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Geochemistry of early mesozoic tholeiites from Morocco
Earth and Planetary Science Letters, 58 (1982) 225-239
225
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[2]
Geochemistry of Early Mesozoic tholeiites from Morocco H. Bertrand
l,
j. Dostal 2 and C. Dupuy 3
I Dbpartement des Sciences de la J'erre, Universitb de Lyon 1, 69622 Villeurbanne Cedex (France) e Department of Geology, Saint Mary's University, Halifax, N.S. (Canada) k~ Centre G~ologique et Gbophysique, U.S.T.L., 34060 Montpellier Cedex (France)
Received June 15, 1981 Revised version received December 14, 1981
Mesozoic dolerites from two areas of Morocco, the High Atlas fold belt between Marrakech and Demnat and the Anti-Atlas belt in the area of Foum Zquid, are mostly high-Ti quartz-normative tholeiites which in many respects resemble Mesozoic dolerite dikes from eastern North America. The dolerites display a wide range of major and trace element compositions, some of which are due to fractional crystallization. The doleritic sequences from High Atlas also show vertical stratigraphic zonation which is chai'acterized by a progressive depletion of lithophile elements toward the top. This trend together with regularities of trace element ratio variations are indicative of a dynamic melting of an initially homogeneous source. It is suggested that the continental upper mantle source for dolerites of Morocco was enriched in several incompatible elements in comparison with the upper mantle source for ocean floor tholeiites.
1. Introduction O n both sides of the Atlantic Ocean, the continental margins contain tholeiitic rocks in the f o r m of n u m e r o u s sills, dikes and lava flows. These tholeiites are associated with elongated basins filled with Triassic continental sediments and might be related to the tectonic event which resulted in the opening of the Atlantic Ocean in the Mesozoic. A l t h o u g h on the N o r t h A m e r i c a n side these rocks have been extensively studied [1-6] very little is k n o w n about dolerites from Europe and Africa. The purpose of this paper is to present geochemical data on the Mesozoic dolerites from M o r o c c o and to discuss the processes involved in the genesis of these rocks.
2. Geological and petrographic notes The geology and p e t r o g r a p h y of the studied dolerites have been described by Bertrand and Prioton [7]. The rocks occur in two distinct areas: the High Atlas fold belt between Marrakech and
D e m n a t , and the Anti-Atlas belt in the area of F o u m Zguid (Fig. 1). In the former area, the dolerites ( 5 0 - 1 5 0 m thick through the total sequence) outcrop as sills and lava flows in late Triassic to early Liassic basins and are interbedded with red beds and evaporites. Those of the latter area appear as sills and dikes intruding into the Paleozoic sedimentary sequence. Their K - A r ages, ranging between 180 and 200 m.y. [7-9], are consistent with the m e a n age of 186 m.y. proposed by Manspeizer et al. [10] for the western High Atlas, and with the ages obtained on the A m e r i c a n margin by Erickson and Kulp [11], A r m s t r o n g and Besancon [12] and G o h n et al. [13]. Most c o m m o n are fine-grained dolerites with intergranular microstructure which pass progressively to a coarser-grained dolerite at the core of the bodies. This latter consists of a granular framework of pyroxene and plagioclase which occur together with large patches of groundmass ranging f r o m a spherufitic fibrous material to subophitic intergrowths of plagioclase and pyroxene. Some bodies contain an olivine-bearing layer (up to 15 m thick) in the lower part, overlain by a coarse-
0012-82 IX/82/0000-0000/$02.75 © 1982 Elsevier Scientific Publishing Company
226
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1~3,51Z~!1 x ........ 1137 [:'!S/?__~IC_O ' r ' o g] l-i nod- d °'-er-it-e. . . . . . . . . .
Om --
1
2
3
4
Fig, 1. Generalized geological map of northwest Morocco. Rectangles delineate the studied areas. 1 = folded metasedimentary rocks; 2=Precambfian and Paleozoic basement; 3 = dolerites; 4=Recent sedimentary rocks. Solid lines represent faults. Short lines marked 1 and 2 correspond to sections shown in Fig. 2.
grained gabbroic dolerite with associated pegmatitic schlieren. A schematic stratigraphic profile of the entire doleritic sequence from the High Atlas is shown in Fig. 2. The petrography of the dolerite is comparatively simple. The dominant microphenocryst phases are plagioclase (Anso_70) showing albite twinning and rare sector zoning, augite (Wo35_25En60_30Fss_40) and sometimes coexisting pigeonite (WOs_loEn7~_40Fs20_so). Olivine which occurs rarely and in subordinate amounts, is always replaced by serpentine. Ti-magnetite with exsolved ilmenite is mainly found in the differenti-
~E'=:_:7_-_~ R . d
Beds
Fig. 2. Schematic stratigraphic profile of sections ] and 2 of Fig. I showing the complete doleritic sequence in the High Atlas. White areas represent strongly altered dolerites. The numbers (e.g. l ll6, 1125) refer to the analyzed samples reported in Table I. In each outcrop, the calcareous layer overlaid by red beds are good stratigraphic markers. The relative position of each group is also shown.
ated rocks. With the exception of olivine, all these minerals are contained in mesostasis together with accessory apatite and orthopyroxene or pigeonite. The mesostasis of the most differentiated rocks also contains intergrowths of quartz and K-feldspar. Locally, the rocks were strongly affected by secondary alteration producing biotite, sericite, chlorite, serpentine and uralite which rims clinopyroxene. In some samples the amount of the secondary minerals reaches up to 7%.
3. Analytical methods From the dolerites, 80 relatively fresh samples were analyzed for major elements by plasma emis-
227 TABLE I Composition of selected dolerites from Anti-Atlas and High Atlas Anti-Atlas
1010 SiO 2 TiO 2 A1203 FezO 3 FeO MnO MgO CaO NazO K 20 P205 LOI Total [Mg]
1033
115
117
1018
1013
1012
51.06 1.06 14.84 0.46 8.87 0.19 8.17 12.25 1.69 0.49 0.16 0.26 99.50
50.97 0.97 14.40 0.85 8.33 0.15 7.80 11.62 1.91 0.33 0.14 0.25 97.69
50.06 1.07 16.82 2.18 6.68 0.15 7.42 11.48 1.78 1.09 0.13 1.06 99.92
50.47 1.20 14.75 0.49 9.24 0.17 8.22 11.66 1.89 0.65 0.14 0.36 99.24
50.33 0.99 14.18 1.45 8.28 0.17 7.45 12.08 1.97 0.25 0.14 0.72 98.01
54.01 1.23 13.86 0.74 9.26 0.24 6.34 10.30 2.41 0.74 0.19 0.35 99.68
54.71 1.39 13.05 1.36 9.88 0.20 4.56 9.33 2.75 I. 16 0.23 0.55 99.17
0.65
0.65
0.65
0.64
0.62
0.58
0.47
Sc V Cr Co Ni Cu Zn
37 274 243 46 99 182 88
36 285 201 45 92 240 124
30 231 165 42 90 93 67
38 276 226 46 91 120 82
35 279 217 45 92 201 I 11
36 301 30 46 54 162 99
33 345 14 42 28 200 114
Li Rb Sr Ba Cs
38 14 185 147 0.7
9 12 167 92 0.3
15 26 249 143 0.7
15 18 179 120 0.3
11 9 192 95 0.7
14 24 190 199 1.1
15 33 198 25 I 1.5
Zr Hf Nb Y Th U Ta
74 1.9 6.9 24 1.2 0.4 0.3
71 1.9 6.5 24 1.2
72 2.0 5.3 21 1.5
80 1.9 6.5 24 1.2
108 2.9 7.8 30 2.5
148 4.0 10.6 39 3.3
0.3
0.3
0.3
79 2.1 6.0 23 1.1 0.4 0.3
0.6
0.7
La Ce Sm Eu Tb Yb Lu
6.79 15.5 2.55 0.91 0.49 1.70 0.31
6.84 14.2 2.52 0.91 0.48 1.76 0.30
6.68 13.9 2.25 0.75 0.43 1.51 0.26
6.88 15.4 2.63 0.89 0.50 1.80 0.31
11.9 24.8 3.55 1.17 0.66 2.23 0.38
14.0 30.8 4.04 1.32 0.76 2.57 0.44
[ M g ] = M g / ( M g + F e 2+) with Fe 2+ standardized at F e 3 + / F e 2+ =0.15.
6.51 13.8 2.40 0.82 0.45 1.64 0.28
228 T A B L E I (continued)
High Atlas group 1 1137 SiO 2 TiO 2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P205 LOI Total [Mg]
group 2 1134
1125
1127
1128
1131
200
1093
49.79 1,19 12.60 4.17 6.82 0.17 10.38 9.44 1.73 0.72 0.15 0.97 98.13
52.96 1.34 13.16 2.23 7.79 0.19 7.66 9.07 2.53 1.42 0.23 0.60 99.18
53.14 1.43 14.39 5.52 4.40 0.13 6.17 9.07 2.45 1.33 0.22 0.73 99.98
50.32 1,14 14.24 3.14 6.06 0.15 8.44 11.23 1.86 0.25 0.16 1.37 98.36
50.35 1.04 14.22 3.52 6.06 0.16 8.10 11.22 2.08 0.27 0.14 1.34 98.50
50,33 1.12 13.70 3.45 6.35 0.14 8.01 10.05 1.75 0.22 0.17 2.46 97.75
51.56 1.16 13.89 3.76 6.21 0.13 7.13 10.07 2.02 0.30 0.19 1.79 98.21
50.80 1.03 14.55 3.48 6.46 0.18 9.85 10.05 2.22 0.40 0.13 1,51 100.66
51.28 1.00 14.14 2.83 6.07 0.19 7.78 11.60 1.84 0.30 0.14 1.09 98.26
0.68
0.62
0.58
0.67
0.65
0.64
0.61
0,67
0.66
Sc V Cr Co Ni Cu Zn
34 247 514 63 200 49 80
32 239
Li Rb Sr Ba Cs Zr Hf Nb Y Th U Ta La Ce Sm Eu Tb Yb Lu
1135
132 223
33 283 100 40 54 92 74
39 273 224 46 87 73 91
42 278 222 45 89 374 198
39 281 304 49 83 181 85
39 267 206 47 83 263 102
38 262 206 44 85 144 66
46.5 278 209 45 85 70 109
14 23 203 t 79 1.2
11 42 200 208 1.1
14 35 225 222 0.6
6 6 177 90 0.6
20 11 178 62 1.2
7 14 174 61 1.5
6 30 191 109 1.5
13 7 177 70 0.5
9 15 176 84 0.9
102 2.7 10.5 23 2.5 0.6 0.6
165 4.1 16.1 33 4.3 1.1 1.0
143 3.7 15.5 32 3.2 0.9 0.9
80 2.2 5.9 24 1.1 0.3 0.3
83 2.2 6.5 24 1,1 0,3 0.3
104 2.8 7.5 27 2.0 0.5 0.5
112 2.9 10.5
80 2.2 5.7 25 1.3 0.3 0.4
79 2.1 6.6 24 1.4 0.3 0.4
8.1 17.1 2.83 0.90 0.53 1.97 0.33
8.0 17.5 2.73 0.92 0.54 2.04 0.34
7.7 16.2 2.78 0.98 0.51 1.85 0.32
8.8 19.5 3.08 1.04 0.59 2.25 0.40
13.8 27.5 3.71 1.12 0.59 1.98 0.32
46
19.2 39.4 4.73 1.39 0.81 2.40 0.40
14.8 30.7 4.03 1.31 0.62 2.00 0.33
[ M g ] = M g / ( M g + F e 2+) with Fe 2+ standardized at F e J + / F e 2+ =0.15.
