Wise, S. W., Jr., Schlich, R., et al., 1992 Proceedings of ... · Ultramafic lamprophyres...

Wise, S. W., Jr., Schlich, R., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 120

2. LOWER CRETACEOUS VOLCANIC ROCKS ON CONTINENTAL MARGINS AND THEIRRELATIONSHIP TO THE KERGUELEN PLATEAU1

M. Storey,2 R. W. Kent,2 A. D. Saunders,2 V. J. Salters,3 J. Hergt,4 H. Whitechurch,5 J. H. Sevigny,6

M. F. Thirlwall,7 P. Leat,8 N. C. Ghose,9 and M. Gifford10

ABSTRACT

Widespread Lower Cretaceous magmatism occurred along the Indian-Australian/Antarctic margins, and in thejuvenile Indian Ocean, during the rifting of eastern Gondwana. The formation of this magmatic province probablybegan around 120-130 Ma with the eruption of basalts on the Naturaliste Plateau and at Bunbury, western Australia.On the northeast margin of India, activity began around 117 Ma with the Rajmahal continental basalts and associatedlamprophyre intrusions. The formation of the Kerguelen Plateau in the Indian Ocean began no later than 114 Ma.Ultramafic lamprophyres (alnoites) were emplaced in the Prince Charles Mountains near the Antarctic continentalmargin at ~ 110 Ma. These events are considered to be related to a major mantle plume, the remnant of which issituated beneath the region of Kerguelen and Heard islands at the present day.

Geochemical data are presented for each of these volcanic suites and are indicative of complex interactionsbetween asthenosphere-derived magmas and the continental lithosphere. Kerguelen Plateau basalts have Sr and Ndisotopic compositions lying outside the field for Indian Ocean mid-ocean ridge basalts (MORB) but, with theexception of Site 738 at the southern end of the plateau, within the range of more recent hotspot basalts fromKerguelen and Heard Islands. However, a number of the plateau tholeiites are characterized by lower 206Pb/204Pbratios than are basalts from Kerguelen Island, and many also have anomalously high La/Nb ratios. These featuressuggest that the source of the Kerguelen Plateau basalts suffered contamination by components derived from theGondwana continental lithosphere. An extreme expression of this lithospheric signature is shown by a tholeiite fromSite 738, suggesting that the southernmost part of the Kerguelen Plateau may be underlain by continental crust.

The Rajmahal tholeiites mostly fall into two distinct geochemical groups. Some Group I tholeiites have Sr andNd isotopic compositions and incompatible element abundances, similar to Kerguelen Plateau tholeiites from Sites749 and 750, indicating that the Kerguelen-Heard mantle plume may have directly furnished Rajmahal volcanism.However, their elevated 207Pb/204Pb ratios indicate that these magmas did not totally escape contamination bycontinental lithosphere. In contrast to the Group I tholeiites, significant contamination is suggested for Group IIRajmahal tholeiites, on the basis of incompatible element abundances and isotopic compositions.

The Naturaliste Plateau and the Bunbury Basalt samples show varying degrees of enrichment in incompatibleelements over normal MORB. The Naturaliste Plateau samples (and Bunbury Basalt) have high La/Nb ratios, afeature not inconsistent with the notion that the plateau may consist of stretched continental lithosphere, near theocean-continent divide.

INTRODUCTION

Kerguelen Plateau and Broken Ridge, which were orig-inally contiguous, have an estimated combined crustal volumeof approximately 57 × I06 km3, comparable to the OntongJava Plateau in the Pacific (Schubert and Sandwell, 1989). Theformation of this major volcanic edifice in the juvenile LowerCretaceous Indian Ocean was accompanied by magmatism

1 Wise, S. W., Jr., Schlich, R., et al., 1992. proc. ODP, Sci. Results, 120:College Station, TX (Ocean Drilling Program).

2 Department of Geology, University of Leicester, University Road,Leicester LEI 7RH, United Kingdom.

3 Lamont-Doherty Geological Observatory, Columbia University, Pali-sades, NY 10562, U.S.A.

4 Department of Earth Sciences, The Open University, Walton Hall, MiltonKeynes MK7 6AA, United Kingdom.

5 Ecole et Observatoire de Physique du Globe, 5 rue Descartes, 67084Strasbourg, France.

6 Department of Geology and Geophysics, University of Calgary, 2500University Drive N.W., Calgary, Alberta T2N 1N4, Canada.

7 Department of Geology, Royal Holloway and Bedford New College,Egham Hill, Egham, Surrey TW20 OEX, United Kingdom.

8 Department of Geological Sciences, University of Durham, ScienceLaboratories, South Road, Durham DH1 3LE, United Kingdom (presentaddress: British Antarctic Survey, High Cross, Cambridge CB3 0ET, UnitedKingdom).

9 Department of Geology, Patna University, Patna-800005, India.10 5/1 Dorset Street, Busselton, West Australia 6280, Australia.

along the continental margins of eastern Gondwana, proximalto the plateau. Specifically, continental basalts were eruptedin northeastern India in the Rajmahal region and in westernAustralia at Bunbury and, offshore, on the Naturaliste Pla-teau. Lamprophyre intrusions occur in India and Antarctica.These occurrences are summarized in Figure 1.

One of the main objectives of Ocean Drilling Program(ODP) Legs 119 and 120 was to recover basement materialthat would help test the various hypotheses that have beenproposed for the origin of the Kerguelen Plateau. However,any model for the origin of the plateau should also account forthe associated continental magmatism. Previously proposedmodels for the origin of the Kerguelen Plateau include:

1. Continental fragment: Dietz and Holden (1970) sug-gested that the plateau contains rifted continental crust, aremnant of the Gondwana supercontinent. Schlich et al. (1971)considered the sedimentary section of the northern plateau tohave continental affinities whereas Ramsay et al. (1986) sug-gested that the Southern Kerguelen Plateau, based on thegranitic material and metasediments in free-fall grab samples,contains continental crust.

2. Uplifted oceanic crust: Houtz et al. (1977) suggested thatthe Kerguelen Plateau was, in part, a remnant of Cretaceousocean basin crust that formed to the west of Australia duringthe separation of India from Australia-Antarctica.

