Flood Basalt from Mont Tourmente in the Central Kerguelen Archipelago

21 Pages·2002·0.57 MB·English

JOURNALOFPETROLOGY VOLUME43 NUMBER7 PAGES1367–1387 2002
Flood Basalt from Mont Tourmente in the
Central Kerguelen Archipelago: the Change
from Transitional to Alkalic Basalt at
>
25 Ma
F. A. FREY1∗, K. NICOLAYSEN1†, B. K. KUBIT2, D. WEIS3 AND
A. GIRET4
1DEPARTMENTOFEARTH,ATMOSPHERICANDPLANETARYSCIENCES,MASSACHUSETTSINSTITUTEOF
TECHNOLOGY,CAMBRIDGE,MA02139,USA
2DEPARTMENTOFGEOSCIENCES,UNIVERSITYOFMASSACHUSETTS,AMHERST,MA01003,USA
3DEPARTMENTOFEARTHANDENVIRONMENTALSCIENCES,CP160/02,UNIVERSITE´ LIBREDEBRUXELLES,
AV.F.D.ROOSEVELT,50,B-1050,BRUSSELS,BELGIUM
4LABORATOIREDEGE´OLOGIE,UNIVERSITE´ JEANMONNET,CNRS–UMR6524,23RUEDUDOCTEURPAUL
MICHELON,42023,SAINT-E´TIENNE,CEDEX2,FRANCE
RECEIVED MAY 7, 2001; REVISED TYPESCRIPT ACCEPTED FEBRUARY 14, 2002
The surface of the Cenozoic Kerguelen Archipelago, constructed on the eastern sections, which have been proposed to be characteristic
theKerguelenPlateauinthesouthernIndianOcean,isdominantly oftheKerguelenplume.Consequently,theMontTourmenteisotopic
flood basalt. With the objective of understanding the Cenozoic data may reflect heterogeneity within the plume or a constant
history of the Kerguelen mantle plume, the age and geochemical proportion of a depleted component mixed with the high 87Sr/86Sr
characteristics of this flood basalt province are being determined by andlow143Nd/144Ndplume.IncontrasttomanyoftheCretaceous
studying stratigraphic sections of basalt flows at several locations. Kerguelen Plateau lavas, there is no evidence in trace element
Sections from the NW, north–central, east and SE parts of the abundancesorSrandNdisotopicratiosthattheCenozoicKerguelen
archipelago have been studied. Here we report results for a 597m Archipelago lavas were influenced by continental lithosphere.
successionoflavasfromMontTourmentefromthePlateauCentral,
aregionofthearchipelagothathasnotbeenstudiedindetail.Mont
Tourmente lavas range from >26Ma dominantly transitional
basaltsinthelower80%ofthesectionto>25·3Madominantly KEYWORDS:KerguelenArchipelago;Kerguelenplume;floodbasalt;igneous
alkalic basalts in the upper part of the section. The timing of this geochemistry
change from transitional to alkalic volcanism within the Mont
Tourmente section is consistent with that defined by the older
>28–29Ma transitional basalts in the north and the >25Ma
alkaliclavaseruptedintheeast.Thischangeinbasaltcomposition INTRODUCTION
mayberelatedtomigrationofthearchipelagoawayfromtheplume TounderstandtheCenozoicvolcanismattributedtothe
or to increasing lithosphere thickness over the >5Myr of flood Kerguelen plume we are studying stratigraphic sections
basaltvolcanism.ThealkalicandtransitionalTourmentelavasare ofthefloodbasaltlavasthatcovermostoftheKerguelen
nearlyhomogeneousinisotopicratiosofSr,NdandPb.Theyhave Archipelago (Fig. 1). Our objective is to define and
lower87Sr/86Srandhigher143Nd/144Ndcomparedwithlavasfrom understand the temporal and spatial variations in the
∗Correspondingauthor.E-mail:[email protected]
†Present address: Geology Department, Kansas State University,
Manhattan,KS66506,USA. OxfordUniversityPress2002
JOURNALOFPETROLOGY VOLUME43 NUMBER7 JULY2002
Fig. 1. Map of the Kerguelen Archipelago showing geographical regions, such as the Plateau Central, the major geological units and the
locationofstudiedstratigraphicsectionsofthefloodbasalts:RavinsduCharbonandJauneintheSEProvince(Freyetal.,2000),MontCrozier
intheNE(Damascenoetal.,2002),MontBureauandMontRabouille`reinthenorth(Yangetal.,1998),MontdesRuchesandMontFontaine
intheNW(Doucetetal.,2002)andMontTourmenteinthePlateauCentral(thisstudy).MontRossistheyoungestedificeinthearchipelago
(Weis et al., 1998). Inset is a map of the eastern Indian Ocean showing volcanic structures attributed to the Kerguelen plume; i.e. the large
igneousprovinceformedbythenowseparatedCretaceousKerguelenPlateauandBrokenRidge;thehotspottrackformedbytheCretaceous
toCenozoicNinetyeastRidgeandtheCenozoicKerguelenArchipelagoandHeardIslands(×s)locatedonthenorthernKerguelenPlateau.