10.3 22.6 3.38 1.14 0.60 2.18 0.37
2.3 0.6 0.5 12.4 26.4 3.33 1.14 0.64 2.33 0.37
229.
group3 1118
1214
1095
1138
1116
1216
50.22 1.03 13.62 2.78 6.62 0.15 8.50 11.30 1.88 0.22 0.14 1.46 97.92
51.50 1.04 14.26 1.58 7.43 0.19 7.54 11.67 1.99 0.22 0.16 0.78 98.36
52.51 1.20 12.83 1.29 7.89 0.17 7.40 10.60 2.19 0.37 0.18 1.09 97.72
51.80 1.21 13.65 2.73 7.67 0.21 7.80 10.80 1.80 0.22 0.15 1.24 99.28
50.49 1.35 12.86 3.64 8.54 0.19 6.74 10.54 2.12 0.44 1.80 98.71
52.91 1.51 13.43 3.07 7.26 0.41 6.80 10.00 2.21 0.32 0.20 1.56 99.63
0.66
0.64
0.64
0.62
0.55
0.54
37
40 271 222 45 88 192 117
34 273 215 46 92 165 100
30 272 243 49 85 160 99
8 9 178 51 0.7
8 20 175 89 1.4
9 27 173 145 1.2
87 2.1 6.6 24 1.2 0.3 0.3
82 2.0 5.9 24 1.1 0.3 0.3
III 2.9 10.1 29 2.4
2.2
0.5
0.5
8.0 17.1 2.94 0.93 0.49 2.00 0.35
6.8 14.9 2.44 0.85 0.45 1.71 0.29
10.3 21.8 3.53 0.98 0.62 1.98 0.34
10.0 22.1 3.45 1.16 0.57 2.14 0.35
48
53 400 102 51 53 87 6 100
50 0.1 2.8
113 2.9 7.9 48 2.4
9.9 23.7 4.66 1.39 1.10 4.80 0.84
54 403 104 51 67 765 390 8 6 99 96 0.5 115 3.1 6.8 47 2.1 0.6 0.4 8.6 20.8 4.06 1.45 1.08 4.58 0.76
230
sion spectroscopy. Twenty-two of these samples were selected for additional trace element analyses.. REE, Sc, Ta, Th, U and Cs were determined by instrumental neutron activation. Based upon six determinations of standard rock W-l, the precision is better than 5% for most elements except Tb and Cs for which it is 10-15%. Zr, Nb and Y were determined by X-ray fluorescence while V, Cr, Co, Ni, Cu, Zn, Li, Rb, Sr and Ba were analyzed by atomic absorption. The precision and accuracy of the data obtained by X-ray fluorescence and atomic absorption were given by Bougault [14] and Dupuy et al. [15] respectively; the precision is usually better than 10%. The major and trace dement compositions of representative samples are given in Table I.
4 ¸
J
o
N3 ÷
O
N
o
Ill 1
I
I
J
I
I
50
I
I
I
S5 Si 0 2 %
Fig. 3. ( N a 2 0 + K 2 0 ) vs. SiO 2 diagramfor Moroccandolerites from Anti-Atlas(triangles) and High Arias (stars). 1, 2 and 3 refer to the fields for the tholeiite (pigeonitic) series, highalumina basalts series and alkali olivinebasalt series respectively, after Kuno[16].
4. Geochemistry 4.1. Major elements Most of the studied rocks may be Classified as high-TiO2 quartz-normative d.olerites [4]. They have variable M g / ( M g + F e 2+) atomic ratios, ([Mg] value with F e a + / F e 2+ standardized to 0.15) ranging from 0.7 to 0.47 suggesting, that many of the rocks have undergone extensive crystal fractioilation. On the ( N a 2 0 + K20 ) vs. SiO2 diagram (Fig. 3) the dolerites fall in the pigeonite field of Kuno [16]. The scattering of the data on the graph is mainly due to the variable K content. Principal component analysis [17] applied to the studied rocks (Fig. 4) shows that most of the samples fall along the F1 axis indicating a distinct fractionation trend marked by an increase of Ti, P, Si and Na towards the most differentiated rocks. In High Atlas, some rocks are enriched in MgO suggesting, in agreement with thin section observations, an accumulation of olivine (e.g. sample 1137). In addition some other samples are characterized by a high Fe content. These iron-rich samples are always located in the uppermost part of the doleritic sequence just above the calcareous layer (Fig. 2). Such rocks are not uncommon among the eastern North American dolerites [4]. Fig. 4 suggests that most dolerites from both studied areas have similar major element compositions with the exception
of K, which tends to be higher in the rocks of Anti-Atlas. The major element contents of dolerites are also comparable to those of several Mesozoic dolerites from Eastern North America [4]. 4.2. Transition elements With decreasing [Mg] values, Cr abundances rapidly decrease while Ni, Co and Sc show only a F1 (45.4%)
"
Fig. 4. Principal c o m p o n e n t analysis [17]: simultaneous projection of factor loadings and factor scores o n t o the two principal axes which represent 69% of the variance in the data set. Symbols are the same as in Fig. 3.
231
gradual depletion. In agreement with the petrographic observations, the limited variation of Ni negates extensive olivine fractionation. On the contrary, the rapid decrease of Cr is consistent with the dominant role of clinopyroxene during fractionation. Ti and V display a typical tholeiitic trend marked by the absence of Ti-magnetite fractionation and an increase of both elements toward the most differentiated rocks. The Ti/V ratio ranges between 20 and 29, values typical of ocean floor basalts [18]. Unlike other transition metals, Cu and Zn vary widely; in fact, the abundances of Cu are about twice that of typical ocean floor basalts. For a given [Mg] value, the contents of Ni, Cr, Co and Sc are roughly constant while Ti like several incompatible elements, is quite variable. All these features are typical of ocean floor basalts.
riOC
20
10-
50-
"7. O c-
o
rO r
20
10-
"'"
4.3. REE, Zr, Hf, Y, Nb and Ta These incompatible elements display a positive correlation between each other (e.g. La vs. Ta, Fig. 5) and their contents increase toward the most differentiated rocks; however, for rocks of a given [Mg] value, their abundances are highly variable. In the High Atlas, three groups of samples can be distinguished according to their REE content and patterns (Fig. 6). The first group, represented by three samples, is characterized by the highest
50-
20.
10r
La Ce
Sm Eu
i
Tb
i
Yb Lu
Fig. 6. REE abundances of Moroccan dolerites normalized to chondrites [40]. Solid lines correspond to dolerites from High Atlas while dashed lines correspond to those from Anti-Atlas. A, B and C correspond to rocks of the three different groups given in Table 1. Note that REE patterns of rocks from AntiAtlas are similar to those of group 2 from High Atlas•
I
E
g
10
20 La
ppm
Fig. 5. Variations of Ta vs. La. The symbols are the same as in Fig. 3. I = M O R B from FAMOUS area, Leg 49 (63°N, 45°N, 36°N) and Azores-Gibraltar fracture zones, H = M O R B from Legs 45 and 46 (22°N), Legs 51, 52 and 53 (25°N) and the Walvis Ridge: I and H trends after Bougault [14].
light-REE (LREE) enrichment and has a L a / Y b ratio around 7-8. The second group which also includes samples from Anti-Atlas, has a La/Yb ratio between 4 and 5 and is very similar to the high-Ti quartz-normative dolerites from Triassic dikes in the eastern United States [19,20]. The third group (samples 1116 and 1216) has relatively flat REE patterns with a L a / Y b ratio ~ 2 and a negative Eu anomaly. These rocks are strongly differentiated with [Mg] ~ 0.55. The dolerites of all three groups are slightly enriched in LREE and
232 are similar to the T-type of M O R B of Sun et al. [34]. They are also comparable in many respects to basalts from the F A M O U S area [18,21] and to those of I P O D Leg 49 [22]. Dolerites of two different groups can occur in the same section; in such cases, rocks of group 1 are always overlain by rocks of group 2. In other areas, rocks of group 2 are always found below group 3 (Fig. 2). The division into the three groups based on R E E patterns and L a / Y b ratios is corroborated by ratios involving "highly incompatible" (e.g. Nb) and "less incompatible" (e.g. Zr) trace elements. On the other hand, two trace elements with the same degree of incompatibility (e.g. Z r - H f or Y-Tb [23]) have the same ratio in the three rock-groups. Several averaged ratios for each group from High Atlas are given together with averaged ratios from Anti-Atlas in Table 2. The data indicate that the almost constant ratios have values close to that of chondrites and the ratios which decrease from group 1 to group 3 generally approach a chondritic value. Similar trends have been observed in ocean
floor basalts [22-24] and for comparison, the values of Leg 49 basalts, divided into three groups according to their L a / Y b ratios, are also given in Table 2. These ocean floor basalts show a striking similarity with Moroccan dolerites especially in their element ratios (Table 2). The very small range of T a / L a variation shown in the Moroccan rocks is also encountered in samples from Leg 49. However, the values of the T a / L a ratios in dolerites differ markedly from values obtained on Leg 49 (Fig. 5) although they are d o s e to that of M O R B at 22°N (Leg 45 and 46 [14]) and 25°N (Leg 51, 52 a n d 5 3 [14]).