33

M. STOREY ET AL.

_ Alkaline— lamprophyres— High-titanium

lamprophyres

^ Alnoites

• Alkalic ring complex

Tholeiitic basalts

Seaward-dipping reflectors

Figure 1. Reconstruction of eastern Gondwana at ca. 120 Ma showing the location and nature of continental Lower Cretaceous volcanismimmediately before, or contemporaneous with, the formation of the Kerguelen Plateau at ca. 110-115 Ma (modified from Kent, 1991). Also shownare the position of rift zones that acted as major drainage pathways for up to 150 m.y. before the beginning of igneous activity (Kent, 1991). Insetshows (A) the present-day location of the Kerguelen-Heard plume (KHP) between Kerguelen and Heard islands and (B) its position at 110 Ma,at the time of formation of the Kerguelen Plateau. RT = Rajmahal Traps; BB = Bunbury Basalt; and PCM = Prince Charles Mountains. It hasbeen suggested (e.g., Davies et al., 1989; Storey et al., 1989) that this entire magmatic province may be attributable to the KHP, which has morerecently produced Dupal-type oceanic basalts (Hart, 1984) on Kerguelen and Heard islands in the southern Indian Ocean (Storey et al., 1988;Barling and Goldstein, 1990; Gautier et al., 1990).

3. Hotspot related: A hotspot or mantle plume origin for theKerguelen Plateau has been suggested by many authors based onplate reconstructions and geochemistry (e.g., Luyendyck andRennick, 1977; Duncan, 1978; Peirce, 1978; Morgan, 1981;Storey et al., 1989; Weis et al., 1989; Davies et al., 1989).

This paper represents a preliminary report of work inprogress in which we review the setting of the KerguelenPlateau and associated continental volcanism, present newmajor and trace element data for each of the volcanic occur-rences, and examine the data in terms of the above models.

GEOLOGICAL SETTING

Kerguelen PlateauThe Kerguelen Plateau, located on the Antarctic Plate

between 46° and 64°S, is one of the most important physio-graphic features on the Indian Ocean floor (Fig. 2). It runsnorth-northwest for approximately 2300 km, is about 500 kmwide, and rises over 3 km above the surrounding ocean floor.It is separated from the Antarctic continent by the 100-km-wide, 35OO-m-deep Princess Elizabeth trough. Seafloor

spreading at the Southeast Indian Ridge (SEIR) separated theKerguelen Plateau and Broken Ridge approximately 45-42Ma ago (e.g., Houtz et al., 1977; Munschy and Schlich, 1987).

The Kerguelen Plateau has been divided into northern andsouthern sectors by Schlich (1975) and Houtz et al. (1977).The northern part generally lies in water depths of less than1000 m and contains the Eocene to Quaternary volcanicislands of Kerguelen, McDonald, and Heard. The boundarybetween the northern and southern sectors of the plateau liesimmediately south of Heard Island. The transition zone ex-hibits a complex bathymetry with a large west-trending spur,the Elan Bank, extending from the main plateau over adistance of some 600 km. The southern portion of the plateauis generally deeper, with water depths between 1000 and 3000m, and has a more subdued topography. It consists of a broadanticlinal arch affected by multiple stages of normal faultingthat result in horst and graben development (e.g., Coffin et al.,1986). The axis of this arch is marked by the 77°E Graben.

Seismic refraction studies on and in the vicinity of theKerguelen Archipelago by Recq et al. (1983), Recq andCharvis (1986), and Recq et al. (1990) suggest a crustalthickness of between 15 and 23 km for this part of the plateau.

34

LOWER CRETACEOUS VOLCANIC ROCKS

46° S

50c

54C

58C

62C

300 km

60° E

Figure 2. Bathymetric map (in meters) of the Kerguelen Plateaushowing the location of ODP drill sites (circles; filled circles denotebasement sites) and MD48 dredge stations (stars).

These authors note that the seismic velocity vs. depth profileis typical of oceanic islands. Houtz et al. (1977) inferred acrustal thickness of 20-23 km for the Southern KerguelenPlateau based on the gravity data.

According to Schlich (1982), the oldest magnetic anomalybordering the plateau is 34 (equivalent to 84 Ma, following thetime scale of Kent and Gradstein, 1985), which occurs in theCrozet Basin. However, a recent revision of the magneticprofiles in the Enderby Basin on the southwestern margin ofthe plateau (Li, 1988) has indicated the presence of magneticanomalies M1-M7 (122-128 Ma).

The first drill samples from the Kerguelen Plateau base-ment were obtained on the central and southern parts of theplateau during ODP Legs 119 and 120 (Fig. 2). In addition,dredge samples (including basalt) from the 77°E Graben andthe northern part of the plateau were obtained by the MarionDufresne in 1986 (Cruise MD48; Leclaire et al., 1987). De-tailed descriptions of the Cruise MD48 dredge samples and theLeg 119 and 120 cores are given by Bassias et al. (1987),Barron, Larsen, et al. (1989), and Schlich, Wise, et al. (1989),respectively. Some of the Kerguelen Plateau basement basaltsappear to have been erupted in a subaerial environment, asevidenced by the presence of oxidized flow tops at Site 747and overlying nonmarine sediments, of probable Albian age,at Site 750 (Schlich, Wise, et al., 1989). The age of theoverlying sediment at the remaining sites is pre-Turonian(limestone) at Site 738, lower Santonian (nannofossil chalk) atSite 747, and early Eocene (nannofossil ooze) at Site 749. Inaddition, at Site 748, some 150-200 m above the seismicbasement, a highly altered alkali-basalt flow was drilled,overlain by an undated conglomerate and then by upperAlbian-Turonian sandstones (Schlich, Wise, et al., 1989).

Radiometric dating of the Kerguelen Plateau basement hasso far been restricted to a few samples from the central-

southern parts of the plateau (Leclaire et al., 1987; White-church et al., this volume). Whole-rock dating on the least-altered Leg 120 basalts give 40Ar/39Ar "plateau ages" of 109.2± 0.7 Ma for Site 749 located on the Banzare Bank and 118.2± 5 Ma for Site 750 in the Raggatt Basin (Whitechurch et al.,this volume). The Leg 120 results show a close correspon-dence to a Plagioclase K-Ar age of 114 ± 1 Ma from a dredgedtholeiite (dredge station 5) on the 77°E Graben (Leclaire et al.,1987). However, all of the three dated samples are within 400km of each other (Fig. 2). These dates, when coupled with theage of the oldest magnetic anomalies in the Crozet Basin,suggest that the plateau may span at least 30 m.y. Furthersampling and dating studies are required, particularly on thenorthern and southern extremities of the plateau.