Χ,drillinganddredgingsitesontheNinetyeastRidge,BrokenRidge,NaturalistePlateauandKerguelenPlateau.
geochemical characteristics of these lavas. Studies of In this paper we focus on a section from Mont Tour-
sections from the SE (Frey et al., 2000), NE (Damasceno mente, which is located in the central part of the ar-
et al., 2002), north–central (Yang et al., 1998) and NW chipelago in a region known as Plateau Central (Fig. 1).
(Doucet et al., 2002) show that there are important InasurveystudyofthefloodbasaltsGautieretal.(1990)
regional geochemical differences in the flood basalts. foundthattheSrandNdisotopicratiosofthetransitional
These differences include: (1) a temporal change from basalts from this region (four samples) are intermediate
older tholeiitic–transitional to younger alkalic lavas and between those of the alkalic basalts in the SE and NE
(2) isotopic (Sr, Nd, Pb) heterogeneity in the oldest, and the tholeiitic to transitional basalts in the NW. A
>28–29Ma basalts (Yang et al., 1998; Doucet et al., goal in this paper is to determine the generality of the
2002), which contrasts with the isotopic homogeneity of survey results by Gautier et al. (1990) and to answer
younger, 25Ma, sections (Weis et al., 1998; Frey et al., the question—are lavas erupted in the Plateau Central
2000). geochemicallydistinctfrombasaltseruptedinotherparts
1368
FREYetal. FLOODBASALTFROMMONTTOURMENTE
Fig. 2. Location of studied samples (black horizons with sample numbers) in the Mont Tourmente section. Ages (40Ar/39Ar) indicated for
samples93-414and83-354arefromNicolaysenetal.(2000).Thewhitelevelsarethosewithoutoutcroporveryweathered.
of the archipelago? Previously, Frey et al. (2000) showed Thisregionischaracterizedbybasalticflowsthatextend
that the trend to more alkalic lavas with decreasing age for very long distances and are only cut locally by
in the Kerguelen Archipelago reflects (1) a decreasing small streams; glaciation, however, has exposed vertical
supplyofmagmafromtheplume,(2)atemporaldecrease sections, up to 800m high. The total volume of mag-
inextentofmeltingand(3)atemporalincreaseindepth matismrepresentedbythePlateauCentralisasignificant
ofmeltsegregation.IfradiogenicisotopicratiosofPlateau portion of the flood basalts. Like the other volcanoes in
Central lavas differ from those of younger alkalic flood thearea,MontTourmentehasageneralovalshapewith
basalts, these data bear on important issues such as the a clear east–west orientation.
heterogeneity of the Kerguelen plume and changes in
mixing ratios between source components derived from
plume, asthenosphere and oceanic lithosphere.