4.4. Alkalies, alkali earths, U and Th The mobile elements, R b , Cs and to a smaller extent Ba, U and Th are variable and have a positive intercorrelation. The variability of the mobile elements is shown by the large standard deviation for their averaged ratios (Table 2) and this behavioUr contrasts with the regular variation
TABLE 2 Average element ratios in dolerites from Anti-Atlas and High-Atlas compared with MORB of Leg 49 and chondrites Anti-Atlas
La/Yb La/Sm Zr/Y Zr/P Nb/Zr Ta/Hf Ta/La P205/TiO2 Ta/Nb Zr/Hf Y/Tb K/Rb Rb/Cs Cs/U Rb/Sr Rb/La Ba/La
4.4 (0.7) 2.9 (0.3) 3.4 (0.3) 0.127 (0.013) 0.079 (0.008) 0.17 (0.01) 0.047 (0.003) 0.14 (0.02) 0.055 (0.006) 38 (1) 49 (2) 264 (31) 29 (17) 1.8 (0.51) 0.099 (0.039) 2.3 (0.8) 17.4 (2.9)
High Atlas group 1
group 2
group 3
7.5 (0.5) 3.8 (0.2) 4.6 (0.3) 0.157 (0.009) 0.103 (0.006) 0.24 (0.01) 0.054 (0.007) 0.15 (0.02) 0.061 (0.004) 39 (1) 44 (6) 285 (28) 38 (19) 1.2 (0.6) 0.160 (0.046) 2.1 (0.4) 13.0 (2.0)
4.4 (0.5) 3.0 (0.3) 3.5 (0.3) 0.134 (0.012) 0.079 (0.009) 0.17 (0.02) 0.045 (0.006) 0.14 (0.01) 0.054 (0.007) 38 (2) 46 (3) 200 (130) 14 (5) 2.8 (0.9) 0.087 (0.047) 1.7 (0.8) 9.0 (3.0)
2. I (0.1) 2.2 (0.1) 2.4 (0.1) 0.132 0.065 (0.007) 0.12 0.038 0.13 0.053 37 (1) 43 (I) 525 (117) 10 0.8 0.06 0.6 11
0: standard deviation; the rocks of High Atlas and Leg 49 are subdivided according to the La/Yb ratio (see text); Leg 49 ratios are from Wood et al. [22]; chondritic values reported by: A=Sun et al. [34]; B=Wood et al. [22]; C=Bouganlt [14].
233 trends of the immobile element ratios (e.g. L a / Y b , Z r / Y ) among the three groups of rocks in the High Atlas. The rocks of group 2 from the High Atlas which are similar to rocks of Anti-Atlas in their immobile element ratios, have different K / R b and C s / U ratios. Compared to ocean floor basalts [22,25,26] the studied rocks have lower K / R b , R b / C s and higher R b / S r , C s / U and R b / L a ratios. Overall they are enriched in Rb, Cs, Ba, Li and to a lesser extent in U and Th. In this respect, the dolerites of Morocco are very similar to continental tholeiites [27,28].
5. D i s c u s s i o n
The large variations of the mobile elements and their corresponding ratios (e.g. K / R b ) cannot be explained by a simple process of mineral fractionation since their partition coefficients for comm o n minerals of upper mantle and doleritic rocks are very small and similar to each other [26]. The
distribution of these mobile elements could, however, be affected by secondary process particularly alteration and contamination. These two processes will be considered in turn. 5.1. A Iteration In a profile such as shown in Fig. 2, the ratios K / R b , and R b / C s tend to decrease with the increase of water content. In this respect, samples with LOI > 1 have an average R b / C s ratio < 20 and K / R b ratios < 2 1 3 while dolerites with L O I < 1 have R b / C s ratios between 38 and 48 and K / R b ratios > 200. In addition, a rough positive relationship appears between K and K / R b in the analyzed samples. A similar relationship encountered in Puerto Rico trench basalts has been attributed to alteration [29]. The effect of alterative is also indicated by the lack of interelement correlations when a mobile element is plotted against rather immobile elements such as La, Zr, T a (e.g. Fig. 7).
Mid-Atlantic Ridge, Leg 49
Chondrites
group I
group 2
group 3
7.7 (1.7) 4.5 (0.6) 4.9 (0.9) O.109 (0.02) 0.28 (0.01) 0.68 (0.06) 0.! 12 (0.062) 0.17 (0.01) 0.057 (0.004) 41 (0.5) 44 (3) 543 (36) 34 (13) 0.66 (0.25) 0.032 (0.01I) 0.42 (0.07) 9.9 (2.3)
2.7 (0.2) 2.5 (0.1) 2.6 (0.1) 0.092 (0.02) 0.20 (0.02) 0.40 (0.02) 0.100 (0.001) 0.15 (0.04) 0.058 (0.002) 37 (1) 43 (6) 395 (28) 72 (40) 0.39 (0.26) 0.038 (0.008) 0.78 (0.21) 9.2 (0.8)
0.9 1.2 1.9 0.088 0.1 l 0.21 0.096 0.13 0.050 37 45 592 0.001 0.29 6.4
A
B
(0.2)
1.5
(0. I)
1.6
(0.3) (0.01)
2.5 0.12 0.07
(0.03) (1) (4) (1)
(0.8)
0.063 O.11 O.051 343 29 0.9 0.032 1.1 11.1
C
1.6
1.6 1.7 2.4
0.062 0.12 0.067
0.10 0.24 0.097
0.039
0.058 40 46
234
Rb ppm
and when several trace elements are plotted normalized to chondrites, this sample shows a distinct similarity with ocean floor basalts (Fig. 8A). This suggests that the contents of alkalies and alkali earths of sample 1137 is not affected by contamination. A similar conclusion is indicated by a comparison (Fig. 8B) of sample 1216 and P-type of ocean floor tholeiites of Sun et al. [34]. Furthermore, the variation a n d / o r constancy of ratios involving the most stable dement (Table2) are directly proportional to the degree of incompatibility of the corresponding elements in major mantle phases. This suggests that the distribution of thesedements in the dolerites are mainly due to solid-liquid equilibria and that both contamination and alteration had a negligeable effect. Therefore, most of the following discussion will deal with elements such as REE, Zr, Nb, Ta, Y and transition metals which are less sensitive to secondary alteration. Two aspects of the petrogenesis of the Moroc-
-I(-
40
30
zx
.1¢.
20
10
20
30
La ppm
1~
(A)
Fig. 7. Variations of Rb vs. La. Symbols are the same as in Fig. 3. I=MORB from Legs 45 and 46 (22°N), and Legs 51 and 52 (25°N), H=MORB from Leg 49 and Azores-Gibraltar fracture zones; I and H trends after Bougault [14]. r~
5.2. Contamination
Crustal contamination of tholeiitic magma emplaced on the continent has frequently been invoked [27,30] and documented by isotopic data [31]. Furthermore, recent studies [32,33] suggest that this process has a greater effect on the mobile elements. In our case it is not clear whether the large spread of the K / R b , R b / C s and some other ratios is due to contamination or to a secondary alteration process. Among samples with water content > 1%, there is a slight increase of R b / L a and T h / L a ratios in some of the most differentiated rocks; this may favor contamination. But overall it may be suggested that this process is local and most samples do not seem to be significantly affected. For example, among the freshest rocks, sample 1137 has the highest concentration of K 2°
2c
iCs
i Rb
i Ba
t TI1
A U
t
i
Nb
Ta
i K
i La
i Ce
i P
, Sm
, Zr
i Eo
i Ti
i Tb
i Yb
i =0
soI
(B)/2ffl
20
I
I
i
i
I
i
i
1
i
i
i
Rb
BI
K
La
Ce
P
$m
Zr
Eu
Tb
Yb
Fig. 8. Abundances of several minor and trace elements arranged according to their degree of incompatibility--after Sun et al. [34] and normalized to chondrites [34]. A. Sample 1137 and Walvis Ridge basalt of Bougault [14]. B. Sample 1216 and P-type MORB of Sun et al. [34].
235
can dolerites will be discussed in turn. The first aspect refers to the relationships among the rocks of single group. The second one deals with the relationship among the different rock groups and the origin of their parental magmas. ....... 5. 3. Relationship among the rocks of a single group Despite the small range of SiO2 variation, several petrographic and geochemical features suggest that these rooks underwent low-pressure fractional crystallization. These features include distinct linear correlations such as the positive covariance of the incompatible elements with increasing TiO 2 and decreasing [Mg] values, and the negative correlation of the compatible elements (e.g. Ni, Cr, Co) with incompatible elements (e.g. La, Zr). Since the samples from Anti-Atlas were collected from a rather limited area, they were used to evaluate fractional crystallization. From the major element data, the process was tested by leastsquares calculations [35] using the whole-rock analysis listed in Table 1 and the phenocryst compositions reported by Bertrand and Prioton [7]. The calculations were done for various combinations of whole rocks and the results indicate that the major element compositions of the sequence of samples from Anti-Atlas (Table l) can be reproduced by the separation of plagioolase and clinopyroxene. For the sake of simplicity, only the weight fractions of the variables and squared residuals (~R 2) are given for two combinations: rook 1010 --- 0.68 rock 1013 + 0.23 cpx + 0.09 plg/YR 2 = 0.07 rock 1013 ----0.85 rock 1012 + 0.12 cpx +0.03 plg/Y~R2 -- 0.11 The calculated proportions of phases are closely comparable to modal compositions. The model was further tested for trace elements using the inverse method of the fractional crystallization process described in detail by Minster et al. [36]. The procedure for the inversion method consists of two stages. The first step involves an estimation of the initial composition of the parental liquid (Co), bulk partition coefficients (D) and degree of solidification (F). It was assumed that
the parental magma contained 200 ~ 20 ppm Ni (cf. [14]). Then the initial concentrations of several trace elements in such a melt (Co) were estimated by graphic extrapolation [37] from covariation ~rends for an Ni content of 200 ppm. The bulk partition coefficients (D) were calculated from the partition coefficients given in Table 3 and weight fractions of phases obtained from the least-squares calculations for major elements. The degrees of fractional crystallization were calculated by the Rayleigh law equation taking into consideration the variations of D and C0. The parameters are given in Table 3 ("starting model"). The second stage of the procedure involves the calculation of the actual inversion model using approximate parameters obtained from "the starting model". The results are given in Table 3. The close similarities between C~ac. and CoJbs. and between the approximate parameters of the starting model and those recalculated by the inversion method further support fractional crystallization as the process relating the different samples of this sequence. 5. 4. Relationship among the different groups of rocks The large variations of the La/Yb and La/Sm ratios and the very similar contents of some transition metals (Ni, Cr, Co, Sc, V) and major elements among the groups rules out the possibility of all the lava groups being related to a single parental liquid by either low- or high-pressure crystal fractionation. Variable degrees of batch partial melting of a spinel lherzolite source cannot account for the large L a / Y b variation observed in the dolerites. These differences would imply garnet-bearing residue after melting. However, assuming that the source has a chondritic composition, the variations of the La/Yb ratios require melting in a range from 5 to 30%. Such a large span of melting is not consistent with the rather constant abundances of major and some transition elements. Furthermore, any sequence produced by batch partial melting should show a correlation between light and heavy REE [18]. such a correlation is not present among the Moroccan samples. Therefore, the distribution of trace elements requires a more complex process
236 TABLE 3 Calculations of inversion method for dolerites of Anti-Atlas Starting model
Nb Ta La Ce Sm Eu Zr Th Yb Sr V CO S¢
Inversion model
D
Co
D
co
c2,,k~.