Naturaliste Plateau and Bunbury BasaltThe Naturaliste Plateau (Fig. 3) is a submarine topographic

high rising about 2500 m above the surrounding ocean floor atthe southwestern corner of continental Australia, and sepa-rated from it by a deep channel, the Naturaliste Trough(Jongsma and Petkovic, 1977). The plateau runs approxi-mately 400 km east-west and 250 km north-south. It dipsslightly to the north with steep flanks to the south and west.On the northern flank a regular continental rise apron hasdeveloped east of 111 °E, continuous with that of the margin ofWestern Australia, but the western part of the northern flankis steep. To the northwest, the Naturaliste Plateau abutsoceanic crust of Lower Cretaceous age M4 (126.5 Ma)whereas Mil (132.5 Ma) represents the onset of seafloorspreading in the Perth Basin.

The Deep Sea Drilling Program drilled on the NaturalistePlateau at Sites 258 (Leg 26) and 264 (Leg 28). Site 258consisted of some 525 m of marine Cenozoic and Cretaceoussediments. The lowest part of the section consisted of glau-conitic sandstone and silty clay, overlain by 251 m of mid-Albian to Cenomanian ferruginous clays. Most of the detritalconstituents of this lower part were probably derived from theerosion of basaltic volcanic rocks, the source probably beingan elevated, subaerially exposed part of the plateau (Davies etal., 1974). At Site 264, 171 m of marine carbonate sedimentswere drilled, ranging in age from Holocene to Santonian. Thedrill bottomed after a further 35 m of altered, nonfossiliferousvolcaniclastic conglomerate, estimated by Hayes, Frakes, etal. (1975) to be Cenomanian or older in age. Coleman et al.(1982) consider that the top of this conglomerate correspondsto a prominent reflector that Jongsma and Petkovic (1977)described and which they were able to trace over much of thesubsurface of the plateau. Dredge samples recovered from thesteep northwestern flanks of the plateau by the Eltanin in 1972were reported by Heezen and Tharp (1973) as containingrocks of continental origin, although reexamination of thedredge haul by Coleman et al. (1982) revealed that the rockclasts were actually altered tholeiitic basalts.

The Bunbury Basalt represents a possible onland correla-tive to the Naturaliste Plateau. It consists of a few thicktholeiitic flows that crop out over 140 km of the southernportion of the Perth basin (Fig. 3). K-Ar ages for the BunburyBasalt range from 88 to 105 Ma (McDougall and Wellman,1976). However, a dolerite from the Perth Basin has beendated at 136 ± 3 Ma (Playford et al., 1976), and a similar agehas been inferred for the Bunbury basalt (e.g., Davies et al.,1989). More precise dating of both the Naturaliste Plateau andthe Bunbury Basalt are required.

Rajmahal Volcanic FormationThe Rajmahal Volcanic Formation is exposed on the

continental margin of northeast India, cropping out over an

35

M. STOREY ET AL.

30° S

32C

34C

USNS Eltanindredge site

NaturalistePlateau

NP

111° 113C 117° EFigure 3. Bathymetric map (in meters) of the southwest Australian continental margin (after Coleman et al., 1982) showing the location of theNaturaliste Plateau, major fracture zones, seafloor magnetic anomalies, and the location of Eltαnin dredge and Bunbury Basalt samples. Alsoshown are DSDP Sites 257, 258, and 264. Y = Yilgarn Block (Archean), L = Leeuwin Block (Proterozoic), and BT = Bunbury trough. Insetshows location of other major plateaus and basins on the west Australian continental margin: NP = Naturaliste Plateau, PB = Perth Basin, WP= Wallaby Plateau, Z = Zenith Seamounts, CB = Cuvier Basin, EP = Exmouth Plateau, AB = Argo Basin, and SP = Scott Plateau.

area of around 4,300 km2 in the Rajmahal Hills, Bihar (Fig. 4).The formation consists dominantly of quartz-normative tholei-ites, with minor olivine-basalts and basaltic andesites, totalingsome 200-300 m in thickness. This wedge of subaerial volca-nics overlies a thin sequence of Early Permian to EarlyCretaceous sediments (the Gondwana Supergroup), in turnresting on a high-grade Precambrian basement complex (theChotanagpur mobile belt). The volcanics are interbedded withthin sedimentary or volcaniclastic horizons, containing locallyabundant Lower Cretaceous plant fragments (Sengupta,1988). Dating of the basalts by 40Ar/39Ar (Baksi, 1989) alsosupports a Early Cretaceous age of 117 ± 1 Ma.

These rocks are believed to be cogenetic with a suite oftholeiites and alkali basalts exposed along the Dauki lineamentnear Sylhet, Meghalaya. In this area, a 60 × 40 km strip oflavas, some 500-600 m thick (?), has been documented byTalukdar (1967) and Talukdar and Murthy (1970). The vol-canic succession comprises alternating tholeiitic and felsicpyroclastic/rhyolite units, overlain by nepheline tephrites,dipping 19°-25° toward the southeast. The sequence is cutby a basic dyke swarm, the extent and nature of whichis uncertain at present. Early Cretaceous alkaline com-plexes also occur in this region (Chattopadhyay and Hashimi,1984).

36


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Figure 4. Simplified geological map of the Rajmahal region showing the locations of the basalts analyzed.

37

M. STOREY ET AL.

During the 1950s, the subsurface geology of the BengalBasin was the focus of an integrated hydrocarbon prospect bythe Standard Vacuum Oil Company (Mobil/Esso). Seismicreflection profiles and borehole data revealed the presence ofa seaward-dipping reflector sequence on the continental mar-gin, comprising a thin wedge of mafic flows (Biswas, 1963;Sengupta, 1966). These flows form the tops of tilted faultblocks, similar in structural style to those on the opposingcontinental margin of southwest Australia (Marshall and Lee,1989), and suggestive of episodic extension and, by inference,transtension. Kent (1991) relates these features to episodicdoming and extension of eastern Gondwana lithosphere fromEarly Permian to Early Cretaceous times.