SAMPLE DESCRIPTION
Sixty-foursamples(93-351to93-414)werecollectedfrom
GEOLOGY a597msection(Fig.2).Onthebasisof40Ar/39Ardating,
Mont Tourmente is located in the northern part of the Nicolaysen et al. (2000) reported eruption ages of 26·0
Plateau Central in the Kerguelen Archipelago (Fig. 1). ±1·0Maforsample93-414atthebottomofthesection
1369
JOURNALOFPETROLOGY VOLUME43 NUMBER7 JULY2002
Table 1: Major (wt % oxides) and trace element abundance (ppm) in lavas from Mont Tourmente
Sample: 93-351 93-352 93-353 93-354 93-355 93-356 93-357 93-358 93-359 93-360 93-361
Height(m):597 577 563 555 545 537 533 529 519 512 504
XRF
SiO 46·62 47·85 48·32 47·57 46·44 46·97 46·61 46·89 47·85 48·07 47·05
2
TiO 3·96 3·84 3·82 4·26 4·24 4·22 3·86 3·83 3·41 3·30 3·59
2
AlO 13·68 13·10 13·12 12·74 13·58 13·33 13·29 13·14 13·79 13·81 13·64
2 3
FeO 16·15 15·80 15·88 16·71 16·16 16·40 16·37 16·47 15·27 14·91 15·94
2 3
MnO 0·36 0·24 0·26 0·26 0·33 0·24 0·27 0·25 0·22 0·23 0·23
MgO 5·38 4·98 5·09 4·80 4·97 4·81 5·48 5·29 5·19 5·42 5·42
CaO 9·85 9·88 9·57 9·18 9·98 9·53 10·02 10·04 10·70 10·64 9·98
NaO 2·79 2·76 3·01 3·12 2·97 3·03 2·85 2·91 2·76 2·80 2·72
2
KO 0·62 0·66 0·68 0·96 0·62 1·11 0·68 0·60 0·39 0·39 0·48
2
PO 0·46 0·45 0·45 0·52 0·50 0·48 0·42 0·42 0·34 0·35 0·44
2 3
Total 99·87 99·56 100·20 100·12 99·79 100·10 99·83 99·84 99·90 99·91 99·49
V 382 352 349 397 392 377 379 383 342 331 345
Cr 70 65 63 38 62 62 57 74 50 67 93
Ni 35 33 30 27 35 36 40 41 35 39 37
Zn 169 154 161 175 169 169 168 163 137 134 160
Ga 26 24 24 25 25 28 26 24 23 23 25
Rb 5·9 6·9 10·7 17·4 3·3 25·8 6·5 3·4 1·9 1·3 4·7
Sr 338 328 330 317 347 323 317 335 338 348 326
Ba 193 209 228 222 222 215 181 195 159 173 199
Y 40·8 37·2 37·5 41·0 40·7 39·2 36·7 36·3 30·6 31·1 38·2
Zr 269 255 280 286 282 282 244 243 205 204 254
Nb 30·1 28·3 31·3 32·1 31·4 31·7 27·0 26·8 21·7 22·1 27·5
Ce 52 51 47 58 54 55 44 44 38 41 53
Sample: 93-362 93-363 93-364 93-365 93-366 93-367 93-368 93-369 93-370 93-371 93-372
Height(m) 496 485 475 466 462 456 445 436 431 424 418
XRF
SiO 46·62 46·72 49·18 47·83 49·40 49·67 49·66 49·41 49·49 49·28 50·38
2
TiO 3·71 3·93 3·47 3·36 3·20 3·36 3·14 3·11 3·13 3·65 3·56
2
AlO 13·53 13·42 13·01 14·03 13·32 13·19 13·49 13·45 13·47 12·76 13·15
2 3
FeO∗ 16·54 16·56 15·51 15·13 14·63 15·04 14·67 14·61 14·59 15·92 14·82
2 3
MnO 0·24 0·24 0·24 0·24 0·22 0·24 0·24 0·24 0·23 0·26 0·24
MgO 5·41 5·35 5·05 5·54 5·31 4·83 5·45 5·49 5·40 4·75 4·22
CaO 10·01 10·40 9·59 10·37 9·91 9·61 9·99 10·05 10·06 9·27 8·83
NaO 2·77 2·66 2·99 2·68 3·15 2·88 2·91 2·85 2·83 2·90 2·95
2
KO 0·36 0·38 0·57 0·40 0·58 0·47 0·53 0·48 0·47 0·48 1·22
2
PO 0·45 0·45 0·39 0·41 0·37 0·42 0·33 0·35 0·34 0·43 0·41
2 3