(1)
(2)
(3)
(4)
1010
1013
1012
0.0670.02 0.0570.01 0.1070.01 0.1270.02 0.4270.07 0.53 7 0 . 0 6 0.08~-0.03 0.4570.05 0.4570.05 0.7 ~0.1 0.6 ~-0.1 1.3 70.1 1.4 7 1
5.0 7 0.1 0.2 7 0.1 5.0 7 0.5 12 7 I 2.0 7 0.2 0.75 7 0 . 0 5 55 7 "5 0 . 3 5 7 0.05 1.5 7 0.2 170 7 1 0 230 ~20 50 "~ 5 40 ~ 3
0.1070.02 0.0570.01 0.1070.01 0.1170.02 0.3770.01 0.50~0.05 0.0870.03 0.4270.05 0.4570.06 0.8270.04 0.67~0.06 1.1770.09 1.1670.05
4.8 7 0.28 7 5.8 7 12.8 7 2.2 7 0.78 7 59 7 0.427 1.5 7 166 7 234 ~ 51 7 40 7
6.0 0.3 7.2 16.0 2.5 0.91 73 0.49 1.71 177 264 48 38
9.0 0.5 11.2 24.0 3.4 1.15 113 0.64 2.24 194 312 44 35
11.5 0.7 14.3 30.4 4.0 1.3 I 145 0.74 2.60 204 341 42 33
F 0 . 7 0 7 0.15 0 . 4 5 7 0.10 0 . 3 5 7 0.08
F 0.7870.04 0.4970.02 0.39~-0.01
Reference samples 1010 1013 1012
0.1 0.02 0.2 0.5 0.1 0.04 3 0.03 0.1 9 12 3 3
For the calculations, three representative samples which show large range of [Mg] values and of trace element abundances were considered. The uncertainties of data correspond to 26 from the duplicate analyses of samples 1010. Partition coefficients used to calculate D were compiled from Lopez-Escobar et al. [43], Shih [44], Schnetzler and Philpotts [45] for REE and Sr and from Pearce and Norry [46] for Zr, N b and Y. The values for Co, V, Sc, and Ta are from our unpublished data. The uncertainties of D for the "starting model" reflect only those of mineral weight fractions, while those of Co represent the range of Ni variations in the initial liquid ( 2 0 0 ~ 2 0 ppm). Results on the inversion model were obtained after 4 trials.
than simple partial melting or fractional crystallization. Among the different mechanisms tested [38] the dynamic partial melting model [18] seems to best explain the geochemical characteristics of the various groups. In order to minimize the effect of low-pressure fractional crystallization on the abundances of trace elements, the calculations of dynamic melting were initially done for element ratios. For these calculations, the degree of partial melting (7%) and proportions of residual phases (ol = 54, opx = 31, cpx = 11 and sp = 4) were obtained from the least-squares calculations for major elements using the compositions of pyrolite (parental source) and residual minerals given by Sun et al. [34]. The composition of the first melt (C l) was taken as that of samole 1137 which is among the oldest doleritic units in the High Atlas. This composition
(Mg/(Mg -~ Fe 2+) = 0.68; Ni = 200 ppm) may roughly approximate the primary upper mantle liquid [14]. The trace dement composition of the upper mantle source prior to melting was then calculated using the batch melting equation and the composition of sample 1137. The composition of liquids produced by dynamic melting of such a source are shown in Fig. 9 for two pairs of trace element ratios (La/Yb vs Z r / Y and L a / Y b vs. Ta/La). The agreement between observed and calculated data suggests that the individual groups of rocks could be derived from an upper mantle source by dynamic melting. Likewise, equivalent calculations for other ratios involving stable trace elements yield results closely comparable to those observed. The calculations also show that group 2 may be produced by 9-13% of melting while group 3 would require 15-17% melting.
237
'%b
La/yb
A
10
:
lo
B 7%
7%
17% 100 x T a / L a I
I
I
I
i
I
1
3
5
2
4
6
Fig. 9. Variations of La/Yb vs. Zr/Y and La/Yb vs. Ta/La ratios in dolerites from Morocco compared to those of calculated liquid compositions produced by dynamic melting. Mineral composition of residue: ol = 54, opx = 31, cpx-- 11, sp = 4; mineral proportion of melt: ol = 20, opx = 20, cpx = 55, sp = 5. REE partition coefficients of Schilling et al. [41] were used. Dzr and Dy were assumed to be equal to Dsm and D-n, respectively while DTa was taken as 1/2 DLa [22]. Melting was calculated in the range of 7-17% in 2% increments with 2% melt always left in the residue (solid circles); compositions of chondrites (solid squares) and of source material (triangle: La/Yb=3.34, Zr/Y=3.54, Ta/La=0.043); solid lines delineate the fields of dolerites. Note that the trends of liquids produced by melting and of dolerites for the La/Yb vs. Zr/Y ratios lead toward chondrites. 4
T h e c o m p o s i t i o n of the o r i g i n a l s o u r c e m a t e r i a l for several e l e m e n t s is r e p o r t e d i n Fig. 10. T h e c h o n d r i t e - n o r m a l i z e d a b u n d a n c e s of several trace and minor elements show progressive enrichment f r o m V w i t h 1.3 t i m e s c h o n d r i t i c c o n c e n t r a t i o n to the most incompatible elements with abundances m o r e t h a n 3 times t h a t of c h o n d r i t e s . T w o except i o n s to this t r e n d are T a a n d N b w h i c h s h o w a r e l a t i v e d e p l e t i o n . C o m p a r e d to the p r i m o r d i a l m a n t l e c o m p o s i t i o n suggested b y W o o d et al. [22], t h e o r i g i n a l s o u r c e of M o r o c c a n dolerites (Fig. 10) w a s d e p l e t e d i n "less" i n c o m p a t i b l e e l e m e n t s (Ba, T h , U , Ta, N b , La, Ce). C o m p a r i s o n w i t h the l i m i t e d a m o u n t of d a t a o n the A r c h a e a n u p p e r m a n t l e [39] suggests s i m i l a r c o n t e n t s o f P, Ti, Y, Y b a n d V. T h e f r a c t i o n a t i o n of the c h o n d r i t e -
~3
2-
o
I
I
Ba Th
I
U
I
I
I
Ta Nb l a
I
I
Ce Hf
I
Zr
I
I
Sm Eu
I
P
I
Ti
I
I
I
I
Tb
Y
Yb
V
Fig. 10. The abundances of several elements in the calculated mantle source dolerite 1137 (solid dots). In the model calculation, equation 15 of Shaw [42] with 7% melting was used together with the partition coefficients of Schilling et al. [41] for REE and V. D of Ba, Th, U and Ta were assumed to be 1/2 DLa and DNt, to be 3/4 DLa [22]. D of P and Ti were taken to be intermediate between DEu and D-n,. Dzr and DHf were assumed to correspond to Dsm [22]. The other parameters for the calculations are the same as in Fig. 7. Stars: primordial upper mantle reported by Wood et al. [22]; circles: the Archaean mantle of Sun and Nesbitt [39].
238 n o r m a l i z e d p a t t e r n for i n c o m p a t i b l e elements (Fig. 10) m a y i n d i c a t e a c o m p l e x h i s t o r y of the u p p e r m a n t l e source of the studied dolerites.
cal d e t e r m i n a t i o n s . This s t u d y was s u p p o r t e d b y C e n t r e G 6 o l o g i q u e et G 6 o p h y s i q u e of M o n t p e l l i e r , the N a t u r a l Sciences a n d Engineering R e s e a r c h C o u n c i l of C a n a d a ( o p e r a t i n g g r a n t A 3782) a n d the C.N.E.X.O. of F r a n c e (grant 78-5704).
6. Conclusion T h e a b u n d a n c e s of R E E , Zr, Hf, Y, N b , T a a n d c o r r e s p o n d i n g ratios in M o r o c c a n dolerites lie within the range of oceanic tholeiites. However, the c o n t e n t s of the m o s t i n c o m p a t i b l e elements (alkalies, Ba, U a n d Th) in the least altered samples are higher t h a n those o f oceanic basalts, but, a l o n g with a relatively low K / R b ratio, are typical o f c o n t i n e n t a l tholeiites. A s it seems that crustal c o n t a m i n a t i o n d i d n o t affect m o s t of o u r a n a l y z e d samples, the relatively high c o n t e n t s of some inc o m p a t i b l e elements suggest that the c o n t i n e n t a l u p p e r m a n t l e source was enriched in these elem e n t s in c o m p a r i s o n with the u p p e r m a n t l e source for oceanic tholeiites. T h e g e o c h e m i c a l d a t a i n d i c a t e that dolerites of the H i g h A t l a s show vertical s t r a t i g r a p h i c z o n a t i o n which is c h a r a c t e r i z e d b y a progressive lithophile e l e m e n t d e p l e t i o n t o w a r d the t o p of the doleritic sequence. Such a n e v o l u t i o n a r y t r e n d is e n c o u n t e r e d in recent oceanic b a s a l t s [22] a n d even in o p h i o l i t i c c o m p l e x e s [47]. This t r e n d together with the regularities of the trace e l e m e n t r a t i o varia t i o n s can b e p r o d u c e d b y a d y n a m i c m e l t i n g of an initially h o m o g e n e o u s source, a s s u m i n g a p r o gressively y o u n g e r time relationship t o w a r d the t o p of the doleritic sequence. Such a time sequence has yet to b e e s t a b l i s h e d b u t leaves o p e n the feasibility of a d y n a m i c melting model. A n o t h e r process, which is consistent with the available d a t a involves the m i x i n g of m a g m a s p r o d u c e d b y different degrees of p a r t i a l melting in different p a r t s o f an u p p e r m a n t l e with vertical heterogeneities [48].