Damodar Valley and Darjeeling LamprophyresRecent work by Sarkar et al. (1980) and Paul and Potts

(1981) provided the first geochemical and geochronologicaldata on Indian Cretaceous lamprophyres. These rocks intrudesediments of the Gondwana Supergroup in the Damodar andDarjeeling coal fields and are also found along lineament zonesin the Proterozoic metamorphic complexes of the ShillongPlateau (Fig. 1). They consist of both alkaline and ultramaficvarieties, emplaced in the form of horizontal cylinders or,more rarely, as thin dykes and sills. K-Ar ages on biotitephenocrysts and whole-rock samples range from 121 to 105Ma (Sarkar et al., 1980). It is presently uncertain as to whetherthese intrusions postdate or are contemporaneous with theeruption of the Rajmahal tholeiites.

Prince Charles Mountains LamprophyresAt the southwest corner of Radok Lake (70°54'S, 67°56'E)

in the Prince Charles Mountains of Antarctica (Fig. 1), twosills occur in conglomerate near the base of an upper Permiansedimentary sequence (Kemp, 1969). They are examples ofthe fairly rare ultramafic lamprophyre alnoite and have beendescribed by Walker and Mond (1971). The lower sill is about4 m thick at its western extremity, thickening eastward to 20m. The upper sill, which is stratigraphically about 300 m abovethe lower, is about 5 m thick. The sills are exposed in verticalcliffs and can be traced horizontally over a distance of 1 km.Thin dykes also occur in the sediments 6.5-11 km northeast ofthe sills. K-Ar dating on phlogopite separates from the sillsgave ages between 108 and 110 Ma (Walker and Mond, 1971).

MAJOR AND TRACE ELEMENT GEOCHEMISTRY

MethodsSamples were cleaned before being crushed in an agate

shatterbox. Trace element analysis was conducted on 46-mm-diameter powder briquettes, made from 15 g of rock powderby adding 20 drops of a 7% solution of polyvinyl alcohol andsubjecting the mixture to a pressure of 15 tons in a steel die.For major element analysis, the rock powders were driedovernight at 120°C before igniting them at 800°C. This wasfollowed by accurately weighing 1 g of ignited rock powderwith 5 g of Johnson Matthey spectroflux 100B into a platinumcrucible and fusing the mixture at 1200°C. The melt waspressed into a glass disk on an aluminum mold.

X-ray fluorescence (XRF) analysis was conducted at theUniversity of Leicester using a Philips PW1400 automaticspectrometer. Major elements and Nb, Zr, Y, Sr, Rb, Th, andNi were determined using a Rh X-ray tube. The trace elementsV, Cr, Ba, La, Ce, and Nd were determined using a W X-raytube. Details of operating conditions and data quality aregiven by Marsh et al. (1980) and Weaver et al. (1983).

Instrumental neutron activation analysis (INAA) of therare-earth elements (REE) and Th, Ta, Hf, and Se was

conducted on selected samples at the University of Durhamusing a method similar to that detailed by Leat et al. (1990),counting samples for 24 hr.

The XRF analyses of standard WI and the INAA analysesof standards BR and BOB-1, performed in conjunction withthis work, are given in Saunders et al. (in press).

Kerguelen PlateauLegs 119 and 120 recovered Kerguelen Plateau basement at

Sites 738, 747, 749, and 750 (Fig. 2), which consisted ofmoderately fresh to highly altered basalt. The secondarymineral assemblage consisted of smectites, zeolites, and cal-cite. The basalts range from aphyric to highly porphyritic,containing phenocrysts of Plagioclase, clinopyroxene andolivine. The lava flow drilled at Site 748 is a highly alteredanalcite-bearing alkali basalt containing Plagioclase and cli-nopyroxene phenocrysts. The MD48 basement dredge sam-ples recovered from the 77°E Graben and the northwest flanksof the Kerguelen Plateau consisted of moderately to highlyaltered basalt (Bassias et al., 1987; Leclaire et al., 1987;Davies et al., 1989).

New major and trace element analyses for Leg 120 andMD48 Kerguelen Plateau basalts are given in Table 1. Mostare olivine normative tholeiites, with MgO, Ni, and Cr con-tents for the Leg 120 samples ranging from 5.8% to 10.2%, 29to 156 ppm, and 83 to 401 ppm, respectively. The MD48dredge samples contain more evolved compositions, withMgO varying between 3.9% to 5.7%. The tholeiites have beenvariably affected by hydrothermal alteration, as indicated bytheir large range in volatile contents (e.g., LOI = 0.1% to7.3%) and the generally nonsystematic behavior of the moremobile elements (e.g., Rb, K, and Na) with magmatic frac-tionation indexes such as MgO or Zr (Schlich, Wise, et al.,1989).

In terms of immobile incompatible element abundancesand ratios, Leg 120 basalts exhibit significant inter- andintrasite variations. For example, Site 747 basalts have lowerTi/Zr ratios (74 to 83) than those of Site 749 (94 to 139) and Site750 (150 to 185), suggesting that the first are derived from amore enriched source (Fig. 5). The Site 749 tholeiites appearto consist of two distinct magma types: most of the data haveTi/Zr ratios clustering around 100, but a second, more de-pleted, group has Ti/Zr ratios ranging from 122 to 139. TheMD48 tholeiites range in Ti/Zr ratios from 74 to 97.

Figure 6 consists of MORB-normalized multi-element andchondrite-normalized REE plots and illustrates the large rangein incompatible element ratios exhibited by Kerguelen Plateautholeiites. Moderate to strong enrichment of Th and the lightREE (LREE) is shown by samples from Site 747 (LaN/YbN =3.2 to 4.7), Site 738 (LaN/YbN = 4.2), and the MD48 basaltsdredged from the 77°E Graben and the northern part of theKerguelen Plateau (LaN/YbN = 2.2 to 6.1). In contrast, basaltsfrom Site 749 are slightly enriched in the LREE (LaN/YbN =1.53 to 1.80), whereas basalts from Site 750 in the RaggattBasin have the lowest LaN/YbN ratios (0.88 to 1.12), being themost similar to normal MORB. Plagioclase accumulation isthe most likely explanation for the Sr spike in the stronglyphyric (30% Plagioclase phenocrysts) Sample 120-749C-16H-7, 69-76 cm (Fig. 6). A notable feature of the KerguelenPlateau tholeiites from Sites 738, 747, 750, and MD48 dredgestations 2, 5, and 6 is their fairly low Nb and Ta concentrationsrelative to adjacent, similarly incompatible elements (e.g., Thand La) in the multi-element plots (Fig. 6). High La/Nb ratios(>l) are more normally associated with subduction-relatedmagmatism (e.g., Saunders et al., 1980; Gill, 1981) and somecontinental flood basalts (e.g., Mantovani et al., 1985) andlamprophyres (e.g., Paul and Potts, 1981).

