The Kerguelen volcanic Plateau: the second largest oceanic Igneous Province (LIP) on earth and a witness of the Indian Ocean opening

by

Jean-Yves Cottin* (1), Gilbert MiChon (1) & Guillaume DelpeCh (2)

AbsTrAcT. - the geological evolution, geomorphological and geophysical data of the Kerguelen plateau are summa- rized from the results of Indian Ocean Drilling Program (ODP) and Institut Paul Emile Victor (IPEV) scientific programs conducted during the last twenty years. this plateau is located in the Antarctic plate oceanic domain between 46 and 64°S, approximately equidistant from Africa and Australia. it is a large igneous province (lip) made up of thick sequences of lava flows rising 2000 to 3000 m above the seafloor, and generated by the activity of a hot spot, which seems to be pres- ently located beneath heard island (where the active volcano Big Ben is located). the birth of the plateau occurred in a mid-oceanic ridge setting (South East Indian Ridge, SEIR) 115 Ma ago. Since 43 Ma, the Kerguelen plateau and Broken Ridge separated and the SeiR moved away northward from the plateau to reach its present intraplate setting. the plateau is subdivided into three parts: (1) the northern part, which carries the Kerguelen islands, (2) the Central part, which rep- resents a basin located between Kerguelen and Heard islands with Upper Cretaceous sediments, and (3) the Southern Part which is characterized by a thick crust and which may contain old continental crust fragments. Available ages on sediments and lavas from several ODP legs indicate a short interval (110-115 Ma) for the formation of the uppermost igneous crust constituting the Kerguelen plateau.

rÉsUMÉ. - le plateau volcanique de Kerguelen : la deuxième plus grande province magmatique océanique terrestre et un témoin de l’ouverture de l’océan indien. l’histoire géologique et les données géomorphologiques et géophysiques acquises sur le plateau de Kerguelen sont résumées à partir des résultats des vingt dernières années des programmes ocean Drilling program (oDp) et des pro- grammes de l’institut polaire Français paul emile Victor (ipeV) sur les îles subantarctiques de l’océan indien. Ce plateau appartient à la partie océanique de la plaque Antarctique, entre le 46e et le 64e parallèle Sud, et il est situé à mi chemin de l’Afrique et de l’Australie. C’est une grande province ignée engendrée par l’activité d’un point chaud qui pourrait être actuellement localisé sous l’île de heard (volcan actif Big Ben). la naissance de ce plateau remonte à 115 Ma à proximité de la ride sud-est indienne, alors que depuis 43 Ma il s’est séparé de Broken Ridge pour acquérir la position intraplaque actuelle. Ce plateau comprend trois parties : (1) le plateau océanique nord qui porte les îles Kerguelen, (2) la partie centrale qui est un basin sédimentaire d’âge Crétacé séparant les îles Kerguelen et Heard, et enfin (3) la partie sud, caractérisée par une croûte épaisse marquant la transition avec la partie continentale de la plaque Antarctique. les âges des sédiments et des laves de différents forages ODP indiquent un intervalle de temps très court (110-115 Ma) pour la formation de la partie magmatique supérieure de la croûte.

Key words. - Kerguelen plateau - Volcanism - ocean drilling - Geology - plate tectonics - Gondwana break-off.

the Kerguelen plateau, located in the Antarctic plate plateau is a large igneous province (lip) made up of thick oceanic domain, extends at 2300 km between 46 and 64°S sequences of lava flows rising 2000 to 3000 m above the (Fig. 1) and represents a large structural height in the south- seafloor. Seismic data reveal that this part of the Kerguelen- ern Indian Ocean. With dimensions of 500 km by 2100 km, it Gaussberg ridge has a 14-23 km thick oceanic crust, thicker covers around 2 x 106 km2, and is the second largest oceanic than that of the northern Kerguelen plateau (Recq and Char- plateau after Ontong-Java in the Pacific Ocean (Coffinet al., vis, 1986; Charvis et al., 1993). 1999; 2002; Delpech, 2004). The combined magmatic vol- ume of the Kerguelen plateau and the Broken Ridge plateau Geological evolution: geomorphological, geophysical and is approximately 57.106 km3. the Kerguelen plateau corre- geochemical data sponds to the northwest extension of the Kerguelen-Gauss- berg aseismic ridge, which extends southward close to the Structure, stratigraphy and age of the Kerguelen Plateau Antarctic continent with a nW-Se trend parallel to that of the Kerguelen plateau is subdivided into three domains the South east indian Ridge (SeiR) (Fig. 1). the Kerguelen and radiometric ages young northward (Figs 2 and 3):

(1) Université Jean Monnet Saint-etienne, membre pReS Université de lyon, UMR CnRS 6524 “Magmas et volcans”, 42023 Saint-Etienne cedex, France. [[email protected]] (2) UMR CNRS 8148 IDES, Université Paris Sud-XI, 91405 Orsay cedex, France. [[email protected] ] * Corresponding author [[email protected]]

The Kerguelen Plateau: marine ecosystem and fisheries: 29-42. Geology of the Kerguelen Plateau cottin et al.