Acknowledgements M a n y t h a n k s are due to Drs. G. Vasseur a n d J. Verni~res for their help with the calculations, to Prof. M. G i r o d for initiating the p r o j e c t a n d to Mrs. L. S a v o y a n t a n d S. Pourtales for their chemi-
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225
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[2]
Geochemistry of Early Mesozoic tholeiites from Morocco H. Bertrand
l,
j. Dostal 2 and C. Dupuy 3
I Dbpartement des Sciences de la J'erre, Universitb de Lyon 1, 69622 Villeurbanne Cedex (France) e Department of Geology, Saint Mary's University, Halifax, N.S. (Canada) k~ Centre G~ologique et Gbophysique, U.S.T.L., 34060 Montpellier Cedex (France)
Received June 15, 1981 Revised version received December 14, 1981
Mesozoic dolerites from two areas of Morocco, the High Atlas fold belt between Marrakech and Demnat and the Anti-Atlas belt in the area of Foum Zquid, are mostly high-Ti quartz-normative tholeiites which in many respects resemble Mesozoic dolerite dikes from eastern North America. The dolerites display a wide range of major and trace element compositions, some of which are due to fractional crystallization. The doleritic sequences from High Atlas also show vertical stratigraphic zonation which is chai'acterized by a progressive depletion of lithophile elements toward the top. This trend together with regularities of trace element ratio variations are indicative of a dynamic melting of an initially homogeneous source. It is suggested that the continental upper mantle source for dolerites of Morocco was enriched in several incompatible elements in comparison with the upper mantle source for ocean floor tholeiites.
1. Introduction O n both sides of the Atlantic Ocean, the continental margins contain tholeiitic rocks in the f o r m of n u m e r o u s sills, dikes and lava flows. These tholeiites are associated with elongated basins filled with Triassic continental sediments and might be related to the tectonic event which resulted in the opening of the Atlantic Ocean in the Mesozoic. A l t h o u g h on the N o r t h A m e r i c a n side these rocks have been extensively studied [1-6] very little is k n o w n about dolerites from Europe and Africa. The purpose of this paper is to present geochemical data on the Mesozoic dolerites from M o r o c c o and to discuss the processes involved in the genesis of these rocks.
2. Geological and petrographic notes The geology and p e t r o g r a p h y of the studied dolerites have been described by Bertrand and Prioton [7]. The rocks occur in two distinct areas: the High Atlas fold belt between Marrakech and
D e m n a t , and the Anti-Atlas belt in the area of F o u m Zguid (Fig. 1). In the former area, the dolerites ( 5 0 - 1 5 0 m thick through the total sequence) outcrop as sills and lava flows in late Triassic to early Liassic basins and are interbedded with red beds and evaporites. Those of the latter area appear as sills and dikes intruding into the Paleozoic sedimentary sequence. Their K - A r ages, ranging between 180 and 200 m.y. [7-9], are consistent with the m e a n age of 186 m.y. proposed by Manspeizer et al. [10] for the western High Atlas, and with the ages obtained on the A m e r i c a n margin by Erickson and Kulp [11], A r m s t r o n g and Besancon [12] and G o h n et al. [13]. Most c o m m o n are fine-grained dolerites with intergranular microstructure which pass progressively to a coarser-grained dolerite at the core of the bodies. This latter consists of a granular framework of pyroxene and plagioclase which occur together with large patches of groundmass ranging f r o m a spherufitic fibrous material to subophitic intergrowths of plagioclase and pyroxene. Some bodies contain an olivine-bearing layer (up to 15 m thick) in the lower part, overlain by a coarse-
0012-82 IX/82/0000-0000/$02.75 © 1982 Elsevier Scientific Publishing Company
226
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id- ~ ;d-s-.-: . . . . . .
.............. 1118
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Group 1
1~3,51Z~!1 x ........ 1137 [:'!S/?__~IC_O ' r ' o g] l-i nod- d °'-er-it-e. . . . . . . . . .
Om --
1
2
3
4
Fig, 1. Generalized geological map of northwest Morocco. Rectangles delineate the studied areas. 1 = folded metasedimentary rocks; 2=Precambfian and Paleozoic basement; 3 = dolerites; 4=Recent sedimentary rocks. Solid lines represent faults. Short lines marked 1 and 2 correspond to sections shown in Fig. 2.
grained gabbroic dolerite with associated pegmatitic schlieren. A schematic stratigraphic profile of the entire doleritic sequence from the High Atlas is shown in Fig. 2. The petrography of the dolerite is comparatively simple. The dominant microphenocryst phases are plagioclase (Anso_70) showing albite twinning and rare sector zoning, augite (Wo35_25En60_30Fss_40) and sometimes coexisting pigeonite (WOs_loEn7~_40Fs20_so). Olivine which occurs rarely and in subordinate amounts, is always replaced by serpentine. Ti-magnetite with exsolved ilmenite is mainly found in the differenti-
~E'=:_:7_-_~ R . d
Beds
Fig. 2. Schematic stratigraphic profile of sections ] and 2 of Fig. I showing the complete doleritic sequence in the High Atlas. White areas represent strongly altered dolerites. The numbers (e.g. l ll6, 1125) refer to the analyzed samples reported in Table I. In each outcrop, the calcareous layer overlaid by red beds are good stratigraphic markers. The relative position of each group is also shown.
ated rocks. With the exception of olivine, all these minerals are contained in mesostasis together with accessory apatite and orthopyroxene or pigeonite. The mesostasis of the most differentiated rocks also contains intergrowths of quartz and K-feldspar. Locally, the rocks were strongly affected by secondary alteration producing biotite, sericite, chlorite, serpentine and uralite which rims clinopyroxene. In some samples the amount of the secondary minerals reaches up to 7%.
3. Analytical methods From the dolerites, 80 relatively fresh samples were analyzed for major elements by plasma emis-
227 TABLE I Composition of selected dolerites from Anti-Atlas and High Atlas Anti-Atlas
1010 SiO 2 TiO 2 A1203 FezO 3 FeO MnO MgO CaO NazO K 20 P205 LOI Total [Mg]
1033
115
117
1018
1013
1012
51.06 1.06 14.84 0.46 8.87 0.19 8.17 12.25 1.69 0.49 0.16 0.26 99.50
50.97 0.97 14.40 0.85 8.33 0.15 7.80 11.62 1.91 0.33 0.14 0.25 97.69
50.06 1.07 16.82 2.18 6.68 0.15 7.42 11.48 1.78 1.09 0.13 1.06 99.92
50.47 1.20 14.75 0.49 9.24 0.17 8.22 11.66 1.89 0.65 0.14 0.36 99.24
50.33 0.99 14.18 1.45 8.28 0.17 7.45 12.08 1.97 0.25 0.14 0.72 98.01
54.01 1.23 13.86 0.74 9.26 0.24 6.34 10.30 2.41 0.74 0.19 0.35 99.68
54.71 1.39 13.05 1.36 9.88 0.20 4.56 9.33 2.75 I. 16 0.23 0.55 99.17
0.65
0.65
0.65
0.64
0.62
0.58
0.47
Sc V Cr Co Ni Cu Zn
37 274 243 46 99 182 88
36 285 201 45 92 240 124
30 231 165 42 90 93 67
38 276 226 46 91 120 82
35 279 217 45 92 201 I 11
36 301 30 46 54 162 99
33 345 14 42 28 200 114
Li Rb Sr Ba Cs
38 14 185 147 0.7
9 12 167 92 0.3
15 26 249 143 0.7
15 18 179 120 0.3
11 9 192 95 0.7
14 24 190 199 1.1
15 33 198 25 I 1.5
Zr Hf Nb Y Th U Ta
74 1.9 6.9 24 1.2 0.4 0.3
71 1.9 6.5 24 1.2
72 2.0 5.3 21 1.5
80 1.9 6.5 24 1.2
108 2.9 7.8 30 2.5
148 4.0 10.6 39 3.3
0.3
0.3
0.3
79 2.1 6.0 23 1.1 0.4 0.3
0.6
0.7
La Ce Sm Eu Tb Yb Lu
6.79 15.5 2.55 0.91 0.49 1.70 0.31
6.84 14.2 2.52 0.91 0.48 1.76 0.30
6.68 13.9 2.25 0.75 0.43 1.51 0.26
6.88 15.4 2.63 0.89 0.50 1.80 0.31
11.9 24.8 3.55 1.17 0.66 2.23 0.38
14.0 30.8 4.04 1.32 0.76 2.57 0.44
[ M g ] = M g / ( M g + F e 2+) with Fe 2+ standardized at F e 3 + / F e 2+ =0.15.
6.51 13.8 2.40 0.82 0.45 1.64 0.28
228 T A B L E I (continued)
High Atlas group 1 1137 SiO 2 TiO 2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P205 LOI Total [Mg]
group 2 1134
1125
1127
1128
1131
200
1093
49.79 1,19 12.60 4.17 6.82 0.17 10.38 9.44 1.73 0.72 0.15 0.97 98.13
52.96 1.34 13.16 2.23 7.79 0.19 7.66 9.07 2.53 1.42 0.23 0.60 99.18
53.14 1.43 14.39 5.52 4.40 0.13 6.17 9.07 2.45 1.33 0.22 0.73 99.98
50.32 1,14 14.24 3.14 6.06 0.15 8.44 11.23 1.86 0.25 0.16 1.37 98.36
50.35 1.04 14.22 3.52 6.06 0.16 8.10 11.22 2.08 0.27 0.14 1.34 98.50
50,33 1.12 13.70 3.45 6.35 0.14 8.01 10.05 1.75 0.22 0.17 2.46 97.75
51.56 1.16 13.89 3.76 6.21 0.13 7.13 10.07 2.02 0.30 0.19 1.79 98.21
50.80 1.03 14.55 3.48 6.46 0.18 9.85 10.05 2.22 0.40 0.13 1,51 100.66
51.28 1.00 14.14 2.83 6.07 0.19 7.78 11.60 1.84 0.30 0.14 1.09 98.26
0.68
0.62
0.58
0.67
0.65
0.64
0.61
0,67
0.66
Sc V Cr Co Ni Cu Zn
34 247 514 63 200 49 80
32 239
Li Rb Sr Ba Cs Zr Hf Nb Y Th U Ta La Ce Sm Eu Tb Yb Lu
1135
132 223
33 283 100 40 54 92 74
39 273 224 46 87 73 91
42 278 222 45 89 374 198
39 281 304 49 83 181 85
39 267 206 47 83 263 102
38 262 206 44 85 144 66
46.5 278 209 45 85 70 109
14 23 203 t 79 1.2
11 42 200 208 1.1
14 35 225 222 0.6
6 6 177 90 0.6
20 11 178 62 1.2
7 14 174 61 1.5
6 30 191 109 1.5
13 7 177 70 0.5
9 15 176 84 0.9
102 2.7 10.5 23 2.5 0.6 0.6
165 4.1 16.1 33 4.3 1.1 1.0
143 3.7 15.5 32 3.2 0.9 0.9
80 2.2 5.9 24 1.1 0.3 0.3
83 2.2 6.5 24 1,1 0,3 0.3
104 2.8 7.5 27 2.0 0.5 0.5
112 2.9 10.5
80 2.2 5.7 25 1.3 0.3 0.4
79 2.1 6.6 24 1.4 0.3 0.4
8.1 17.1 2.83 0.90 0.53 1.97 0.33
8.0 17.5 2.73 0.92 0.54 2.04 0.34
7.7 16.2 2.78 0.98 0.51 1.85 0.32
8.8 19.5 3.08 1.04 0.59 2.25 0.40
13.8 27.5 3.71 1.12 0.59 1.98 0.32
46
19.2 39.4 4.73 1.39 0.81 2.40 0.40
14.8 30.7 4.03 1.31 0.62 2.00 0.33
[ M g ] = M g / ( M g + F e 2+) with Fe 2+ standardized at F e J + / F e 2+ =0.15.