38


Naturaliste Plateau and Bunbury Basalt

Analyses of Naturaliste Plateau basalts are given by Cole-man et al. (1982), who report major and trace element data onseven dredge samples recovered by the Eltanin. They consistof altered Plagioclase- and clinopyroxene-phyric basalts.Analyses of the Bunbury Basalt are given by Burgess (1978),averages of which are given by Coleman et al. (1982). TheBunbury Basalt consists of moderately fresh Plagioclase- andpyroxene-phyric flows.

New major and trace element data for five Eltanin dredgebasalts from the Naturaliste Plateau and seven samples of theBunbury Basalt are given in Table 2. The location of thesamples are shown in Figure 3. Eltanin Samples EL5512-7 andEL5512-8 were also analyzed by Coleman et al. (1982), andtheir data are shown in Table 2. The two data sets show goodagreement, with the exception of Rb.

In contrast to the Kerguelen Plateau basement, the Natu-raliste Plateau basalts are all strongly quartz-normative tholei-ites (normative quartz = 12.9 to 15.9). Their large normativequartz content is probably, in part, a consequence of alter-ation, as is also the presence of normative corundum (Table2). The Bunbury Basalt samples are weakly quartz-normativetholeiites. In terms of their compatible elements, the Natural-iste Plateau samples are moderately evolved with MgO, Ni,and Cr ranging from 4% to 5.1%, from 111 to 139 ppm, andfrom 188 to 555 ppm, respectively. The Bunbury Basaltsamples have slightly higher MgO contents (5.5% to 6.5%) butlower Ni (37 to 58 ppm) and Cr (39 to 146 ppm) values(Coleman et al., 1982, this volume).

The Naturaliste Plateau basalts show stronger LREE enrich-ment (LaN/YbN = 3.1 to 4.7) than do the Bunbury Basalt samples(LaN/YbN = 1.54 to 2.25) (Fig. 7). Both are enriched in highlyincompatible elements (e.g., Th, LREE) relative to normalMORB. A feature of the multi-element plots for both theNaturaliste Plateau and Bunbury Basalt samples are the troughsat Nb and Ta, with La/Nb ratios ranging from 1.4 to 3.3.

Rajmahal Basalts and Indian Lamprophyres

Previous studies on the volcanic rocks of the RajmahalTraps and Sylhet Traps (e.g., Mahoney et al., 1983; Baksi etal., 1987) have shown that they consist predominantly ofquartz-normative tholeiites and basaltic andesites with a phe-nocryst assemblage of Plagioclase, augite, Fe-Ti oxides, and,more rarely, olivine. Analysis of a few samples from theBengal Basin has also revealed the presence of alkali basaltsand olivine-normative tholeiites (Baksi et al., 1987).

Table 3 presents 25 new analyses of the Rajmahal basalts;their locations are shown in Figure 4. They are all quartz-normative tholeiites and exhibit varying contents of MgO, Ni,and Cr, ranging from 5.3% to 8%, from 18 to 147 ppm, andfrom 105 to 673 ppm, respectively.

With the exception of Sample RB 88/34, the Rajmahaltholeiites define two distinct groups in terms of their incom-patible element abundances and ratios (Groups I and II; Figs.5C, 8, and 9). Group I tholeiites are characterized by higherTi/Zr (94 to 118), and lower Zr/Y (3.1 to 3.9), K2O (0.15% to0.55%), and Rb (0.3 to 7.2 ppm) than the Group II tholeiites(Ti/Zr = 61 to 73; Zr/Y = 4.4 to 4.8; K2O = 0.56% to 1.2%, Rb= 17 to 33 ppm). The K/Rb ratios of Group I tholeiites (K/Rb= 242 to 1038) overlap with Group II tholeiites (K/Rb = 248 to422), but are more variable as they range to higher values. Thegeneral enrichment in highly incompatible elements (e.g., Th,LREE) of Group II tholeiites over Group I tholeiites isillustrated in Figure 9. It is also interesting that overall GroupI tholeiites have lower La/Nb ratios (La/Nb = 0.8 to 1.6) thanGroup II tholeiites (La/Nb = 1.5 to 2.4). The La/Nb ratios of

some Group I tholeiites (e.g., La/Nb 1) are typical ofoceanic-island basalts.

The geochemistry of the Lower Cretaceous Indian lampro-phyres that intrude into the Gondwana sediments has beendescribed by Paul and Potts (1981) and Middlemost et al. (1988).The Indian lamprophyres are characterized by variable SiO2

(39.0% to 54.6%), moderate to very high K2O (2.2% to 5.9%),MgO (4.7% to 16.8%), Ni (30 to 930 ppm), and Cr (31 to 977 ppm)and low to high A12O3 contents (4.3% to 15.1%). The lampro-phyres show variable enrichment in LREE (LaN/YbN = 29 to110), while exhibiting relative depletion in Ta (and Nb) (Fig. 9).

Prince Charles Mountains Lamprophyres

The petrography of the Radok Lake lamprophyres in thePrince Charles Mountains (Fig. 1) has been described byWalker and Mond (1971). The samples show varying degreesof deuteric alteration, the least altered rocks consisting mostlyof phlogopite, melilite, olivine, and nepheline with minoramounts of opaque iron minerals, perovskite, and apatite.Secondary minerals make up 30% to 70% of the rock andinclude serpentine, chlorite, talc, prehnite, calcite, dolomite,and possibly antigorite (Walker and Mond, 1971; N.C.N.Stephenson pers. comm., 1989). Based on their mineralogyand major element chemistry, they classify as alnoites withinthe ultramafic lamprophyre group (Walker and Mond, 1971).