Figure 2. - Bathymetry of the Kerguelen plateau (Coffin et al., 1999; Proceedings of the ODP Leg 183 Initial Reports).

started about 120 Ma ago (Coffin et al., 2002). However, Figure 1. - Geographic map of the indian ocean showing the geo- Gagnevin et al. (2003) described explosive volcanic prod- dynamic position of the Kerguelen plateau and related geological ucts in the Rallier du Baty peninsula dated at 26,000 years features (after Kent et al., 2002). Bp. the recent discovery of pumice layers covering glacial deposits allows the estimation of the last volcanic activity of i. the northern part of the Kerguelen plateau (nKp) the Kerguelen Archipelago to be historical. extends from 46 to 53°S. The altitudes of the Kerguelen ii. the Central part of the Kerguelen plateau (CKp) is islands, which are the apex of this part of the plateau, are a basin located between Kerguelen and heard-McDonald mostly lower than 1000 m and the highest summit reaches Islands (~50-55°S). Sediment ages are Cretaceous (Schlich, 1852 m above sea level (Mount Ross) (Fig. 2). 1982; Quilty et al., 1983; Munschy and Schlich, 1987), but On the northernmost edge of the NKP, at site 1140 (LEG younger than the southern part of the Kerguelen plateau with ODP 183), 40Ar/39Ar radiometric dating on drilled sub- Santonian age (~85 Ma) for cherts and calcareous oozes marine basaltic rocks yielded ages of 34-35 Ma (Duncan, (Frölich et al., 1983; Frölich and Wicquart, 1989). This age 2002), these ages can be considered as the youngest mag- is similar to those of the 83-88 Ma old lavas from Broken matic activity for the northern part of the plateau. the oldest Ridge (Duncan and Richards, 1991), an oceanic eW-trend- 40Ar/39Ar ages are given by basalts from Skiff Bank, located ing plateau located east of the Kerguelen plateau, although west of the Kerguelen islands, which are 68-69 Ma (Duncan, more recent 40Ar/39Ar dating on Broken Ridge basalts (Dun- 2002). Therefore, the timing of formation of the NKP can be can, 2002) yielded an older age of 95 Ma at site 1141/1142. constrained between 34 and 69 Ma. On the southern part of the CKP, at site 1138 (Leg ODP 183), Radiometric ages for the exposed Kerguelen islands 40Ar/39Ar ages on drilled basalts underlying the sediments basalts are younger and range from 30 to 0.1 Ma (Nico- yielded an age estimate of ~100-101 Ma (Duncan, 2002). laysen et al., 2000). These volcanic rocks are related to the Seismic data reveal that this part of the Kerguelen-Gauss- youngest magmatic activity of the Kerguelen plume, which berg ridge has a 14-23 km thick oceanic crust, thicker than

30 The Kerguelen Plateau: marine ecosystem and fisheries cottin et al. Geology of the Kerguelen Plateau

Figure 3. - Gravity map of the different parts of the Kerguelen plateau and asso- ciated magmatism with the Kerguelen plume. the Kerguelen islands belongs to the northern part of the plateau and heard island to the central part of the plateau. Map from Weis et al. (2002), after Coffin et al. (1999, 2002). The locations of the ODP legs (sites 738, 744, 745, 746, 747, 749, 1137, 1138, 1140) are also shown. that of the northern Kerguelen plateau (Recq and Charvis, by 40Ar loss, resulting in an apparent older age. A reliable 1986; Charvis et al., 1993). conservative estimation on the timeframe of emplacement iii. the Southern part of the Kerguelen plateau (SKp) is of the southernmost SKP would thus be between 110 and located between 55 and 64°S. the crust beneath this part of 120 Ma (Coffinet al., 2002; Duncan, 2002). the plateau is about 25 km thick (Goslin and Diament, 1987) evidence from basalts and overlying sediments at sites and may contain old continental crust fragments (Ramsay et 738, 747, 748, 749 and 750 (Fig. 5), combined with the al., 1986; Alibert, 1991; operto and Charvis, 1995, 1996). results of subsidence modelling (Coffinet al., 1999) shows Available ages from sediments and lavas from several oDp that much of the magmatic rocks of the SKp and CKp were legs show that the uppermost igneous crust in this region erupted in a sub-aerial environment. Multi-channel seis- formed during a relatively short interval at ~110-115 Ma mic reflection profiles indicate that the volcanic rocks at all (leclaire et al., 1987; Schlich et al., 1989), although more sites except site 748 (Raggat Basin) were recovered from recent 40Ar/39Ar dating by Duncan (2002) gave an age of the uppermost igneous basement of the plateau, which lies ~118-119 Ma. these older ages seem to have been affected beneath younger sedimentary cover (Coffin et al., 1999).

The Kerguelen Plateau: marine ecosystem and fisheries 31 Geology of the Kerguelen Plateau cottin et al.

Figure 4. - Measured and predicted seafloor topography of Indian Ocean from Smith and Sandwell, 1997. NKP: North Kerguelen Plateau, CKp: Central Kerguelen plateau, SKp: South Kerguelen plateau, eB: elan Bank, SeiR: South east indian Ridge, SWiR: South West indian Ridge, BR: Broken Ridge.

Figure 5. - Summary of drill logs which recovered volcanic rocks on the Kerguelen Plateau and Broken Ridge (Coffin et al., 1999; pro- ceedings of the ODP, Leg 183). Data are shown for sites 738, 747, 748, 749 and 750 (Southern part), 1136, 1137 (Elan Bank), 1138, 1139 (Central Part) and 1140 (Northern Part), 1141 and 1142 (Broken Ridge) (See Figs 1, 2). Radiometric ages for basalts are shown in italics. Biostratigraphic ages of sediments overlying the volcanic basement are also indicated.