Total 99·64 100·10 100·00 99·99 100·09 99·70 100·41 100·05 100·01 99·69 99·78
V 363 356 345 340 317 313 327 310 320 349 354
Cr 64 58 58 79 65 47 67 69 69 26 28
Ni 39 41 41 50 46 39 47 49 51 33 28
Zn 168 166 152 151 139 142 141 138 141 158 160
Ga 24 24 23 23 23 23 22 22 22 24 24
Rb 1·5 2·1 9·7 1·3 10·9 13·7 9·4 7·2 9·0 14·9 42·5
Sr 335 340 324 333 326 320 323 329 325 320 322
Ba 181 203 200 168 192 183 171 173 173 184 197
Y 39·9 38·3 34·1 35·0 32·2 34·8 30·7 31·3 31·4 37·1 35·3
Zr 261 270 248 244 233 250 212 213 215 260 251
Nb 28·1 28·7 26·6 26·1 24·8 26·6 23·0 22·8 23·3 28·0 27·1
Ce 50 53 45 48 43 50 38 43 42 52 46
1370
FREYetal. FLOODBASALTFROMMONTTOURMENTE
Sample: 93-373 93-374 93-375 93-376 93-377 93-378 93-379 93-380 93-381 93-382 93-383
Height(m):412 406 396 392 363 345 345 341 340 329 324
XRF
SiO 49·86 50·24 49·28 49·29 49·22 49·04 49·30 47·75 48·87 49·43 50·03
2
TiO 3·45 3·41 3·63 3·06 3·73 3·03 2·92 3·70 2·88 2·88 3·18
2
AlO 13·09 12·96 13·20 14·29 13·21 14·93 14·49 13·91 14·29 13·88 13·47
2 3
FeO∗ 15·05 14·72 15·41 13·49 15·42 13·24 13·50 15·02 13·57 13·61 14·62
2 3
MnO 0·23 0·22 0·24 0·20 0·23 0·22 0·21 0·27 0·20 0·20 0·23
MgO 4·89 4·72 4·72 5·45 4·83 5·47 5·32 5·57 5·99 5·65 4·64
CaO 9·37 9·24 8·84 10·29 9·03 10·38 10·34 10·22 10·50 10·74 9·23
NaO 2·95 3·01 2·88 2·82 3·08 2·82 2·73 2·76 2·80 2·76 3·26
2
KO 0·79 0·68 0·85 0·47 0·76 0·75 0·58 0·29 0·43 0·53 0·59
2
PO 0·40 0·38 0·42 0·38 0·46 0·37 0·35 0·39 0·29 0·31 0·43
2 3
Total 100·08 99·58 99·46 99·74 99·96 100·25 99·73 99·87 100·28 99·99 99·68
V 347 342 362 333 353 299 291 372 313 292 308
Cr 46 61 40 107 40 109 107 61 137 126 37
Ni 37 38 28 43 26 51 48 41 60 58 27
Zn 156 148 161 142 153 136 131 166 124 122 147
Ga 23 23 24 23 24 24 23 25 22 23 24
Rb 9·6 18·4 10·3 14·3 8·2 6·1 4·5 0·6 2·2 5·0 13·4
Sr 336 322 335 342 340 353 346 358 343 326 333
Ba 205 198 219 192 229 187 192 143 153 160 224
Y 36·0 34·1 37·7 32·8 38·0 33·9 31·3 36·1 26·9 27·1 36·0
Zr 242 250 275 232 273 227 218 257 180 181 261
Nb 26·0 27·1 29·4 25·6 29·3 24·5 23·6 28·6 19·2 19·5 28·5
Ce 51 46 50 46 55 47 45 49 29 35 54
Sample: 93-384 93-385 93-386 93-387 93-388 93-389 93-390 93-391 93-392 93-393 93-394
Height(m):316 308 296 287 281 272 256 248 232 222 216
XRF
SiO 50·67 49·09 50·57 49·05 49·36 48·52 50·85 50·49 49·78 49·57 48·95
2
TiO 3·21 3·63 3·54 3·05 3·19 3·67 4·08 3·88 3·24 3·25 3·76
2
AlO 13·49 13·15 13·33 14·28 13·46 13·41 12·78 13·26 14·02 13·31 13·90
2 3
FeO 14·33 15·31 14·51 13·47 14·41 14·72 14·76 14·28 13·74 14·34 14·17
2 3
MnO 0·22 0·23 0·22 0·20 0·22 0·24 0·21 0·23 0·21 0·22 0·24
MgO 4·52 4·89 4·39 5·48 5·46 5·39 4·50 4·80 5·52 5·28 5·02
CaO 8·79 9·59 8·61 10·30 9·77 9·87 8·67 8·84 9·80 10·18 9·09
NaO 3·06 3·02 3·23 2·88 2·91 2·87 3·27 3·10 2·88 2·88 3·03