Two whole-rock samples collected from the lower sill atRadok Lake have been analyzed for major and trace elements(Table 4). They have low SiO2 (37.2%), high MgO (13.9 % and16.1%) and Ni (239 and 281 ppm). The alnoites are enriched inLREE, Ta, Nb, Th, K, and Rb. Unlike the ultra-potassicIndian lamprophyres, they have low La/Nb ratios (0.58 and0.75) and other trace element similarities to ocean islandbasalts (Fig. 10).

ISOTOPE GEOCHEMISTRY

Kerguelen PlateauPreliminary Sr, Nd, and Pb isotopic data for Kerguelen

Plateau and Rajmahal basalts are shown in Figures 11-13; thecomplete data set will be given elsewhere. Samples from theKerguelen Plateau show a remarkably large variation inisotopic composition; for example, εNd ranges from 5.2 to-8.5. In terms of individual locations, Site 747 basalts havehigh 87Sr/86Sr ratios (0.7056 to 0.7058), whereas εNe rangesfrom 2.7 to -4, overlapping with the most enriched basaltsfrom Kerguelen Island (Fig. 11). In comparison, εNd is higherand more variable for basalts from Sites 749 (εN d = 1.9 to 4.5)and 750 (εN d= 1.4 to 5.2). Site 750 tholeiites differ from thoseof Site 749 in having higher 87Sr/86Sr ratios. Acid-leachingexperiments (Fig. 11) and the high Sr contents of Site 750tholeiites (Fig. 6) suggest that this is only partly caused byalteration. In marked contrast to the tholeiites from the centraland northern part of the Kerguelen Plateau, which have Sr andNd isotopic compositions within the mantle array, isotopicanalysis of a tholeiite from Site 738 (Alibert, 1991), at thesouthern tip of the Kerguelen Plateau, gave extreme 87Sr/86Srand εNd values of 0.7090 and 8.5, respectively (Fig. 13), welloutside the range of oceanic (hotspot-related) basalts.

Although the Pb isotopic compositions of basalts fromSites 748 and 749 overlap with data for Kerguelen Island, thetholeiites from Sites 738, 747, and 750, in comparison, arecharacterized by lower ^Pb/^Pb (17.47 to 17.74), despitemajor differences in their Nd and Sr isotopic compositions.The single analysis of the Site 738 tholeiite (Alibert, 1991) hasa very high 207Pb/204Pb ratio (15.71), similar to values observedin continental lamproites from western Australia and leucititesfrom Gaussberg volcano, Antarctica (Fig. 12).

39

è Table 1. Major and trace element analyses of Leg 120 drill and Marion Dufresne (MD 48) dredge samples from the Kerguelen Plateau, Indian Ocean.

HoleCore, sectionInterval (cm)