32 The Kerguelen Plateau: marine ecosystem and fisheries cottin et al. Geology of the Kerguelen Plateau

Basalts at site 748 were recovered 200 m above the seis- the crust beneath the CKp consists of three layers, with mically defined basement and overlie a poorly recovered an average thickness of 19-21 km. the upper layer is 1.2- zone that contains smectitic clay and highly altered basalt. 2.3 km thick with velocities from 3.8-4.9 km/s. The middle An unexpected result of the Leg 183 drillings (Coffin et layer is 2.3-3.3 km thick with velocities ranging from 4.7- al., 1999) was the finding of highly evolved, felsic vol- 6.7 km/s at the base of this layer. the lower layer is ~17 km canic, derived from the intense eruption of the latest mag- thick and seismic velocities range from 6.6-7.4 km/s at the matic activity of the northern part of the Plateau (sites 1138, base of the crust (Charvis et al., 1995). Heard Islands and 1139, Skiff Bank) and in its central part the crust beneath the SKp consists of an upper crus- (site 1137, Elan Bank). They occur as thin sanidine-rich tuffs tal layer ~5.3 km thick with velocities ranging from 3.8- intercalated between basaltic lavas flows. 6.5 km/s and a lower crustal layer ~11 km thick with veloci-

Crustal structure beneath the Kerguelen Plateau and Archipelago

The Kerguelen Plateau Many geophysical studies were carried out on the Kerguelen pla- teau to determine the structure of the crust beneath the plateau and beneath the Kerguelen islands (Bra- dley and Frey, 1988; Recq et al., 1990; Charvis et al., 1995; operto and Charvis, 1996). importantly, these geophysical studies indicate that the crustal structure beneath the Kerguelen islands is not repre- sentative of the whole Kerguelen plateau (e.g., Charvis et al., 1995). these studies also highlighted the different structure and nature of the crust beneath the northern Kergue- len plateau and Southern Kergue- len plateau. the northern Kergue- len plateau displays typical oceanic crust characteristics, although it is thicker than the average oceanic crust (e.g., Bradley and Frey, 1988; Schaming and Rotstein, 1990). Geophysical data also indicate that the crust beneath the SKp contains older continental crust fragments (operto and Charvis, 1996). the crust beneath the nKp consists of two layers, an upper oceanic layer 2 with a thickness of ~8-10 km (average seismic p-Waves veloci- ties (Vp) of 5.5 km/s; Recq and Charvis, 1986; Recq et al., 1990; Charvis et al., 1993) and a lower, thinner oceanic crust layer 3 with a thickness ranging from 4-10 km, Figure 6. - (a) Vp (km/s) versus Depth (km) and (b) the architecture of the lithospheric mantle and an average Vp = 6.6 km/s. beneath Kerguelen islands. After Grégoire et al. (2001).

The Kerguelen Plateau: marine ecosystem and fisheries 33 Geology of the Kerguelen Plateau cottin et al.

Figure 7. - plate reconstruction model for the Kerguelen Archipelago and its present oceanic intraplate position within the Antarctic plate (after Coffinet al., 2002). K = Kerguelen islands, H = Heard Island, IND = India, AUS = Australia, NP = Naturalist Plateau, ANT = Antarc- tic, BB = Bunbury Basalt, lAM = lamprophyres of Rajmahal traps, BR = Broken Ridge, CKp = Central Kerguelen plateau, SKp = South Kerguelen plateau, nKp = north Kerguelen plateau.

