029
0.091
0.13
0.075
0.078
0.035
0.032
0.04
0.056
0.089
0.076
0.108
0.144
0.194
0.008
0.032
0.027
Starting composition1
0.55
17.8
5.1
180
8.3
3.4
960
0.56
0.55
1.436
1.06
0.347
0.131
0.459
0.572
0.374
0.372
0.064
0.04
0.27
Calculated trace-element
composition (ppm)
1
15.6
74.83
100.17
4433 (0.53%)
82.49
24.45
11307 (1.88%)
6.37
11.91
33.11
20.95
5.23
1.32
5.34
4.87
2.44
1.83
2.3
0.92
6.96
Calculated trace-element
composition (ppm)
1.5
12.58
73.57
88.71
3741 (0.45%)
78.42
23.63
10623 (1.77%)
6
10.35
28.4
18.55
4.8
1.25
5.02
4.67
2.37
1.8
1.22
0.79
5.8
Tholeiitic basalt composition
(ERC98-1), ppm
5.97
365.6
160.5
4731 (0.57%)
166.5
23.2
10200(1.7%)
7.02
13.3
29.5
17.8
4.7
1.57
5.64
4.83
2.86
2.48
1.5
0.8
4.1
1Primitive-mantle trace-element composition, ppm (Taylor and McLennan, 1985). 2F = degree of partial melting.
calculated by using CIPW norm values of rocks and the element's mineral/melt (Kd) partition coefficient. D0 values of the elements for starting composition are calculated by the formula (Rollinson, 1993) given below:
D0 = ΣWiKDi,
where W is the weight proportion of each mineral (calculated from major-element composition) and KDi is the individual mineral-liquid partition coefficient. The preference for using KDi data in modeling stems from the use of microprobe and whole-rock trace-element compositions in their calculation. Athough microprobe analyses have been carried
out, trace element-based results are very limited. These results only may be used to compare calculated and reference KDi values. A comparison shows that some calculated KDi values (KPlg = 0.28; TiPlg = 0,07; TiO1 = 0.036; KCPX = 0.042) are similar to reference data suggested by several authors (Table 3). These similarities permit the use of the referenced KDi values in calculation. Mineral-liquid partition coefficients used in melting models are given in Table 3. The chemical compositions, calculated CIPW norms, and D0 values of each element for whole-rock composition of primitive mantle (Taylor and Mc Lennan, 1985) also are given in Tables 3 and 4. Different element contents may be calculated
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MT. ERCIYES STRATOVOLCANO 517
FIG. 7. A-B. Chondrite-normalized spider and REE plots of tholeiitic basalts and trace-element modeling, calculated by fractional melting from a primitive mantle source composition. C-D. Chondrite-normalized spider and REE plots of a calculated model of trace elements, from a tholeiitic basalt end member and andesitic products of Mt. Erciyes. Normalization values are from Thompson (1982) and Nakamura (1974).
for different degrees of partial melting (F). These calculations also provide the best approach for studying the genesis conditions of tholeiitic basalts. Table 4 lists trace-element contents calculated from the starting composition using the partial melting process with varying values of F.
Numerous calculations were performed for different degrees of partial melting, the closest values between the trace element model and the real chemical data on tholeiitic basalts being obtained for a low degree of partial melting (F = 1 - 1.5%). These similarities are illustrated in a chondrite-normalized spider diagram (Figs. 7A-7B). Figure 7A shows that calculated trace-element compositions compare favorably with analyzed data from tholeiitic basalts, except for Sr and Zr. These differences may be explained by the low initial concentration of these elements in the starting composition. Moreover, we note the high D0 values, especially for Sr, and consequently the minimal effects of possible crustal contribution. Note that, for 1% F, there is a similar
ity between the real chondrite-normalized REE pattern of tholeiitic basalt and the calculated values (Fig. 7B). So trace-element modeling of fractional melting processes emphasizes that tholeiitic basalts are generated with 1-1.5% partial melting of a depleted asthenospheric mantle source.
Similar calculations were also performed for calc-alkaline samples. The less differentiated tholeiitic basalt composition (ERC96-116) was chosen as the starting composition for the calc-alkaline basalt. CIPW norms and D0 values for the source component (ERC96-116) are calculated and listed in Tables 3 and 5.