747C11R-16-8

Major elements:SiO2

TiO2

A12O3

Fe2O3

MnOMgOCaONa2OK2Op 2 θ 5TotalLOI

CIPW norms:QtzCorOrAbAnXT

INeDiHy01MtIlmAp

XRF:NbZrYSrRbGaZnNiVCrBaLaCeNd

INAA:LaCeNdSmEuTbYbLuTaHfThWSe

51.862.03

17.4210.170.176.229.802.730.6730.214

101.291.88

1.83

3.9823.1033.30

11.3820.71

1.763.860.50

11.8159.028.1

265.38.4

22.9130.455.6

377.3182.2142.9

14.036.019.1

13.232.018.34.71.380.672.350.300.663.841.296.1

48.0

747C12R-2

119-121

50.541.55

15.6411.380.177.53

11.042.430.5290.187

101.001.26

3.1320.5630.21

19.0919.092.591.962.940.43

7.4112.7

23.0232.1

11.220.092.169.1

306.1278.6160.7

11.724.514.3

747C12R-4

36

50.611.63

15.0512.010.207.65

11.572.310.2380.200

101.470.67

1.4119.5530.00

21.3722.16

0.302.073.100.46

9.3122.6

26.9243.9

2.419.193.465.5

299.3224.0147.7

10.730.315.1

12.528.617.74.221.290.782.760.370.543.051.097.6

43.5

747C13R-3

144-148

50.541.65

16.5710.870.157.85

10.552.590.5110.213

101.491.46

3.0221.9232.09

15.3718.014.641.883.130.49

10.2132.424.2

259.29.2

21.397.975.0

250.082.6

250.315.532.917.5

747C14R-130-34

49.721.68

16.5311.420.157.97

10.162.500.5880.21

100.932.05

3.4721.1632.15

13.7717.226.511.973.190.49

9.2135.227.5

248.86.7

19.793.273.8

265.490.5

246.714.837.818.5

747C14R-216-21

50.771.97

16.8712.170.146.947.892.761.1100.211

100.833.00

6.5623.3630.37

6.0725.00

2.082.103.740.49

10.5152.837.0

227.918.922.798.650.9

326.2114.2228.8

21.248.224.3

747C15R-126-32

49.681.26

16.7610.880.178.509.182.011.2080.199

99.855.20

7.1417.0133.15

9.0322.62

5.221.882.390.46

7.9101.123.3

233.317.517.279.2

107.9194.6351.2187.5

13.033.317.0

747C16R-278-80

50.351.24

16.5210.760.218.389.601.921.7780.172

100.933.66

10.5116.2531.22

12.4617.45

7.511.862.360.40

8.198.122.8

250.48.1

18.477.9

100.8187.7371.1205.5

14.630.315.6

747C16R-450-53

49.541.29

16.3310.270.219.11

10.492.130.8930.179

100.442.73

5.2818.0232.37

15.0115.268.971.772.450.41

7.7100.4

18.9240.4

20.318.471.1

107.0211.3359.6175.9

12.430.515.1

13.328.617.73.591.070.632.170.310.562.511.086.0

36.7

747C16R-5

103-106

50.901.21

17.2310.620.167.479.591.881.9980.158

101.223.63

11.8115.9132.68

11.3319.404.661.832.300.37

7.096.723.2

233.518.522.065.971.6

201.0401.4229.3

13.431.316.4

12.325.513.53.501.030.651.890.280.462.340.805.3

33.8

748C79R-5

137-139

50.402.81

18.068.690.087.016.734.610.4971.128

100.025.66

0.412.94

39.0126.02

12.788.661.505.342.61

123.4605.0

26.41226.9

7.318.276.6

224.3179.0274.2

1351.0109.5214.9

89.0

748C79R-690-94

49.242.74

17.508.770.097.546.714.720.5571.122

98.997.14

3.2939.9424.93

0.834.34

15.581.515.202.60

121.9599.3

27.71130.8

7.818.779.0

181.7169.5166.0

1661.0108.9218.3

88.0

105.0224.0103.0

14.203.311.251.80.269.08

12.4011.5016.418.7

748C82R-CC

0-2

51.232.72

17.837.610.096.986.814.801.3181.103

100.494.86

7.7940.6223.22

2.692.04

14.431.315.172.56

125.5621.6

30.01125.2

7.819.486.5

210.7168.1218.1

1449.0109.3220.3

89.4

106.0230.0104.0

12.803.581.382.210.267.50

10.8010.5015.719.4

749C12R-264-69

54.921.11

12.9911.000.118.186.943.272.2110.161

100.892.60

13.0727.6714.24

15.6221.98

2.981.902.110.37

4.354.522.5

137.618.216.173.252.2

199.3235.0171.7

4.510.96.2

749C12R-382-84

52.821.54

15.5811.470.166.808.363.310.6280.178

100.851.14

0.67

3.7128.0125.81

11.9124.43

1.982.920.41

5.793.630.1

217.512.822.7

121.129.7

278.8274.3120.8

6.116.411.5

749C12R-3

117-120

52.601.40

14.0113.260.175.829.472.921.1790.168

101.001.72

0.54

6.9724.7121.65

20.1220.52

2.292.660.39

4.784.031.2

202.441.120.9

103.430.6

234.0237.8

98.88.2

14.69.2

749C12R-4

138-140

52.331.40

14.1712.500.155.859.122.771.0040.162

99.461.13

2.27

5.9323.4423.27

17.3220.94

2.162.660.38

5.287.930.2

212.828.522.297.730.0

247.3235.4104.7

5.814.810.9

749C15R-2

118-123

53.751.44

14.4811.550.196.359.792.950.4720.171

101.140.13

3.47

2.7924.9624.88

18.5820.33

1.992.730.40

5.891.231.3

228.39.9

23.997.431.2

243.5229.2107.4

6.219.3

9.7

Table 1 (continued).

HoleCore, sectionInterval (cm)