34 The Kerguelen Plateau: marine ecosystem and fisheries cottin et al. Geology of the Kerguelen Plateau ties of 6.6-6.9 km/s. A seismically reflective transition zone sitional zone representing a layered crust-mantle boundary of 4-6 km with velocities of 6.7-6.9 km/s has been recog- (e.g., Recq et al., 1990; Grégoire et al., 1994; 1998; Figs 2, nised at the base of the crust (operto and Charvis, 1995, 3). Grégoire et al. (2001) obtained geochemical and petro- 1996). this transition zone was not recognised beneath the physical data on a set of representative deep-seated mantle nKp and the CKp. it was interpreted by operto and Charvis xenoliths. the comparison of available geophysical data (1996) as a stretched continental crust fragment overlain by with the results of Grégoire et al. (2001) led these authors basaltic flows and isolated from the Antarctic margin during to propose a model for the crust-mantle boundary under the the early opening of the indian ocean. Kerguelen Islands (Fig. 6). The 2-3 km transition zone is best explained by varying proportions of two-pyroxene granulites The Kerguelen archipelago and ultramafic rocks such as peridotites (e.g., harzburgites), The Kerguelen Islands are located 1500 km southwest of with the proportion of ultramafic rocks increasing with the Southeast Indian Ridge and 500 km north of the Heard depth, down to ~30 km beneath the surface. The occurrence and McDonald islands. they represent the northern apex of of garnet-bearing granulites in the mantle xenoliths suite fur- the Kerguelen-Gaussberg ridge and around 2% of the volume ther indicates that at depths greater than ~30 km (where gar- of the entire Plateau (Fig. 2 and 3). These 400 islands have net would be stable in the granulitic assemblage), deep-seat- a total surface of 6500 km2, and thus form the third largest ed mafic magmas are intercalated in the lithospheric mantle oceanic archipelago in the world, after iceland and hawaii. column down to ~55 km below the surface. the petrophysi- plate tectonic reconstructions indicate that the Kerguelen cal data obtained by Grégoire et al. (2001) also provide an Islands formed around 44-43 Ma on or near the SEIR, in a explanation for the lateral variations observed in the velocity mid-oceanic ridge setting (Fig. 1). At this time, the Kergue- gradient and for the low p-wave velocities (7.7-7.95 km/s) len plume was situated close to the SeiR. the geodynamic of the upper mantle under the Kerguelen Archipelago (Recq setting of the Kerguelen islands 44 Ma ago is thus similar to et al., 1990). These variations may be explained by variable the present geodynamic setting of iceland, where the iceland proportions of mafic (garnet-bearing granulites and garnet- plume is superimposed onto the mid-oceanic Atlantic ridge. bearing pyroxenites) and ultramafic (harzburgites) litholo- the Kerguelen islands occupy an intraplate position since gies within the upper mantle, at least down to 55 km below the SEIR moved away from Kerguelen Plateau 43 Ma ago. the surface (the lowest depth determined for garnet-bearing the present day setting of the Kerguelen islands is therefore granulites from geothermobarometric considerations). the similar to that of the hawaiian islands. With a Magmatic calculated density of the harzburgite from Grégoire et al. activity that started about ~30 Ma to present day, the Ker- (2001) is 3.34 g/cm3 and is significantly lower than the den- guelen islands are the oldest oceanic islands in the world in sity of young subcontinental lithospheric mantle (3.37 g/ the youngest ocean (Giret et al., 1999). cm3; poudjom Djomani et al., 2001). The calculated densi- Geophysical data, coupled with petrological and geo- ties for garnet-bearing granulites and pyroxenites (3.27 g/ chemical data, allows us to propose a model for the crust cm3) are lower than the calculated density of the mantle harz- and upper mantle structure beneath the Kerguelen Archi- burgites. therefore, the mixture of garnet-bearing granulites pelago. especially, deep-seated mantle and crustal xenoliths and pyroxenites and mantle harzburgites in the lithospheric from Kerguelen islands, mainly granulites and peridotites column beneath the Kerguelen Archipelago provides an (harzburgites), provide informations about the deep lithos- explanation for the relatively low seismic velocities (7.70- phere rock types and their spatial distribution. hence, their 7.95 km/s; Grégoire et al., 2001). study enables a geological interpretation of the structure of the crust and mantle under the Archipelago. the granulitic Geodynamic history of the Kerguelen Plateau xenoliths represent mafic magmas underplated at the crust- Based on the plate tectonic reconstructions from Royer mantle boundary or intruded within the mantle as a result of and Sandwell (1989) and Royer and Coffin (1992), the geo- the Kerguelen plume activity (Grégoire et al., 1998). dynamic history of the Kerguelen Plateau shown on figure 7 Seismic refraction studies indicate a thick oceanic crust can be summarised as follows: beneath the archipelago (Recq et al., 1990; Charvis et al., - During the Upper Jurassic (the oldest age recorded 1995). the crust consists of two main layers: an oceanic for the indian oceanic crust is 165 Ma), Gondwana began layer 2, which is 8-9 km thick and an oceanic layer 3 which to break-up into two lithospheric plates: east Gondwana is 5-10 km thick, and extends from a depth of ~10 km to a (Madagascar, Seychelles, india, Australia, Antarctic) and depth of 15-20 km at its base. A seismically different zone West Gondwana (Africa). ~2-3 km thick is however present beneath layer 3 under the - During the lower Cretaceous (~119 Ma), Australia and archipelago. the seismic velocities (Vp = 7.2-7.5 km/s) are Antarctica began to separate. lower than the usual Vp for the oceanic lithosphere at this - Around 115-110 Ma, the southern part of the Kergue- depth (~8 km/s). this zone has been interpreted as a tran- len plateau formed just near the new oceanic ridge separat-

The Kerguelen Plateau: marine ecosystem and fisheries 35 Geology of the Kerguelen Plateau cottin et al.

Figure 8. - Simplified geological map of the Kerguelen archipelago. From Nougier (1970), modified after Michon, Cottin, Giret (Richet et al., 2007) ing Australia/Antarctica and india (SeiR). the Rajmahal plateau. the Kerguelen plateau and the Broken Ridge began traps in the north east india, also formed during this period to separate and the SeiR moved away from the Kerguelen (108 Ma), have been interpreted as the first surface expres- plume. Since that time, magmatism on Broken Ridge pro- sion of the Kerguelen plume (Ray et al., 1999). this first gressively decreased, but magmatic activity on the Kergue- stage of the formation of the plateau is thus similar to the len plateau reached its climax. it allowed a rapid growth present-day geodynamic setting of the African Rift. of the plateau, resulting in the formation of the Kerguelen - Around 85 Ma, the central part of the plateau formed islands. At this time, the Kerguelen hotspot was located together with the Broken Ridge plateau just near the SeiR or beneath the Kerguelen Islands. Seafloor spreading between superimposed to the SeiR. A Major plate boundary reorgani- Australia and Antarctica accelerated to a rate of 22 mm/yr. sation probably occurred during the mid-Cretaceous, leading - After 43 Ma, the volcanic activity was restricted to the to a subdivision into four new tectonic plates in the indian northern part of the plateau and in the southern part of the ocean; Africa/Madagascar, india/Seychelles, Australia and Ninety-East Ridge. Since 43 Ma, volcanism has been con- Antarctica. tinuous on Kerguelen islands and huge volumes of magma - Around 72 Ma, a major tectonic event stretched the have been emplaced. the last volcanic event (in the Rallier basement of the plateau and allowed the formation of the du Baty Peninsula) is dated at 26,000 years BP (Gagnevin 77°e Graben. this event was associated with the accelera- et al., 2003). Nevertheless, it is likely that the last volcanic tion of the displacement of india towards the north and also events occurred a few thousand years ago, and even possibly with the opening of a fracture system between Australia and during the historic period, particularly in the Rallier du Baty Antarctica. peninsula. Currently, only fumarolic activity is documented - Around 64 Ma, seafloor spreading continued between on the western side of the Rallier du Baty peninsula, as well Australia and Antarctica (SeiR) and the transform move- as hot springs in Val travers. the Kerguelen hotspot seems ment continues between the northern and the southern presently located beneath the heard islands as evidenced by parts of the Kerguelen plateau. the ninety-east Ridge and the episodic volcanic activity of the Big Ben volcano. the northeastern part of the Kerguelen plateau were then emplaced over the Kerguelen hotspot. Geochemical characteristics of the magmatism of the - Around 44-43 Ma, the transform movement ended Kerguelen Plateau between the northern and southern parts of the Kerguelen the northern and western parts of the archipelago have a