The trace-element modeling procedure was performed for different F values and calculated and measured trace-element compositions compared. Moreover, compositional similarities are observed between calculated and chemically analyzed calc-alkaline basalts. Some compositional differences also are noted in LIL elements in the case of 25% partial melting from the accepted end-member. Rb,
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5 1 8 KÜRKCÜOGLU ET AL.
TABLE 5. Model of Trace-Element Compositions Calculated Using the Fractional Melting Process from a Tholeiitic Basalt End Member
F (%)2
Rb
Sr
Ba
K,K 2O
Zr
Y
Ti
Nb
La
Ce
Nd
Sm
Eu
Gd
Dy
Er
Yb
Th
Ta
Hf
Calculated whole-rock
partition coefficient, D0
0.064
1.21
0.28
0.107
0.105
0.131
0.185
0.104
0.2
0.19
0.21
0.23
0.4
0.25
0.19
0.31
0.19
0.012
0.11
0.069
Starting composition1
5.6
457.6
282.6
5893 (0.71%)
214.8
29.7
10620 (1.77%)
12.3
16.4
36.9
18.4
4.45
1.41
4.93
4.7
2.69
2.82
1.9
0.9
4.6
Calculated trace-element composition
(ppm)
0.25 (25%)
1.3
397.13
481.56
4999.9 (0.6%)
176.34
33.66
16189 (2.69%)
5.48
19.68
42.5
22.91
5.87
2.06
6.76
5.41
3.93
3.24
0.79
1.37
ERC95-64 (Basalt)
13.5
528.2
238.1
7802 (0.94%)
228.2
30.1
9300 (1.55%)
14.6
19.7
41
19.4
4.32
1.3
4.56
4.11
2.39
2.43
ERC96-139 (Basaltic andesite)
21.44
392.6
359.9
10209(1.23)
209.8
21.3
6780(1.13)
13.52
26.9
50.7
24.7
5.4
1.47
5.23
4.35
2.51
2.21
4.4
1.3
5.1
ERC96-141 (Basaltic andesite)
24.72
447.6
299.2
9711(1.17)
175.4
18.3
6660(1.11)
12.77
22.6
47.6
21.1
4.8
1.33
5.56
4.04
2.21
1.89
4.5
1.2
4.3
ERC91-29 (Andesite)
30
421.7
312.3
10956 (1.32%)
215.8
29
6720 (1.12%)
18.7
27.9
55.9
24.3
4.75
1.4
4.57
4.59
2.57
2.58
1ERC96-116 (tholeiitic basalt). 2 F = degree of partial melting.
K, Th, Nb, and Ta are higher in calc-alkaline basalts than in the calculated model (Table 5, Fig. 7C). Although partial melting is the primary process for producing basaltic magma, these compositional differences may be explained by either: (1) very low initial concentrations of the end member in these elements; or (2) possible involvement of crustal materials during the magma's ascent. This second hypothesis can be tested or controlled by AFC modeling. Globally, similar trends were obtained between calc-alkaline basalt and 25% partially
melted modeled compositions from the tholeiitic source (Fig. 7D).
AFC modeling Chemical variations during assimilation and
fractional crystallization (AFC) have been pointed out by DePaolo (1981). Modeling of trace elements is applied in order to evaluate whether crustal material is involved in the generation of calc-alkaline basalts. For calculation purposes, the C88-79 tholeiitic basalt sample has been selected as the basic
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MT. ERCIYES STRATOVOLCANO 519
TABLE 6. Calculated Rb and Sr Concentrations and 87Sr/86Sr Isotopic Ratios Using AFC Modeling from a Tholeiitic Basalt End Member1
F
Sr
Rb
Rb/Sr 87Sr/86Sr
Sr
Rb
Rb/Sr 87Sr/86Sr
0.2
391.509
5.915
0.015108
0.703526
390.535
7.725
0.019781
0.703779
End member C88-79 Contaminant: upper crust
r = 0.1 DSr = 1.06 DRb = 0.064
0.4
382.8%
11.831
0.030899
0.703735
380.893
16.594
0.043566
0.704329
0.6
371.48
23.37
0.062911
0.704464
r = 0.15
368.416
33.865
0.091921
0.705084
0.8
353.773
56.601
0.1599924
0.70529
349.779
83.487
0.238685
0.706311
0.9
338.119
119.866
0.3545083
0.706092
334.075
177.683
0.5318656
0.707452
1Contaminant chemical compositions are from Taylor and McLennan, (1985). F = remaining melt fraction; r = ratio of assimilation rate to fractionation rate.