2
KO 1·15 0·45 0·90 0·47 0·65 0·60 0·63 0·65 0·50 0·59 1·38
2
PO 0·43 0·41 0·45 0·37 0·42 0·42 0·55 0·49 0·43 0·38 0·45
2 3
Total 99·86 99·77 99·74 99·54 99·86 99·71 100·21 100·02 100·11 100·00 99·99
V 313 348 339 304 320 366 366 337 305 315 374
Cr 33 54 50 126 133 72 34 90 160 82 105
Ni 26 36 32 56 56 38 32 57 59 52 50
Zn 152 147 147 131 144 143 156 151 137 141 146
Ga 24 24 25 23 23 23 25 25 23 23 26
Rb 30·3 13·1 13·3 3·2 9·6 7·4 24·6 17·8 8·5 9·1 31·5
Sr 330 327 352 363 328 347 325 342 333 328 320
Ba 227 184 257 192 201 191 233 237 199 182 197
Y 36·1 35·1 38·7 31·8 34·3 34·2 41·2 40·3 36·3 32·8 37·8
Zr 263 247 294 225 243 235 314 311 259 234 265
Nb 28·4 27·2 31·4 24·5 26·9 26·4 34·2 33·8 27·3 25·1 29·0
Ce 54 50 58 49 50 47 63 60 51 48 50
1371
JOURNALOFPETROLOGY VOLUME43 NUMBER7 JULY2002
Table 1: continued
Sample: 93-395 93-396 93-397 93-398 93-399 93-400 93-401 93-402 93-403 93-404 93-405
Height(m):211 207 197 187 183 177 170 144 137 130 125
XRF
SiO 49·38 49·86 49·93 50·52 50·22 49·39 50·11 48·94 48·48 48·36 50·47
2
TiO 3·71 3·63 3·65 3·67 3·46 3·51 3·18 2·79 2·84 2·89 3·41
2
AlO 13·74 13·43 13·50 13·47 12·98 13·30 13·93 13·99 14·20 14·36 13·25
2 3
FeO∗ 14·07 13·98 13·96 13·95 15·39 15·66 13·27 13·12 13·41 13·42 14·59
2 3
MnO 0·21 0·22 0·22 0·21 0·23 0·23 0·22 0·20 0·21 0·21 0·22
MgO 5·09 4·94 5·01 4·34 4·49 4·61 5·09 6·01 6·20 6·38 4·64
CaO 9·47 9·20 9·27 8·35 8·80 8·85 10·10 11·09 11·13 11·04 8·89
NaO 3·06 3·11 3·07 3·28 3·24 2·97 2·82 2·67 2·66 2·62 3·12
2
KO 0·67 0·77 0·75 1·25 0·57 0·93 0·78 0·44 0·41 0·25 0·64
2
PO 0·44 0·44 0·44 0·56 0·44 0·45 0·36 0·30 0·30 0·31 0·43
2 3
Total 99·84 99·58 99·79 99·60 99·81 99·89 99·85 99·55 99·84 99·84 99·63
V 364 362 354 355 330 342 298 291 291 294 315
Cr 101 101 100 68 24 24 152 166 163 229 25
Ni 51 50 51 39 20 20 60 70 69 86 24
Zn 140 141 139 153 152 155 127 119 119 124 143
Ga 24 24 24 26 24 24 24 21 22 23 24
Rb 5·3 10·4 11·3 26·7 14·5 13·1 15·7 3·2 2·2 1·1 13·7
Sr 350 336 339 335 335 342 338 350 356 348 337
Ba 210 209 207 242 217 212 173 153 153 131 234
Y 37·1 36·5 36·5 43·3 55·7 37·1 32·2 26·4 27·3 28·6 35·9
Zr 259 257 255 315 268 271 222 183 185 192 263
Nb 28·1 28·1 27·8 34·0 28·8 29·0 23·5 19·7 20·0 20·5 28·2
Ce 52 51 53 65 54 55 46 39 37 39 53
Sample: 93-406 93-407 93-408 93-409 93-410 93-411 93-412 93-413 93-414
Height(m):112 104 101 92 81 56 45 33 16
XRF
SiO 50·42 51·15 51·20 49·60 49·91 49·79 49·95 50·47 50·25
2
TiO 3·51 3·33 3·39 2·98 3·07 3·33 3·31 3·27 3·30
2
AlO 13·43 13·96 13·48 13·85 13·48 13·46 13·42 13·20 13·57
2 3
FeO∗ 15·07 14·06 14·28 13·78 13·99 14·97 15·04 14·85 14·57
2 3
MnO 0·24 0·22 0·22 0·22 0·22 0·22 0·23 0·22 0·21
MgO 4·20 4·05 4·40 5·57 5·43 4·88 4·71 4·68 4·63