36 The Kerguelen Plateau: marine ecosystem and fisheries cottin et al. Geology of the Kerguelen Plateau

more elevated topography than its southern part. the south- ern part is affected by subsidence associated with the sedi- mentary basin which separates the Kerguelen Archipelago from the heard islands. The flood basalts represent 80-85% of the volume of the Kerguelen islands. they outcrop as thick sequences of typi- cal flood basalts forming cliffs up to 1,000 m high, and they range from tholeiitic to alkaline types (Gautier, 1987; Storey et al., 1988; Gautier et al., 1990; Weis et al., 1993; Delpech 2004). These basalts are sub-horizontal with 2-3° dip toward the southeast. their compositional variations have been interpreted as the result of variable mixing between a depleted and an enriched sources. they evolve through time towards alkaline and highly alkaline compositions. this temporal evolution suggests an increasing contribution of the enriched plume component. Available radiometric dat- ings for the Kerguelen Archipelago are shown in figure 8. The oldest flood basalts are transitional between tholeiitic and alkaline basalts. they were emplaced during the plume- ridge interaction stage around 38 Ma ago (Weis et al., 1998a), which prevailed until 25 Ma. they form the basement of the northern part of plateau and are mostly found in the north- western part of the Archipelago. From 25 Ma to historic times, the erupted flood basalts rather show an alkaline affini- ty. they were emplaced in an intraplate oceanic setting above the Kerguelen plume. the alkaline series display two distinct trends. the first trend is characterised by mildly alkaline basalts (2.5 < na2o + K2o < 5 wt%) and the second is highly alkaline (4 < na2o + K2o < 8 wt%) (Gautier et al., 1990; Weis et al., 1993). The most alkaline lavas are restricted to the southeastern part of the archipelago (Se province; Jeanne d’Arc and Ronarc’h peninsulas and Mount Ross). the vol- canic alkaline rock types range from basalts to trachytes for the mildly alkaline series and from basanites to phonolites for the highly alkaline lavas (Weis et al., 1993; Delpech, 2004). these two series also display different ages of emplacement: the mildly alkaline lavas are older (lower Miocene Series; 20-22 Ma) than the highly alkaline ones (Upper Miocene series; 6.6-10.2 Ma) (Weis et al., 1993; Delpech, 2004). the occurrence of numerous volcano-plutonic complex- es (large bodies of plutonic rocks) intruding the flood basalts sequences is quite unusual for oceanic islands. it is one of Figure 9. - Sr-nd-pb isotopic compositions of the Kerguelen pla- the striking features of the Kerguelen islands (Marot and teau and archipelago volcanics. A: 87Sr/86Sr versus 143nd/144nd Zimine, 1976; Giret, 1983; Gagnevin et al., 2003). In con- 206 204 207 204 isotopic compositions; b: pb/ pb versus pb/ pb; c: trast to the flood basalts, volcano-plutonic complexes exhibit 208pb/204pb versus 206pb/204pb. Black circles denote basalts from heard island (Barling and Goldstein, 1990). Data sources for Indian highly evolved rocks including granites and nepheline-bear- MORB from Michard et al. (1986), Price et al. (1986), Dosso et al. ing syenites. two series have been recognised similarly to (1988), Schiano et al. (1997). Data for Kerguelen volcanics from Storey et al. (1988), Gautier et al. (1990), Weis et al. (1993), Weis the case of the flood basalts. the first one is of tholeiitic- et al. (1998a, 1998b), Yang et al. (1998), Frey et al. (2000), Doucet transitional type and the second is alkaline. the tholeiitic- et al. (2002), Frey et al. (2002). Data for Leg ODP 120 (sites 747, transitional plutonic series is characterised by sills of gab- 749, 750) from Frey et al. (2002); data for Leg ODP 183 (site 1140) from Weis and Frey (2002), site 1137 (Weis et al., 2001; Ingle et bros, which intrude the flat-lying basalt sequences and are al., 2002), site 1139 (Kieffer et al., 2002). Data are age-corrected usually small (0.5-1 km in diameter) (Giret, 1983; Weis and for in situ Rb, Sm and U-th decays. Giret, 1993; Scoates et al., 2007; Chevet, 2009). The alka-