10.3 22.6 3.38 1.14 0.60 2.18 0.37
2.3 0.6 0.5 12.4 26.4 3.33 1.14 0.64 2.33 0.37
229.
group3 1118
1214
1095
1138
1116
1216
50.22 1.03 13.62 2.78 6.62 0.15 8.50 11.30 1.88 0.22 0.14 1.46 97.92
51.50 1.04 14.26 1.58 7.43 0.19 7.54 11.67 1.99 0.22 0.16 0.78 98.36
52.51 1.20 12.83 1.29 7.89 0.17 7.40 10.60 2.19 0.37 0.18 1.09 97.72
51.80 1.21 13.65 2.73 7.67 0.21 7.80 10.80 1.80 0.22 0.15 1.24 99.28
50.49 1.35 12.86 3.64 8.54 0.19 6.74 10.54 2.12 0.44 1.80 98.71
52.91 1.51 13.43 3.07 7.26 0.41 6.80 10.00 2.21 0.32 0.20 1.56 99.63
0.66
0.64
0.64
0.62
0.55
0.54
37
40 271 222 45 88 192 117
34 273 215 46 92 165 100
30 272 243 49 85 160 99
8 9 178 51 0.7
8 20 175 89 1.4
9 27 173 145 1.2
87 2.1 6.6 24 1.2 0.3 0.3
82 2.0 5.9 24 1.1 0.3 0.3
III 2.9 10.1 29 2.4
2.2
0.5
0.5
8.0 17.1 2.94 0.93 0.49 2.00 0.35
6.8 14.9 2.44 0.85 0.45 1.71 0.29
10.3 21.8 3.53 0.98 0.62 1.98 0.34
10.0 22.1 3.45 1.16 0.57 2.14 0.35
48
53 400 102 51 53 87 6 100
50 0.1 2.8
113 2.9 7.9 48 2.4
9.9 23.7 4.66 1.39 1.10 4.80 0.84
54 403 104 51 67 765 390 8 6 99 96 0.5 115 3.1 6.8 47 2.1 0.6 0.4 8.6 20.8 4.06 1.45 1.08 4.58 0.76
230
sion spectroscopy. Twenty-two of these samples were selected for additional trace element analyses.. REE, Sc, Ta, Th, U and Cs were determined by instrumental neutron activation. Based upon six determinations of standard rock W-l, the precision is better than 5% for most elements except Tb and Cs for which it is 10-15%. Zr, Nb and Y were determined by X-ray fluorescence while V, Cr, Co, Ni, Cu, Zn, Li, Rb, Sr and Ba were analyzed by atomic absorption. The precision and accuracy of the data obtained by X-ray fluorescence and atomic absorption were given by Bougault [14] and Dupuy et al. [15] respectively; the precision is usually better than 10%. The major and trace dement compositions of representative samples are given in Table I.
4 ¸
J
o
N3 ÷
O
N
o
Ill 1
I
I
J
I
I
50
I
I
I
S5 Si 0 2 %
Fig. 3. ( N a 2 0 + K 2 0 ) vs. SiO 2 diagramfor Moroccandolerites from Anti-Atlas(triangles) and High Arias (stars). 1, 2 and 3 refer to the fields for the tholeiite (pigeonitic) series, highalumina basalts series and alkali olivinebasalt series respectively, after Kuno[16].
4. Geochemistry 4.1. Major elements Most of the studied rocks may be Classified as high-TiO2 quartz-normative d.olerites [4]. They have variable M g / ( M g + F e 2+) atomic ratios, ([Mg] value with F e a + / F e 2+ standardized to 0.15) ranging from 0.7 to 0.47 suggesting, that many of the rocks have undergone extensive crystal fractioilation. On the ( N a 2 0 + K20 ) vs. SiO2 diagram (Fig. 3) the dolerites fall in the pigeonite field of Kuno [16]. The scattering of the data on the graph is mainly due to the variable K content. Principal component analysis [17] applied to the studied rocks (Fig. 4) shows that most of the samples fall along the F1 axis indicating a distinct fractionation trend marked by an increase of Ti, P, Si and Na towards the most differentiated rocks. In High Atlas, some rocks are enriched in MgO suggesting, in agreement with thin section observations, an accumulation of olivine (e.g. sample 1137). In addition some other samples are characterized by a high Fe content. These iron-rich samples are always located in the uppermost part of the doleritic sequence just above the calcareous layer (Fig. 2). Such rocks are not uncommon among the eastern North American dolerites [4]. Fig. 4 suggests that most dolerites from both studied areas have similar major element compositions with the exception
of K, which tends to be higher in the rocks of Anti-Atlas. The major element contents of dolerites are also comparable to those of several Mesozoic dolerites from Eastern North America [4]. 4.2. Transition elements With decreasing [Mg] values, Cr abundances rapidly decrease while Ni, Co and Sc show only a F1 (45.4%)
"
Fig. 4. Principal c o m p o n e n t analysis [17]: simultaneous projection of factor loadings and factor scores o n t o the two principal axes which represent 69% of the variance in the data set. Symbols are the same as in Fig. 3.
231
gradual depletion. In agreement with the petrographic observations, the limited variation of Ni negates extensive olivine fractionation. On the contrary, the rapid decrease of Cr is consistent with the dominant role of clinopyroxene during fractionation. Ti and V display a typical tholeiitic trend marked by the absence of Ti-magnetite fractionation and an increase of both elements toward the most differentiated rocks. The Ti/V ratio ranges between 20 and 29, values typical of ocean floor basalts [18]. Unlike other transition metals, Cu and Zn vary widely; in fact, the abundances of Cu are about twice that of typical ocean floor basalts. For a given [Mg] value, the contents of Ni, Cr, Co and Sc are roughly constant while Ti like several incompatible elements, is quite variable. All these features are typical of ocean floor basalts.
riOC
20
10-
50-
"7. O c-
o
rO r
20
10-
"'"
4.3. REE, Zr, Hf, Y, Nb and Ta These incompatible elements display a positive correlation between each other (e.g. La vs. Ta, Fig. 5) and their contents increase toward the most differentiated rocks; however, for rocks of a given [Mg] value, their abundances are highly variable. In the High Atlas, three groups of samples can be distinguished according to their REE content and patterns (Fig. 6). The first group, represented by three samples, is characterized by the highest
50-
20.
10r
La Ce
Sm Eu
i
Tb
i
Yb Lu
Fig. 6. REE abundances of Moroccan dolerites normalized to chondrites [40]. Solid lines correspond to dolerites from High Atlas while dashed lines correspond to those from Anti-Atlas. A, B and C correspond to rocks of the three different groups given in Table 1. Note that REE patterns of rocks from AntiAtlas are similar to those of group 2 from High Atlas•
I
E
g
10
20 La
ppm
Fig. 5. Variations of Ta vs. La. The symbols are the same as in Fig. 3. I = M O R B from FAMOUS area, Leg 49 (63°N, 45°N, 36°N) and Azores-Gibraltar fracture zones, H = M O R B from Legs 45 and 46 (22°N), Legs 51, 52 and 53 (25°N) and the Walvis Ridge: I and H trends after Bougault [14].
light-REE (LREE) enrichment and has a L a / Y b ratio around 7-8. The second group which also includes samples from Anti-Atlas, has a La/Yb ratio between 4 and 5 and is very similar to the high-Ti quartz-normative dolerites from Triassic dikes in the eastern United States [19,20]. The third group (samples 1116 and 1216) has relatively flat REE patterns with a L a / Y b ratio ~ 2 and a negative Eu anomaly. These rocks are strongly differentiated with [Mg] ~ 0.55. The dolerites of all three groups are slightly enriched in LREE and
232 are similar to the T-type of M O R B of Sun et al. [34]. They are also comparable in many respects to basalts from the F A M O U S area [18,21] and to those of I P O D Leg 49 [22]. Dolerites of two different groups can occur in the same section; in such cases, rocks of group 1 are always overlain by rocks of group 2. In other areas, rocks of group 2 are always found below group 3 (Fig. 2). The division into the three groups based on R E E patterns and L a / Y b ratios is corroborated by ratios involving "highly incompatible" (e.g. Nb) and "less incompatible" (e.g. Zr) trace elements. On the other hand, two trace elements with the same degree of incompatibility (e.g. Z r - H f or Y-Tb [23]) have the same ratio in the three rock-groups. Several averaged ratios for each group from High Atlas are given together with averaged ratios from Anti-Atlas in Table 2. The data indicate that the almost constant ratios have values close to that of chondrites and the ratios which decrease from group 1 to group 3 generally approach a chondritic value. Similar trends have been observed in ocean
floor basalts [22-24] and for comparison, the values of Leg 49 basalts, divided into three groups according to their L a / Y b ratios, are also given in Table 2. These ocean floor basalts show a striking similarity with Moroccan dolerites especially in their element ratios (Table 2). The very small range of T a / L a variation shown in the Moroccan rocks is also encountered in samples from Leg 49. However, the values of the T a / L a ratios in dolerites differ markedly from values obtained on Leg 49 (Fig. 5) although they are d o s e to that of M O R B at 22°N (Leg 45 and 46 [14]) and 25°N (Leg 51, 52 a n d 5 3 [14]).