CaO 8·10 7·87 8·45 10·16 9·70 9·20 8·84 8·96 8·86
NaO 3·15 3·23 3·23 2·77 2·95 3·30 3·22 3·28 3·12
2
KO 0·96 1·31 0·89 0·38 0·56 0·46 0·75 0·78 0·88
2
PO 0·51 0·41 0·48 0·35 0·35 0·45 0·45 0·45 0·45
2 3
Total 99·58 99·59 100·02 99·65 99·66 100·06 99·91 100·15 99·83
V 286 310 293 297 312 288 310 292 321
Cr 12 6 22 69 45 17 16 14 20
Ni 15 18 17 39 32 24 22 22 24
Zn 166 159 144 125 133 142 143 135 147
Ga 25 25 25 22 22 23 23 24 24
Rb 10·1 16·0 10·1 7·4 8·9 12·7 13·5 11·1 15·9
Sr 330 317 337 337 340 348 332 326 338
Ba 259 282 254 162 186 215 221 225 228
Y 55·6 43·7 36·5 28·2 29·8 37·2 35·5 34·9 36·2
Zr 311 331 276 199 208 251 251 249 254
Nb 32·5 34·5 28·9 21·3 22·5 27·1 26·9 26·7 27·4
Ce 73 64 55 39 43 56 51 53 54
1372
FREYetal. FLOODBASALTFROMMONTTOURMENTE
n].eyA.07es
3404406407409414 71301121049216 ±3·00·132·328·025·431·030·3 ±6124210106820 ±7·30·118·339·331·318·825·8 ±2·60·244·584·871·645·561·4 ±5·00·624·944·339·827·734·9 ±6·080·026·2210·59·136·448·21 ±2·020·012·063·152·552·102·56 ±1·010·031·031·691·430·991·27 ±2·130·042·244·093·562·232·89 ±0·330·020·320·560·530·330·45 ±4·490·114·577·057·374·745·84 ±1·670·021·983·414·252·192·97 uresanddiscussionofaccuracyandprecisioTechnologyfollowingproceduresofIla&FrbundancesweredeterminedbyXRFandINAmpleswiththelowestCrcontent(406and4eewithin3%.Ingeneral,therelativeCevalu
Table1:continued Sample:35135235635836036537638038139039239440240 Height (m):59757753752951246639234134025623221614413 INAA ±±Sc33·031·531·733·933·433·631·034·433·6628·830·70·330·432·33 ±±Cr66645968677710259135439158110216316 ±±La24·824·025·421·818·423·621·924·216·50·128·524·20·225·717·11 ±±Ce59·556·661·252·945·954·650·760·140·10·166·757·01·360·841·44 ±±Nd36·532·934·331·727·332·231·234·725·30·639·033·40·633·923·72 ±±Sm8·507·968·377·846·637·727·337·965·740·139·238·020·058·416·00 ±±Eu2·702·462·642·542·202·432·382·611·950·32·712·470·072·631·95 ±±Tb1·321·161·311·151·041·181·191·250·960·061·921·190·051·310·95 ±±Yb3·113·193·452·792·352·902·713·152·220·043·453·030·023·112·19 ±±Lu0·490·480·460·450·360·420·390·450·320·040·460·450·010·460·31 ±±Hf6·165·816·075·514·735·615·436·054·210·017·016·020·126·094·22 ±±Th2·632·422·512·161·822·502·242·401·520·023·22·60·22·451·59 TheXRFanalyseswerecarriedoutattheUniversityofMassachusetts[seeRhodes(1996)fordescriptionofproced∗FeOindicatesallironreportedasFeO.TheINAAanalyseswerecarriedoutattheMassachusettsInstituteof2323(1984,2000).Meansanddeviationfromaverageareindicatedforthreesamplesanalyzedinduplicate.CrandCeaBecauseonlyasubsetofsampleswereanalyzedbyINAA,weusetheXRFdatainfigures.Exceptforthetwosa>>with10ppmCr),Crvaluesdeterminedbythetwotechniquesagreewithin10%and11of17comparisonsagr>determinedbythetwotechniquesagreeverywellbutXRFdataare16%lower.