The Kerguelen Plateau: marine ecosystem and fisheries 37 Geology of the Kerguelen Plateau cottin et al. line plutonism began around 25 Ma ago and continued until tle reservoir similar to the indian ocean Mid-ocean Ridge historic times (Gabbros from Mount Ross: 0.2 Ma; Weis Basalt (MoRB) source (Storey et al., 1988; Davies et al., and Giret, 1993). The size of these alkaline volcano-pluton- 1989; Weis and Frey, 2002). Neal et al. (2002) and Ingle et ic complexes varies from 1-15 km and they form annular al. (2002) suggested that 100 Ma old basalts dredged from structures in which different structural levels outcrop. the site 1138 (Figs 1, 2) on the Central Kerguelen Plateau may two most important intrusions are the Mount Ross intrusion represent the Kerguelen plume-head isotopic composition (Giret et al., 1988) in the southern part of the plateau Cen- with 206pb/204pb ~18, trace element ratios and Sr, nd, and hf tral, and the Rallier du Baty intrusion in the southwest part isotopic compositions similar to primitive mantle estimates, of the islands (lameyre et al., 1976; Gagnevin et al., 2003). with little or no influence of a continental component. in Similarly to the case of the flood basalts, these alkaline vol- the southern part of the Kerguelen Plateau, sites 1136, 738, cano-plutonic complexes display two different evolution 750, 749 Plateau basalts (Figs 1, 2) display lower Pb isotopic trends. The first one is characterised by silica-oversaturated compositions and trace elements ratios which are inferred rocks and the second by silica-undersaturated rocks. in the to be indicative of assimilation of old continental crust dur- “oversaturated series”, extrusive rocks evolve from basalts ing ascent of the magmas en route to the surface (e.g., Frey to trachytes and rhyolites and plutonic rocks from gabbros et al., 2002) ~120 Ma ago. Assimilation of continental crust to alkaline granites. Volcano-plutonic complexes belonging material by plume melts is also supported by the discovery to this series are mainly restricted to the west of the archi- of garnet biotite gneiss clasts in the Elan Bank Leg 183 site pelago (Rallier du Baty, îles nuageuses, île de l’ouest). in 1137. These clasts were dated between 534 Ma and 2547 Ma the “undersaturated series”, rocks evolve from basanites to (nicolaysen et al., 2000), suggesting that old continental phonolites for volcanic rocks and from gabbros to nephe- crust pieces were incorporated into the shallow Kerguelen linic syenites for plutonic rocks. they are only found in the oceanic lithosphere during the Gondwana breakup. however, central and eastern provinces of the Archipelago (Société de the geochemical characteristics of 34 Ma old Plateau basalts Géographie, Mont Ross, Montagnes Vertes, Monts Ballons). from the NKP (site 1140) do not display any evidence for a the Mont Ross volcano-plutonic complex is the most recent component derived from continental crust, but only for mix- intrusion as most of its plutonic and volcanic rocks are less ing between a SeiR MoRB-type component and a Kergue- than 1 Ma old (Giret et al., 1988). len plume component (Weis and Frey, 2002). The Kerguelen Archipelago volcanic rocks have isotopic ratios that show a Isotopic characteristics of volcanic rocks and the nature of wide range in composition; the 87Sr/86Sr ratios range from Kerguelen mantle sources 0.70394 to 0.70633 and the 143nd/144nd ratios vary from Many major, trace elements and isotopic studies have 0.51249 to 0.51279. For Pb isotopes, 206pb/204pb ranges been carried out on the Kerguelen flood basalts (Storeyet al., from 18.025 to 18.607, 207pb/204pb from 15.47 to 15.59 and 1988; Gautier et al., 1990; Weis et al., 1993, 1998a, 1998b; 208pb/204Pb from 38.36 to 39.21 (Gautier et al., 1990; Weis et Delpech, 2004, Delpech et al., 2004), on the Kerguelen Pla- al., 1993, 1998a, 1998b; Yang et al., 1998). in contrast to the teau basalts (Davies et al., 1989; Mahoney et al., 1995, as plateau basalts from the SKp, the Kerguelen archipelago vol- well as on the main geological structures related to the Gond- canics do not show any evidence for continental crust assimi- wana breakup (Broken Ridge, naturaliste plateau, ninety- lation (Fig. 9). east Ridge, and the SeiR and the South West indian Ridge). the oldest tholeiitic basalts (29-28 Ma) exhibit the less Most of these studies concerned the Sr, nd and pb isotopic enriched signature with 87Sr/86Sr, 143nd/144nd and pb iso- systems (e.g., Gautier et al., 1990; Weis et al., 1993, 1998a, topic ratios requiring a depleted asthenospheric component 1998b). More recently, data on the os and hf isotopic systems in their source, such as for the SeiR MoRBs (Storey et al., were obtained (Yang et al., 1998; ingle et al., 2002; Mattielli 1988; Gautier et al., 1990; Doucet et al., 2002). However, et al., 2002; Delpech, 2004; Janney et al., 2005). Kerguelen Yang et al. (1998) attributed the heterogeneous depleted plateau basalts have been dredged from several oDp leg sites component in the 29 Ma Mont Bureau and Mont Raboullière on the southern to the northern part of the Kerguelen plateau basalts to assimilation of gabbroic cumulates from the SeiR (e.g., site 1140 on the northern Kerguelen Plateau, sites 1136, by plume melts through ascent, leading to lower pb and Sr 738, 749 and 750 on the southern Kerguelen Plateau, and and higher nd isotopic compositions. in contrast, the young- sites 747 and 1138 on the central part of the Kerguelen Pla- er alkaline to highly alkaline series display more enriched teau, Figs 1 and 2). The first Kerguelen Plateau basalts (and signatures, pointing towards enriched geochemical reser- Broken Ridge basalts) formed when the Kerguelen plume voirs defined by Zindler and Hart (1986). These are predom- head impinged upon the recently formed indian ocean litho- inantly eM ii in an Sr-nd isotopic space and mixes of eM i sphere at ~115 Ma (Frey et al., 2002). The geochemical char- and eM ii in pb isotopic systems (Fig. 9). the eM i enriched acteristics of the plateau basalts indicate that their sources signature is given by the high 87Sr/86Sr and low 143nd/144nd derived from mixing of plume materials with a depleted man- and is attributed to an ocean island Basalt (oiB) component