4.4. Alkalies, alkali earths, U and Th The mobile elements, R b , Cs and to a smaller extent Ba, U and Th are variable and have a positive intercorrelation. The variability of the mobile elements is shown by the large standard deviation for their averaged ratios (Table 2) and this behavioUr contrasts with the regular variation
TABLE 2 Average element ratios in dolerites from Anti-Atlas and High-Atlas compared with MORB of Leg 49 and chondrites Anti-Atlas
La/Yb La/Sm Zr/Y Zr/P Nb/Zr Ta/Hf Ta/La P205/TiO2 Ta/Nb Zr/Hf Y/Tb K/Rb Rb/Cs Cs/U Rb/Sr Rb/La Ba/La
4.4 (0.7) 2.9 (0.3) 3.4 (0.3) 0.127 (0.013) 0.079 (0.008) 0.17 (0.01) 0.047 (0.003) 0.14 (0.02) 0.055 (0.006) 38 (1) 49 (2) 264 (31) 29 (17) 1.8 (0.51) 0.099 (0.039) 2.3 (0.8) 17.4 (2.9)
High Atlas group 1
group 2
group 3
7.5 (0.5) 3.8 (0.2) 4.6 (0.3) 0.157 (0.009) 0.103 (0.006) 0.24 (0.01) 0.054 (0.007) 0.15 (0.02) 0.061 (0.004) 39 (1) 44 (6) 285 (28) 38 (19) 1.2 (0.6) 0.160 (0.046) 2.1 (0.4) 13.0 (2.0)
4.4 (0.5) 3.0 (0.3) 3.5 (0.3) 0.134 (0.012) 0.079 (0.009) 0.17 (0.02) 0.045 (0.006) 0.14 (0.01) 0.054 (0.007) 38 (2) 46 (3) 200 (130) 14 (5) 2.8 (0.9) 0.087 (0.047) 1.7 (0.8) 9.0 (3.0)
2. I (0.1) 2.2 (0.1) 2.4 (0.1) 0.132 0.065 (0.007) 0.12 0.038 0.13 0.053 37 (1) 43 (I) 525 (117) 10 0.8 0.06 0.6 11
0: standard deviation; the rocks of High Atlas and Leg 49 are subdivided according to the La/Yb ratio (see text); Leg 49 ratios are from Wood et al. [22]; chondritic values reported by: A=Sun et al. [34]; B=Wood et al. [22]; C=Bouganlt [14].
233 trends of the immobile element ratios (e.g. L a / Y b , Z r / Y ) among the three groups of rocks in the High Atlas. The rocks of group 2 from the High Atlas which are similar to rocks of Anti-Atlas in their immobile element ratios, have different K / R b and C s / U ratios. Compared to ocean floor basalts [22,25,26] the studied rocks have lower K / R b , R b / C s and higher R b / S r , C s / U and R b / L a ratios. Overall they are enriched in Rb, Cs, Ba, Li and to a lesser extent in U and Th. In this respect, the dolerites of Morocco are very similar to continental tholeiites [27,28].
5. D i s c u s s i o n
The large variations of the mobile elements and their corresponding ratios (e.g. K / R b ) cannot be explained by a simple process of mineral fractionation since their partition coefficients for comm o n minerals of upper mantle and doleritic rocks are very small and similar to each other [26]. The
distribution of these mobile elements could, however, be affected by secondary process particularly alteration and contamination. These two processes will be considered in turn. 5.1. A Iteration In a profile such as shown in Fig. 2, the ratios K / R b , and R b / C s tend to decrease with the increase of water content. In this respect, samples with LOI > 1 have an average R b / C s ratio < 20 and K / R b ratios < 2 1 3 while dolerites with L O I < 1 have R b / C s ratios between 38 and 48 and K / R b ratios > 200. In addition, a rough positive relationship appears between K and K / R b in the analyzed samples. A similar relationship encountered in Puerto Rico trench basalts has been attributed to alteration [29]. The effect of alterative is also indicated by the lack of interelement correlations when a mobile element is plotted against rather immobile elements such as La, Zr, T a (e.g. Fig. 7).
Mid-Atlantic Ridge, Leg 49
Chondrites
group I
group 2
group 3
7.7 (1.7) 4.5 (0.6) 4.9 (0.9) O.109 (0.02) 0.28 (0.01) 0.68 (0.06) 0.! 12 (0.062) 0.17 (0.01) 0.057 (0.004) 41 (0.5) 44 (3) 543 (36) 34 (13) 0.66 (0.25) 0.032 (0.01I) 0.42 (0.07) 9.9 (2.3)
2.7 (0.2) 2.5 (0.1) 2.6 (0.1) 0.092 (0.02) 0.20 (0.02) 0.40 (0.02) 0.100 (0.001) 0.15 (0.04) 0.058 (0.002) 37 (1) 43 (6) 395 (28) 72 (40) 0.39 (0.26) 0.038 (0.008) 0.78 (0.21) 9.2 (0.8)
0.9 1.2 1.9 0.088 0.1 l 0.21 0.096 0.13 0.050 37 45 592 0.001 0.29 6.4
A
B
(0.2)
1.5
(0. I)
1.6
(0.3) (0.01)
2.5 0.12 0.07
(0.03) (1) (4) (1)
(0.8)
0.063 O.11 O.051 343 29 0.9 0.032 1.1 11.1
C
1.6
1.6 1.7 2.4
0.062 0.12 0.067
0.10 0.24 0.097
0.039
0.058 40 46
234
Rb ppm
and when several trace elements are plotted normalized to chondrites, this sample shows a distinct similarity with ocean floor basalts (Fig. 8A). This suggests that the contents of alkalies and alkali earths of sample 1137 is not affected by contamination. A similar conclusion is indicated by a comparison (Fig. 8B) of sample 1216 and P-type of ocean floor tholeiites of Sun et al. [34]. Furthermore, the variation a n d / o r constancy of ratios involving the most stable dement (Table2) are directly proportional to the degree of incompatibility of the corresponding elements in major mantle phases. This suggests that the distribution of thesedements in the dolerites are mainly due to solid-liquid equilibria and that both contamination and alteration had a negligeable effect. Therefore, most of the following discussion will deal with elements such as REE, Zr, Nb, Ta, Y and transition metals which are less sensitive to secondary alteration. Two aspects of the petrogenesis of the Moroc-
-I(-
40
30
zx
.1¢.
20
10
20
30
La ppm
1~
(A)
Fig. 7. Variations of Rb vs. La. Symbols are the same as in Fig. 3. I=MORB from Legs 45 and 46 (22°N), and Legs 51 and 52 (25°N), H=MORB from Leg 49 and Azores-Gibraltar fracture zones; I and H trends after Bougault [14]. r~
5.2. Contamination
Crustal contamination of tholeiitic magma emplaced on the continent has frequently been invoked [27,30] and documented by isotopic data [31]. Furthermore, recent studies [32,33] suggest that this process has a greater effect on the mobile elements. In our case it is not clear whether the large spread of the K / R b , R b / C s and some other ratios is due to contamination or to a secondary alteration process. Among samples with water content > 1%, there is a slight increase of R b / L a and T h / L a ratios in some of the most differentiated rocks; this may favor contamination. But overall it may be suggested that this process is local and most samples do not seem to be significantly affected. For example, among the freshest rocks, sample 1137 has the highest concentration of K 2°
2c
iCs
i Rb
i Ba
t TI1
A U
t
i
Nb
Ta
i K
i La
i Ce
i P
, Sm
, Zr
i Eo
i Ti
i Tb
i Yb
i =0
soI
(B)/2ffl
20
I
I
i
i
I
i
i
1
i
i
i
Rb
BI
K
La
Ce
P
$m
Zr
Eu
Tb
Yb
Fig. 8. Abundances of several minor and trace elements arranged according to their degree of incompatibility--after Sun et al. [34] and normalized to chondrites [34]. A. Sample 1137 and Walvis Ridge basalt of Bougault [14]. B. Sample 1216 and P-type MORB of Sun et al. [34].
235
can dolerites will be discussed in turn. The first aspect refers to the relationships among the rocks of single group. The second one deals with the relationship among the different rock groups and the origin of their parental magmas. ....... 5. 3. Relationship among the rocks of a single group Despite the small range of SiO2 variation, several petrographic and geochemical features suggest that these rooks underwent low-pressure fractional crystallization. These features include distinct linear correlations such as the positive covariance of the incompatible elements with increasing TiO 2 and decreasing [Mg] values, and the negative correlation of the compatible elements (e.g. Ni, Cr, Co) with incompatible elements (e.g. La, Zr). Since the samples from Anti-Atlas were collected from a rather limited area, they were used to evaluate fractional crystallization. From the major element data, the process was tested by leastsquares calculations [35] using the whole-rock analysis listed in Table 1 and the phenocryst compositions reported by Bertrand and Prioton [7]. The calculations were done for various combinations of whole rocks and the results indicate that the major element compositions of the sequence of samples from Anti-Atlas (Table l) can be reproduced by the separation of plagioolase and clinopyroxene. For the sake of simplicity, only the weight fractions of the variables and squared residuals (~R 2) are given for two combinations: rook 1010 --- 0.68 rock 1013 + 0.23 cpx + 0.09 plg/YR 2 = 0.07 rock 1013 ----0.85 rock 1012 + 0.12 cpx +0.03 plg/Y~R2 -- 0.11 The calculated proportions of phases are closely comparable to modal compositions. The model was further tested for trace elements using the inverse method of the fractional crystallization process described in detail by Minster et al. [36]. The procedure for the inversion method consists of two stages. The first step involves an estimation of the initial composition of the parental liquid (Co), bulk partition coefficients (D) and degree of solidification (F). It was assumed that
the parental magma contained 200 ~ 20 ppm Ni (cf. [14]). Then the initial concentrations of several trace elements in such a melt (Co) were estimated by graphic extrapolation [37] from covariation ~rends for an Ni content of 200 ppm. The bulk partition coefficients (D) were calculated from the partition coefficients given in Table 3 and weight fractions of phases obtained from the least-squares calculations for major elements. The degrees of fractional crystallization were calculated by the Rayleigh law equation taking into consideration the variations of D and C0. The parameters are given in Table 3 ("starting model"). The second stage of the procedure involves the calculation of the actual inversion model using approximate parameters obtained from "the starting model". The results are given in Table 3. The close similarities between C~ac. and CoJbs. and between the approximate parameters of the starting model and those recalculated by the inversion method further support fractional crystallization as the process relating the different samples of this sequence. 5. 4. Relationship among the different groups of rocks The large variations of the La/Yb and La/Sm ratios and the very similar contents of some transition metals (Ni, Cr, Co, Sc, V) and major elements among the groups rules out the possibility of all the lava groups being related to a single parental liquid by either low- or high-pressure crystal fractionation. Variable degrees of batch partial melting of a spinel lherzolite source cannot account for the large L a / Y b variation observed in the dolerites. These differences would imply garnet-bearing residue after melting. However, assuming that the source has a chondritic composition, the variations of the La/Yb ratios require melting in a range from 5 to 30%. Such a large span of melting is not consistent with the rather constant abundances of major and some transition elements. Furthermore, any sequence produced by batch partial melting should show a correlation between light and heavy REE [18]. such a correlation is not present among the Moroccan samples. Therefore, the distribution of trace elements requires a more complex process
236 TABLE 3 Calculations of inversion method for dolerites of Anti-Atlas Starting model
Nb Ta La Ce Sm Eu Zr Th Yb Sr V CO S¢
Inversion model
D
Co
D
co
c2,,k~.