1373
JOURNALOFPETROLOGY VOLUME43 NUMBER7 JULY2002
Fig.3. NaO+KOvsSiO classificationplotshowingthattheMontTourmentelavas(Χand×)straddlethealkalic–tholeiiticdividingline
2 2 2
ofMacdonald&Katsura(1964).×,13samplesfromtheupper112mofthesection.MajorelementdatawereadjustedtoaFeO/FeO molar
2 3
ratioof0·85.Ingeneral,these>26–25MaMontTourmentelavasarenotasalkalicastheslightlyyounger>25Mafloodbasaltseruptedin
theSEProvince(Freyetal.,2000)oratMontCrozier(Damascenoetal.,2002).MostMontTourmentelavasoverlapwiththeolder,28–29Ma,
lavaseruptedatMontBureauandMontRabouille`reinthenorth–centralpartofthearchipelago(Yangetal.,1998).
Fig.4. AlkalinityindexandSiO abundance(wt%)vsstratigraphicheight(meters)intheMontTourmentesection.Alkalinityindex[NaO
2 2
+KO−0·37(SiO −39)]isameasureofthedeviationinwt%NaO+KOfromtheMacdonald&Katsura(1964)lineinFig.3with
2 2 2 2
positive values (Α) indicating samples within the alkalic field. It should be noted that nine of the uppermost 13 lavas are alkalic and all 13
sampleshaverelativelylowSiO contents.Samplenumbersareindicatedforthetwoalkaliclavaslowerinthesectionandtransitionalsample
2
380,whichhasalowSiO content.
2
and 25·3 ± 0·7Ma for sample 93-354 from near the plagioclase and 3–4% clinopyroxene phenocrysts and
top of the section (Fig. 2); therefore this section formed microphenocrysts. The scarcity of phenocrysts in most
within1Myr.Exceptfortwosamples,allofthesamples oftheMontTourmentelavascontrastswiththeabundant
contain <5% phenocrysts and microphenocrysts, dom- phenocrysts, especially plagioclase, in the younger,
inantlyplagioclasewithlesseramountsofclinopyroxene. 24–25Ma, alkalic flood basalts erupted to the east at
In contrast, samples 93-401 and 93-404 contain 8–10% Mont Crozier (Damsaceno et al., 2002).
1374
FREYetal. FLOODBASALTFROMMONTTOURMENTE
Fig.5. TiO,PO,CaO,AlO,SiO andNaOvsMgOcontent(allinwt%).ΧandΑ,transitionalandalkalicsamples,respectively.Most
2 2 5 2 3 2 2
ofthealkaliclavashavelowerSiO andhigherTiO andPO contentsthanthetholeiiticlavas,especiallytheninealkaliclavasfromtheupper
2 2 2 5
112m,labelledasUpperAlkalicGroup.Alsolabelledarethefourtransitionalsampleswith>6%MgO.
SE (Ravin Jaune and du Charbon sections), and they
ANALYTICAL TECHNIQUES
overlap with the transitional flood basalts forming the
Abundances of major and several trace elements (Rb,
Mont Bureau and Mont Rabouille`re sections in the
Sr, Ba, V, Cr, Ni, Zn, Ga, Y, Zr, Nb and Ce) in 64
north–central part of the Kerguelen Archipelago (Fig.
samples were determined by X-ray fluorescence (XRF),
3).WithintheMontTourmentesectionthereis,however,
and in a subset of 19 samples abundances of Sc, REE,
a temporal change from transitional to alkalic basalt.
Hf and Th were determined by instrumental neutron
The uppermost 13 lavas, 93-351 to 93-363 from 597 to
activation analysis (INAA) (Table 1). Sixteen samples
485m,arecharacterizedbyrelativelylowSiO contents,
were analyzed for Sr and Nd isotopes and 13 for Pb 2
and11ofthe13arealkalicorveryclosetothetholeiitic–
isotopes (Table 2).
alkalic boundary (Figs 3 and 4). We conclude that these
11lavasformadistinctivegeochemicalgroup,whichwe
designateasthe‘UpperAlkalicGroup’.Twootheralkalic
RESULTS lavas (93-394, 93-398) occur lower in the section; they
Major elements differ from the uppermost samples in having relatively
In a total alkalis vs SiO classification plot, Mont Tour- high KO (>1·3–1·4%, Table 1) and SiO (>49%)
2 2 2
mentelavasstraddlethealkalic–tholeiiticboundary(Fig. contents (Fig. 4b).