38 The Kerguelen Plateau: marine ecosystem and fisheries cottin et al. Geology of the Kerguelen Plateau carried by the Kerguelen plume. Gautier et al. (1990) and hemisphere Reference line (nhRl) for the indian MoRBs Weis et al. (1993) suggested that a two component mixing in a pb-pb isotope space. this Dupal anomaly is the larg- between a depleted indian MoRB component and the Ker- est isotopic anomaly in the indian ocean and South Atlantic guelen plume-type component can account for the evolution ocean. Rehkämper and hofmann (1997) suggested that the of Sr, nd and pb isotopic compositions among the archi- isotopic characteristics of the indian ocean MoRBs, cou- pelago lavas at their time of eruption. however, Yang et al. pled with variations in specific trace element ratios (Nb/U, (1998) ruled out this mixing model since the 29 Ma Mont Ce/pb), provide evidence for recycling of old altered ocean- Bureau and Mont Raboullière basalts have isotopic com- ic crust (~1.5 Ga old) and variable amounts of pelagic sedi- positions encompassing almost all the Sr-nd isotopic range ments in the source of indian MoRBs. Weis and Frey (1996) of the Kerguelen archipelago lavas, whatever their time of noticed that the first occurrence of the Dupal anomaly in the eruption. in fact, the depleted indian MoRB component is Indian Ocean appears to coincide with the first manifestation only seen in the 29-26 Ma old flood basalts from the Loran- of the Kerguelen plume in generating the Kerguelen plateau chet peninsula (Weis et al., 1993; Yang et al., 1998; Doucet 110-118 Ma ago. They suggested that the Dupal anomaly is et al., 2002), whereas young flood basalts erupted before a deep-seated isotopic anomaly residing in the Kerguelen 25 Ma mostly carry a major plume-type imprint. Weis et al. plume and that the Kerguelen plume is responsible for this (1998b) suggested that the Kerguelen plume composition is specific isotopic signature in the Indian Ocean (Weiset al., characterised by moderately radiogenic pb isotopic compo- 1989, 1991; Weis and Frey, 1996). Castillo (1988) suggested sitions (206pb/204Pb ~18.6) and εNd ~0 to -2, and is best rep- that the Dupal anomaly possibly finds its origin in the lower resented by the lavas belonging to the 25 Ma basalts from mantle and may be subsequently carried to the surface by the Mount Crozier. Based on hf-nd-pb isotope systematics mantle plumes. Mattielli et al. (2002) and Ingle et al. (2002) of Kerguelen Archipelago lavas, Mattielli et al. (2002) also discussed the origin of the Kerguelen plume signature on the suggested that the Mount Crozier basalts represent melts basis of nd-pb-hf and pb-hf isotopes, respectively. Mattiel- from the Kerguelen plume core (with εHf 1.4 to 3.8). How- li et al. (2002) interpreted the isotopic signature of the 25 Ma ever, the marked change in pb and hf isotopic compositions Mount Crozier basalts as representative of the Kerguelen in the youngest archipelago lavas suggests a major change in plume core composition, where the Dupal anomaly resides. the source composition of the young volcanics (Mount Ross They find isotopic evidence that the EM I (Enriched Mantle) and Upper Miocene Series, < 10 Ma). The εNd-εHf array for component could be attributed to recycled oceanic crust or these young lavas is parallel to the array for the older flood plateaus (Gasperini et al., 2000), which would explain the 87 86 basalts, but evolving towards lower εhf. values (εhf = -0.8 radiogenic Sr/ Sr and moderately radiogenic pb isotopic to –3.5). Mattielli et al. (2002) attributed these changes to compositions found in the Mount Crozier basalts. however, a major decrease in melting degrees, from large degrees of ingle et al. (2002) described Nd-Pb-Hf isotopic variations melting of the plume head before 20 Ma to small degrees in the Kerguelen plume composition (based on the Kergue- of melting of the outer zone of the plume (low 206pb/204pb len archipelago volcanics) by different mixing proportions isotopic composition of ~18-18.2). Based on hf and pb iso- between pelagic sediments and recycled oceanic crust in topic systematics, ingle et al. (2002) argued that the basalts the Kerguelen plume, following the idea of Rehkämper and from the Mount Crozier best represent the Kerguelen plume hofmann (1997) for indian ocean MoRBs. the current lit- tail composition. they interpreted the lower 206pb/204pb and erature supports the idea that the Dupal anomaly is an intrin- 176hf/177Hf isotopic compositions of the < 10 Ma lavas in sic characteristic of the Kerguelen plume source composi- terms of assimilation of Kerguelen plateau materials, during tion, and that this isotopic anomaly is present in most of the ascent to the surface. therefore, a simple temporal evolution Indian seafloor basalts. Recycling in the mantle of either old with a two mixing component process, where the propor- oceanic lithosphere in the mantle or portions of oceanic pla- tion of an ‘enriched source’ (the Kerguelen plume) increases teaus, along with pelagic sediments, later incorporated in the through time cannot solely account for the whole variation Kerguelen mantle plume, are suggested as a possible origin of isotopic ratios of Kerguelen islands and plateau (Chevet, of the Dupal characteristics of the Kerguelen plume source 2009), and of Heard Islands and Ninety-East Ridge as well. (ingle et al., 2002; Mattielli et al., 2002).

Kerguelen plume characteristics and the peculiar geochemical DUPAL anomaly of the Indian Ocean cOncLUsIOns magmatic rocks The Dupal anomaly (Dupré and Allègre, 1983; Hart, As shown by geophysical studies, the Kerguelen pla- 1984) is characterised by high initial 87Sr/86Sr, 208pb/204pb, teau experienced different geodynamic environments. First, 207pb/204pb for a given ‘low’ 206pb/204pb, and low the birth of the Kerguelen plateau 115 Ma ago occurred in a 143nd/144nd. it causes the shift to the right of the northern mid-oceanic ridge setting, just near or on top of the SeiR.