(1)
(2)
(3)
(4)
1010
1013
1012
0.0670.02 0.0570.01 0.1070.01 0.1270.02 0.4270.07 0.53 7 0 . 0 6 0.08~-0.03 0.4570.05 0.4570.05 0.7 ~0.1 0.6 ~-0.1 1.3 70.1 1.4 7 1
5.0 7 0.1 0.2 7 0.1 5.0 7 0.5 12 7 I 2.0 7 0.2 0.75 7 0 . 0 5 55 7 "5 0 . 3 5 7 0.05 1.5 7 0.2 170 7 1 0 230 ~20 50 "~ 5 40 ~ 3
0.1070.02 0.0570.01 0.1070.01 0.1170.02 0.3770.01 0.50~0.05 0.0870.03 0.4270.05 0.4570.06 0.8270.04 0.67~0.06 1.1770.09 1.1670.05
4.8 7 0.28 7 5.8 7 12.8 7 2.2 7 0.78 7 59 7 0.427 1.5 7 166 7 234 ~ 51 7 40 7
6.0 0.3 7.2 16.0 2.5 0.91 73 0.49 1.71 177 264 48 38
9.0 0.5 11.2 24.0 3.4 1.15 113 0.64 2.24 194 312 44 35
11.5 0.7 14.3 30.4 4.0 1.3 I 145 0.74 2.60 204 341 42 33
F 0 . 7 0 7 0.15 0 . 4 5 7 0.10 0 . 3 5 7 0.08
F 0.7870.04 0.4970.02 0.39~-0.01
Reference samples 1010 1013 1012
0.1 0.02 0.2 0.5 0.1 0.04 3 0.03 0.1 9 12 3 3
For the calculations, three representative samples which show large range of [Mg] values and of trace element abundances were considered. The uncertainties of data correspond to 26 from the duplicate analyses of samples 1010. Partition coefficients used to calculate D were compiled from Lopez-Escobar et al. [43], Shih [44], Schnetzler and Philpotts [45] for REE and Sr and from Pearce and Norry [46] for Zr, N b and Y. The values for Co, V, Sc, and Ta are from our unpublished data. The uncertainties of D for the "starting model" reflect only those of mineral weight fractions, while those of Co represent the range of Ni variations in the initial liquid ( 2 0 0 ~ 2 0 ppm). Results on the inversion model were obtained after 4 trials.
than simple partial melting or fractional crystallization. Among the different mechanisms tested [38] the dynamic partial melting model [18] seems to best explain the geochemical characteristics of the various groups. In order to minimize the effect of low-pressure fractional crystallization on the abundances of trace elements, the calculations of dynamic melting were initially done for element ratios. For these calculations, the degree of partial melting (7%) and proportions of residual phases (ol = 54, opx = 31, cpx = 11 and sp = 4) were obtained from the least-squares calculations for major elements using the compositions of pyrolite (parental source) and residual minerals given by Sun et al. [34]. The composition of the first melt (C l) was taken as that of samole 1137 which is among the oldest doleritic units in the High Atlas. This composition
(Mg/(Mg -~ Fe 2+) = 0.68; Ni = 200 ppm) may roughly approximate the primary upper mantle liquid [14]. The trace dement composition of the upper mantle source prior to melting was then calculated using the batch melting equation and the composition of sample 1137. The composition of liquids produced by dynamic melting of such a source are shown in Fig. 9 for two pairs of trace element ratios (La/Yb vs Z r / Y and L a / Y b vs. Ta/La). The agreement between observed and calculated data suggests that the individual groups of rocks could be derived from an upper mantle source by dynamic melting. Likewise, equivalent calculations for other ratios involving stable trace elements yield results closely comparable to those observed. The calculations also show that group 2 may be produced by 9-13% of melting while group 3 would require 15-17% melting.
237
'%b
La/yb
A
10
:
lo
B 7%
7%
17% 100 x T a / L a I
I
I
I
i
I
1
3
5
2
4
6
Fig. 9. Variations of La/Yb vs. Zr/Y and La/Yb vs. Ta/La ratios in dolerites from Morocco compared to those of calculated liquid compositions produced by dynamic melting. Mineral composition of residue: ol = 54, opx = 31, cpx-- 11, sp = 4; mineral proportion of melt: ol = 20, opx = 20, cpx = 55, sp = 5. REE partition coefficients of Schilling et al. [41] were used. Dzr and Dy were assumed to be equal to Dsm and D-n, respectively while DTa was taken as 1/2 DLa [22]. Melting was calculated in the range of 7-17% in 2% increments with 2% melt always left in the residue (solid circles); compositions of chondrites (solid squares) and of source material (triangle: La/Yb=3.34, Zr/Y=3.54, Ta/La=0.043); solid lines delineate the fields of dolerites. Note that the trends of liquids produced by melting and of dolerites for the La/Yb vs. Zr/Y ratios lead toward chondrites. 4
T h e c o m p o s i t i o n of the o r i g i n a l s o u r c e m a t e r i a l for several e l e m e n t s is r e p o r t e d i n Fig. 10. T h e c h o n d r i t e - n o r m a l i z e d a b u n d a n c e s of several trace and minor elements show progressive enrichment f r o m V w i t h 1.3 t i m e s c h o n d r i t i c c o n c e n t r a t i o n to the most incompatible elements with abundances m o r e t h a n 3 times t h a t of c h o n d r i t e s . T w o except i o n s to this t r e n d are T a a n d N b w h i c h s h o w a r e l a t i v e d e p l e t i o n . C o m p a r e d to the p r i m o r d i a l m a n t l e c o m p o s i t i o n suggested b y W o o d et al. [22], t h e o r i g i n a l s o u r c e of M o r o c c a n dolerites (Fig. 10) w a s d e p l e t e d i n "less" i n c o m p a t i b l e e l e m e n t s (Ba, T h , U , Ta, N b , La, Ce). C o m p a r i s o n w i t h the l i m i t e d a m o u n t of d a t a o n the A r c h a e a n u p p e r m a n t l e [39] suggests s i m i l a r c o n t e n t s o f P, Ti, Y, Y b a n d V. T h e f r a c t i o n a t i o n of the c h o n d r i t e -
~3
2-
o
I
I
Ba Th
I
U
I
I
I
Ta Nb l a
I
I
Ce Hf
I
Zr
I
I
Sm Eu
I
P
I
Ti
I
I
I
I
Tb
Y
Yb
V
Fig. 10. The abundances of several elements in the calculated mantle source dolerite 1137 (solid dots). In the model calculation, equation 15 of Shaw [42] with 7% melting was used together with the partition coefficients of Schilling et al. [41] for REE and V. D of Ba, Th, U and Ta were assumed to be 1/2 DLa and DNt, to be 3/4 DLa [22]. D of P and Ti were taken to be intermediate between DEu and D-n,. Dzr and DHf were assumed to correspond to Dsm [22]. The other parameters for the calculations are the same as in Fig. 7. Stars: primordial upper mantle reported by Wood et al. [22]; circles: the Archaean mantle of Sun and Nesbitt [39].
238 n o r m a l i z e d p a t t e r n for i n c o m p a t i b l e elements (Fig. 10) m a y i n d i c a t e a c o m p l e x h i s t o r y of the u p p e r m a n t l e source of the studied dolerites.
cal d e t e r m i n a t i o n s . This s t u d y was s u p p o r t e d b y C e n t r e G 6 o l o g i q u e et G 6 o p h y s i q u e of M o n t p e l l i e r , the N a t u r a l Sciences a n d Engineering R e s e a r c h C o u n c i l of C a n a d a ( o p e r a t i n g g r a n t A 3782) a n d the C.N.E.X.O. of F r a n c e (grant 78-5704).
6. Conclusion T h e a b u n d a n c e s of R E E , Zr, Hf, Y, N b , T a a n d c o r r e s p o n d i n g ratios in M o r o c c a n dolerites lie within the range of oceanic tholeiites. However, the c o n t e n t s of the m o s t i n c o m p a t i b l e elements (alkalies, Ba, U a n d Th) in the least altered samples are higher t h a n those o f oceanic basalts, but, a l o n g with a relatively low K / R b ratio, are typical o f c o n t i n e n t a l tholeiites. A s it seems that crustal c o n t a m i n a t i o n d i d n o t affect m o s t of o u r a n a l y z e d samples, the relatively high c o n t e n t s of some inc o m p a t i b l e elements suggest that the c o n t i n e n t a l u p p e r m a n t l e source was enriched in these elem e n t s in c o m p a r i s o n with the u p p e r m a n t l e source for oceanic tholeiites. T h e g e o c h e m i c a l d a t a i n d i c a t e that dolerites of the H i g h A t l a s show vertical s t r a t i g r a p h i c z o n a t i o n which is c h a r a c t e r i z e d b y a progressive lithophile e l e m e n t d e p l e t i o n t o w a r d the t o p of the doleritic sequence. Such a n e v o l u t i o n a r y t r e n d is e n c o u n t e r e d in recent oceanic b a s a l t s [22] a n d even in o p h i o l i t i c c o m p l e x e s [47]. This t r e n d together with the regularities of the trace e l e m e n t r a t i o varia t i o n s can b e p r o d u c e d b y a d y n a m i c m e l t i n g of an initially h o m o g e n e o u s source, a s s u m i n g a p r o gressively y o u n g e r time relationship t o w a r d the t o p of the doleritic sequence. Such a time sequence has yet to b e e s t a b l i s h e d b u t leaves o p e n the feasibility of a d y n a m i c melting model. A n o t h e r process, which is consistent with the available d a t a involves the m i x i n g of m a g m a s p r o d u c e d b y different degrees of p a r t i a l melting in different p a r t s o f an u p p e r m a n t l e with vertical heterogeneities [48].
Acknowledgements M a n y t h a n k s are due to Drs. G. Vasseur a n d J. Verni~res for their help with the calculations, to Prof. M. G i r o d for initiating the p r o j e c t a n d to Mrs. L. S a v o y a n t a n d S. Pourtales for their chemi-
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