FIG. 8. Modeling the AFC process using the equations of De Paolo (1981). Calculation parameters are given in Tables 1 and 6 with measured isotope and trace-element values. Symbols: r = degree of assimilation/degree of fractionation.
end member because of its depleted composition. Upper continental crust, according to Taylor and McLennan (1985) is selected as the contaminant. For AFC modeling, chemical compositions of the contaminant were assumed to have Rb = 112 ppm, Sr = 350 ppm (Taylor and Mc Lennan, 1985), and 87Sr/86Sr = 0.71600 (Veizer and Compston, 1974). Chemical and isotopic compositions, calculated D0 values, and CIPW norms for the end member are
given in Tables 1, 3, and 6. A variation diagram of 87Sr/86Sr with Rb/Sr during the AFC process is shown in Figure 8. A positive correlation of these ratios is related to a possible involvement of crustal material. The tholeiitic end member is the source component for the calc-alkaline basalt, by way of a 10-15% assimilation of crustal material. Both of the AFC trends (Fig. 8) are the most likely trajectories for the calc-alkaline samples (Fig. 8).
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520 KÜRKCÜOGLU ET AL.
D i s c u s s i o n and Conc lus ions
The 87Sr/86Sr, 143Nd/144Nd isotopic compositions and incompatible-trace-element ratios of tholeiitic basalts, as well as trace-element modeling of partial melting, indicate that they were derived from a d e p l e t e d a s t h e n o s p h e r i c (MORB-l ike) man t l e source. Following that process, new calc-alkaline magmas were generated by lithospheric contribution, during the ascent to the surface, involving the AFC process from an initially tholeiitic basalt end member . T h e la rge compos i t iona l d i f ferences observed for Rb and Nb between calculated (25% partially melted) and observed calc-alkaline compositions are strongly related to the depleted nature of the tholeiitic end member. Rb, Nb, and K are also depleted in the start ing composition. Extracted products from this source also exhibit depleted values.
AFC and partial-melting models also indicate that the AFC process is primarily responsible for the generation of calc-alkaline basalts. But the role of a syngenetic partial melting process during such an event cannot be ruled out, because crustal involvement is not a unique process linked to calc-alkaline basalt genesis.
The a b s e n c e of s ensu s t r ic to ca l c - a lka l i ne basalts may be related to the partial melting process. The rate of partial melting is directly related to changes in the rate of extension. The positive correlation between Sr isotopic ratios and ages (11.2 Ma to 2.8 Ma) suggests the decreasing contribution of the crust through time, as a result of increasing extension in central Anatolia (Temel et al., 1998). The increase in extension rate promoted extraction of tholeiitic basalts rather than calc-alkaline melts. Furthermore, evidence for the increase in exten-sional tectonic features in central Anatolia is also supported by the occurrence of young alkal ine basalts at Mt. Erciyes (Kürkcüoglu et al., 1998), Mt. Hasan stratovolcano (Deniel et al., 1988), and as monogenetic vents in central Anatolia (Notsu et al., 1995).
In light of all the geochemical data, we conclude that the basaltic products of Mt. Erciyes have a complex origin. Incompatible elements, Sr and Nd isotop i c r a t i o s , a n d t r a c e - e l e m e n t m o d e l i n g a l l demonstrate that small amounts of tholeiitic basalts have been derived from a depleted asthenospheric mantle source by a 1-1.5% level of partial melting, with no crustal contamination. This is also suggested by low Sr and high Nd isotopic ratios and low
LIL-element contents. Subsequently, calc-alkaline basalts were produced by 10-15 % crustal assimilation of a tholeiitic basalt source. Furthermore, partial melting from that source component may also be responsible in some way for the generation of the ca lc-a lkal ine basal ts of the Mt. Erciyes s tratovolcano.
A c k n o w l e d g m e n t s
This study was financially supported by Hac-ettepe University, The Scientific and Technical Research Council of Turkey (TUBITAK), and the Centre National Recherche Scientifique (CNRS), France. The authors thank Pinar Alici for scientific contributions during the course of the research. We also thank V. O'Dwyer for improving the English exposition.
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