The Kerguelen Plateau: marine ecosystem and fisheries 39 Geology of the Kerguelen Plateau cottin et al. the Kerguelen plateau evolved in such a context for about BRADleY l.M. & FReY h., 1988. - Constraints on the crustal 100 Ma, until the separation between the Kerguelen Pla- nature and tectonic history of the Kerguelen plateau from com- parative magnetic modelling using MaGSAt data. Tectonophy- teau and the Broken Ridge 44-43 Ma ago. Then, the SEIR sics, 145: 243-251. moved away northward from the Kerguelen plateau, leaving ChARViS p., opeRto S., KonneCKe l.K., ReCQ M., hello the plateau in its present intraplate position. the geological Y., hoUDRY F., leBelleGARD p., loUAt R. & SAGe F., evolution of the Kerguelen plateau records a long history 1993. - Structure profonde du domaine nord du plateau de Ker- guelen (océan indien austral) : résultats préliminaires de la of interactions between two major geological structures, a campagne MD66/KeoBS. C. R. Acad. Sci. Paris, 316: 341- mantle plume and a mid-oceanic ridge. evidences for this 347. complex history are carried by the geochemical signatures ChARViS p., ReCQ M., opeRto S. & BReFoRt D., 1995. - of the erupted lavas. the geodynamic setting was favourable Deep structures of the northern Kerguelen plateau and hotspot- related activity. Geophys. J. Int., 122: 899-924. to interactions between two geochemical mantle reservoirs, CHEVET J., 2009. - Gabbros, dolerites and associated ultramafic the enriched mantle source of the Kerguelen hotspot and the cumulates from the thickened oceanic crust of the Kerguelen depleted mantle source of the indian oceanic asthenosphere. Archipelago. thèse de Doctorat d’État, 165 p. Cotutelle Mac- especially, these interactions occurred during the genesis of quarie Univ., Australie et Univ. Saint-etienne, France. both the Kerguelen plateau and the ninety-east Ridge, when CAStillo p., 1988. - the Dupal anomaly as a trace of the the hotspot was close to the spreading ridge, between 115 upwelling lower mantle. Nature, 334: 667-670. and 43 Ma. Moreover, the continental components revealed CoFFin M.F., FReY F.A., WAllACe p.J. et al. (24 authors), 1999. - Kerguelen plateau-Broken Ridge: a large igneous prov- by the geochemistry of the Kerguelen plateau basalts sug- ince. Sites 1135-1142. In: proc. oDp, initial Reports: Covering gest that the Gondwana mantle lithosphere was mixed with Leg 183 of the Cruises of the Drilling Vessel Joides Resolution. asthenospheric components of the Kerguelen plume (hot Vol. 183, 101 p. Texas A & M Univ., USA. spot) 115 Ma ago. Similar evidences are documented in the CoFFin M.F., pRinGle M.S., DUnCAn R.A, GlADCZenKo t.p., StoReY M., MUelleR R.D. & ChAhAGAn l.A., indian ocean, e.g., on the Southwest indian Ridge (Mahoney 2002. - Kerguelen hotspot magma output since 130 Ma. J. Pet- et al., 1991) where interactions between continental Mada- rol., 43(7): 1121-1139. gascar lithosphere and the Marion plume occurred 80 Ma DAVieS h.l, SUn S.S., FReY F.A., GAUtieR i., McCUlloCh ago. Another important consequence of the emplacement M.t., pRiCe R.C., BASSiAS Y., KlootWiJK C.t. & of the Kerguelen plateau-Broken Ridge system is the erup- leClAiRe l., 1989. - Basalt basement from the Kerguelen plateau and the trail of a Dupal plume. Contrib. Mineral. tion of huge volumes of basaltic magmas in a short period of Petrol., 103: 457-469. time. this process probably had effects on the climate and DELPECH G., 2004. - Études des enclaves ultramafiques des îles atmosphere chemistry during the start of the building of the Kerguelen: caractéristiques du métasomatisme sous un plateau Plateau but also during its final (felsic subaerial events) evo- océanique. Thèse de Doctorat d’État, 404 p. Cotutelle Macqua- rie Univ., Australie et Univ. Saint-etienne, France. lution. DelpeCh G., GRÉGoiRe M., o’ReillY S., Cottin J.Y., MOINE B. & MICHON G., 2004. - Felspar from carbonate Acknowledgements. - We thank G. Duhamel for his proposal to rich metasomatism in the oceanic mantle under Kerguelen include geological topics in the symposium “the Kerguelen pla- islands (South indian ocean). Lithos, 75: 209-237. teau, Marine ecosystems and Fisheries” and André Giret, Bertrand DoSSo l., BoUGAUlt h., BeUZARD p., CAlVeZ J.Y. & Moine, Michel Grégoire, S.Y. o’Reilly and pascal Richet for con- JoRon J.l., 1988. - the geochemical structure of the South- structive discussions. We thank also R. pollard and R. Maury for east indian Ridge. Earth. Planet. Sci. Lett., 88: 47-59. their reviews and comments on the manuscript. For the fieldwork in Kerguelen island this research was supported by French polar DoUCet S., WeiS D., SCoAteS J.S., niColAYSen K., FReY institute (ipeV, Brest, France) tAAF and CnRS (UMR 6524-Mag- F.A. & GIRET A., 2002. - The depleted mantle component in mas et Volcans-) during the programs Cartoker (1990-2000), Geo- Kerguelen archipelago basalts: petrogenesis of tholeiitic-transi- tional basalts from the loranchet peninsula. J. Petrol, 43(7): chronoker (2001), Carbonatoker (2002) and Dylioker (2006-2010). 1341-1366. 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42 The Kerguelen Plateau: marine ecosystem and fisheries