3). Most of the Mont Tourmente lavas are less alkalic The64MontTourmentelavashavelowMgOcontents
thantheslightlyyounger(>25Ma)alkaliclavasforming (Table1),rangingfrom4·05%(sample93-407)to6·38%
the flood basalts in the NE (Mont Crozier section) and (sample93-404).AtagivenMgOcontent,relativetothe
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JOURNALOFPETROLOGY VOLUME43 NUMBER7 JULY2002
Trace elements
Like TiO and PO, the abundances of incompatible
2 2 5
traceelements,suchasNb,defineabroadinversetrend
withMgOcontentwiththeupperalkaliclavasgenerally
havingthehighestabundancesofincompatibleelements
at a given MgO content (Figs 5 and 6). Consistent with
theirrelativelylowMgOcontents,theMontTourmente
lavas have low Ni contents (<100 ppm). The positive
correlations of Ni and Cr with MgO are consistent
with fractionation of mafic phases, such as olivine and
pyroxene(Fig.6);however,olivineisnotpresentinthese
lavas. Abundance of Sc ranges from 30 to 34 ppm for
MgO >5·5%, but MgO and Sc contents are positively
correlated at lower MgO contents (Fig. 6). Like CaO,
Sc is compatible in clinopyroxene; therefore both the
CaO–MgO and Sc–MgO trends are consistent with
clinopyroxeneasamajorfractionatingphaseformagmas
with <5·5% MgO.
Plagioclaseisthemajorphenocrystphaseintheselavas.
AbundanceofSr,anelementcompatibleinplagioclase,is
not positively correlated with abundances of an in-
compatibleelement,suchasNb;infact,thereisapoorly
defined negative trend (Fig. 7). An important role for
plagioclase fractionation is indicated by Sr/Ce ratios,
which are close to primitive mantle ratios in the highest
MgO lavas but decrease to (Sr/Ce) >0·4 with de-
PM
creasing MgO (Fig. 8). This relative depletion of Sr is
apparent in primitive mantle (PM) normalized plots
whereSrabundancesarenearlyconstantat15–17times
the PM abundance (Fig. 9); the mean Sr content is 335
± 11 ppm for 64 lavas from 597m of section. The
limited range in Sr and Sc contents (>13% and 30%,
respectively) relative to the range of Nb abundances, a
factorof>1·8,impliesthatthebulksolid–meltpartition
coefficientsforSrandScwerenearunity.Thisinference
requires a fractionating assemblage dominated by the
observedphenocrystphases,qualitatively>50%feldspar
and >30% clinopyroxene (assuming that the mineral–
meltpartitioncoefficientforSris>2forplagioclaseand
Fig.6. AbundancesofNi,Cr,ScandNb(inppm)vsMgOcontent >3 for clinopyroxene); these proportions are typical
(wt %). Symbols as in Fig. 5. As for TiO and PO in Fig. 5, the
2 2 5 for low-pressure evolution of basaltic magma (Toplis &
UpperAlkalicGroupisenrichedinNbatagivenMgO;twotransitional
lavas93-390and93-391alsohavehighNbcontent. Carroll, 1995).
Other important features in the primitive mantle-
transitional lavas the upper alkalic lavas have higher normalized plots are the negative slopes from Nb to Yb
abundances of PO and TiO and lower SiO contents (Fig.9);thatis,unlikesomebasaltsfromtheCretaceous
2 5 2 2
(Fig.5).Incontrast,allsamplesdefineasimilarnegative KerguelenPlateau(e.g.Mahoneyetal.,1995;Freyetal.,
trendforNaO–MgOandapositivetrendforCaO–MgO 2002), these Cenozoic Kerguelen Archipelago lavas are
2
thatlevelsoffat>11%CaOwhenMgOexceeds>5·5% not relatively depleted in Nb (or Ta). In fact, none of
(Fig. 5). The latter trend is consistent with control of the flood basalt sections in the Kerguelen Archipelago
lava compositions by the onset of clinopyroxene frac- includelavasthatarerelativelydepletedinNb(e.g.Yang
tionationasMgOdecreasedbelow5·5%.Abundanceof et al., 1998; Frey et al., 2000; Doucet et al., 2002). There
AlO also decreases with decreasing MgO content, but are strong positive correlations between abundances of
2 3
there is considerable scatter with no distinction between incompatibleelements,suchasNb,Ce,ZrandY,which
alkalic and transitional lavas (Fig. 5). arerelativelyimmobileduringpost-magmaticalteration;
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