749C15R-5

125-127

Major elements:SiO 2

T i O 2

A1 2 O 3

F e 2 O 3

MnOMgOCaON a 2 OK 2 OP 2 O 5

TotalLOI

CIPW norms:QtzCorOrAbAn

JNeDiHy01MtIlmAp



52.731.52

15.1811.95

0.177.228.313.420.5430.177

101.221.35

3.2128.9424.47

12.8024.47

0.932.062.890.41

5.891.129.3

213.512.322.7

115.129.5

271.3259.5113.8

6.515.210.6

6.816.111.9

3.641.240.692.710.390.402.160.676.2

34.5

749C15R-6

97-100

50.551.75

14.9013.410.196.829.963.210.2280.189

101.210.94

1.3527.1625.58

18.6515.02

6.202.313.320.44

6.3100.032.4

227.76.9

21.298.939.3

317.0149.6

93.96.5

21.913.1

749C16R-769-76

49.980.92

19.019.150.137.01

12.112.480.0810.086

100.960.90

0.4820.9940.51

15.5014.09

5.071.581.750.20

3.941.217.4

225.04.4

1860.669.6

187.8278.4

49.44.19.05.5

3.29.66.32.060.830.531.500.220.151.080.241.9

30.8

750B16R-447-51

51.040.73

15.4411.960.199.33

11.001.970.0920.065

101.820.98

0.5416.6733.02

17.2528.30

1.402.061.390.15

26.919.1

119.65.4

18.579.0

103.0234.7100.047.2

5.2

750B14R-147-50

42.650.93

17.7526.12

0.066.430.340.455.2830.087

100.107.30

10.8831.22

3.811.12

27.0517.26

4.511.770.20

30.013.640.822.017.476.3

156.0278.4131.8192.3

6.1

750B15R-288-92

50.380.71

15.4211.950.19

10.159.971.810.1090.055

100.742.34

0.6415.3233.64

12.5932.41

1.572.061.350.13

24.218.3

112.94.9

18.079.9

108.5220.5

92.739.8

3.5

2.55.64.51.410.590.432.040.300.100.760.213.3

40.9

750B16R-654-58

50.330.76

16.0111.170.188.47

11.362.050.0950.061

100.491.18

0.5617.3534.21

17.7724.63

1.491.931.440.14

26.719.6

128.44.5

17.782.7

115.6241.4100.647.1

5.46.82.1

750B17R-15-7

50.400.75

15.7111.680.188.99

11.311.990.0530.069

101.131.26

0.3116.8433.79

17.8625.25

2.462.021.420.16

26.320.6

122.52.0

19.180.8

112.9238.9106.934.5

2.63.93.6

750B17R-323-26

49.211.17

15.7714.620.188.369.011.990.1890.113

100.614.31

1.1216.8433.55

8.5832.56

1.692.522.220.26

4.246.924.6

192.59.0

19.893.4

120.3269.4192.530.4

6.28.07.0

4.08.96.32.320.840.582.570.340.191.160.494.0

38.9

750B17R-350-54-_

49.751.23

16.5513.960.139.876.552.090.2590.108

100.504.22

0.331.181.53

17.6931.79

41.76

2.412.340.25

3.446.521.3

129.36.6

21.284.0

108.7290.3241.4

20.94.1

MD48-86. DR 02-2

50.742.47

17.929.710.175.726.653.622.3320.447

99.781.97

13.7830.6325.77

3.516.73

11.111.684.691.04

12.7199.836.1

368.269.426.8

213.327.6

268.6168.6140.4

17.947.527.5

17.847.021.0

5.301.740.802.110.330.544.102.501.8

29.9

MD48-86DR 05-1

50.021.72

16.6611.790.194.999.413.321.0830.332

99.511.44

6.4028.0927.37

14.239.117.222.043.270.77

5.6106.730.0

226.325.824.6

184.871.6

294.3340.3

98.89.7

20.214.8

MD48-86DR 05-2

50.671.66

15.8411.750.196.189.823.440.5690.211

100.330.90

3.3629.1126.11

17.499.897.672.033.150.49

5.6107.133.2

232.29.2

20.6142.349.5

286.6309.5

57.27.2

20.413.5

10.421.013.44.701.570.812.850.470.402.401.503.9

42.2

MD48-86DR 05-3

50.611.61

16.0010.260.145.809.583.261.0200.228

98.510.97

6.0327.5926.02

16.4411.474.721.773.060.53

6.0102.034.4

217.539.322.7

175.951.9

262.5333.2

94.99.1

17.913.6

MD48-86DR 06

49.052.38

17.0714.140.203.888.763.670.8950.531

100.581.84

5.2931.0627.47

10.537.449.372.444.521.23

11.3179.447.1

342.414.028.5

222.551.8

348.5186.1137.6

15.434.723.5

19.852.028.0

7.402.191.074.000.700.684.202.60

MD48-86DR 07

50.081.70

16.3912.440.235.949.573.490.6320.208

100.681.16

3.7329.5327.20

15.646.62

11.012.153.230.48

7.2107.328.6

239.211.323.9

175.454.7

305.7315.3

67.39.1

19.613.7

7.017.311.63.771.290.822.270.380.392.410.703.2

35.6

MD48-86DR 08-4

49.742.26

15.1313.850.214.919.563.071.4400.292

100.460.83

8.5125.9823.26

18.577.438.152.394.290.68

12.5175.042.6

169.128.024.6

191.664.7

378.5170.1180.2

17.442.520.0

16.036.019.65.601.491.103.300.550.854.092.002.2

40.7

(-1

O

wsm

ΘocCΛ

b

of i)C

KS

Note: LOI = loss on ignition.

M. STOREY ET AL.

200

N

F

N

F

N

160

120

80

90

80

70

60

50

120

100

80

60

40

* MD48 dredge samples

Site 750

Site 749 (II)

* ^ ^ 9 ‰ Q D ^ Site 747

B

Bunbury Basalts

, , i i , , i , i i , . , i , . , , i , , , . i , , , i i , ,

o £§o° /Group I

Group

6 7

MgO

10 11

Figure 5. Plots of the MgO vs. Ti/Zr ratio for samples from (A)Kerguelen Plateau, (B) Naturaliste Plateau and Bunbury Basalt, and(C) Rajmahal tholeiites.

On a Pb-Sr diagram (Fig. 13 A), all of the Kerguelen Plateausamples lie above the Indian Ocean MORB field, with somefalling in the Kerguelen Island field. In the Pb-εNd plot, anumber of the plateau samples fall between the MORB andKerguelen Island fields. The low 206Pb/204Pb tholeiites fromSites 738, 747, and 750 plot roughly between the fields ofIndian Ocean MORB and Western Australian lamproites (Fig.13).

Rajmahal Basalts

The Sr and Nd isotopic data for the Rajmahal basalts definetwo distinct groups, with 87Sr/86Sr and εNd values ranging from0.7037 to 0.7053 and 5.1 to 2.9 (Group I) and 0.7064 to 0.7075

and -1.5 to -4.4 (Group II), respectively. This mirrors thevariations in incompatible elements (Fig. 8). Most of theGroup I tholeiites have Sr and Nd isotopic compositionssimilar to Site 749 basalts from the Kerguelen Plateau, al-though two Rajmahal samples are distinguished by higher87Sr/86Sr values (Fig. 11). The Rajmahal basalts are charac-terized by low 206Pb/204Pb (17.30 to 17.69) and high ^Pb/^Pb(15.59 to 15.62) ratios (Fig. 12).

DISCUSSION

Kerguelen Plateau basalts have Sr and Nd isotopic com-positions lying outside the field for Indian Ocean MORB, thusexcluding the possibility that the plateau represents a piece ofold, uplifted normal ocean crust (Fig. 13). With the exceptionof Site 738, the Sr and Nd isotopic data are within the range ofhotspot-related oceanic basalts, those from Sites 747 and 748overlapping with the field for the most recent basalts fromKerguelen Island (Fig. 13). However, basalts from KerguelenIsland are characterized by higher 206Pb/204Pb ratios thanplateau samples from Sites 738, 747, 750, and MD48 DR2 (Fig.13). The low 206Pb/204Pb values of these particular samplesmay indicate the involvement of components derived from thecontinental lithosphere (discussed below). Despite thegeochemical complications that have apparently arisenthrough interactions of the plateau source with continentallithosphere, a number of samples do show evidence for theinvolvement of Kerguelen Island-type mantle (Fig. 13), sup-porting the hotspot (Kerguelen-Heard plume [KHP]) hypoth-esis for most of the plateau, as do the plate reconstructions(Fig. 1, inset; Davies et al., 1989; Storey et al., 1989). Oneimportant difference between the younger basalts of Ker-guelen and Heard islands, which were erupted in an intraplateenvironment (e.g., Storey et al., 1988), and the plateau sam-ples is that the latter have lower abundances of such incom-patible elements as Th, Nb, Ta, Zr, and LREE, implyingmuch higher degrees of melting of the KHP source duringformation of the Kerguelen Plateau. In this respect, theKerguelen Plateau samples are similar to basalts from otherlarge-volume, hotspot-related oceanic magmatic provinces,such as the Caribbean Plateau, the Ontong Java Plateau, andNauru Basin (e.g., Floyd, 1989). These observations areconsistent with a near- or on-ridge setting for the KerguelenPlateau (perhaps analogous to Iceland), an environment thatwould favor extensive melting of an upwelling mantle plume(McKenzie and Bickle, 1988; Storey et al., 1991). It has alsobeen suggested that the largest oceanic plateaus (e.g., Ker-guelen Plateau, Ontong Java Plateau) may have formed bymelting of the large heads thought to form during the initiationof mantle plumes (Richards et al., 1989).

Low spb/ <Pb and sometimes high 2°7Pb/204Pb ratios are afeature of old continental mantle lithosphere (e.g., Nelson etal., 1986; Hawkesworth et al., 1990). An extreme expressionof this lithospheric signature is shown by western Australianlamproites (Figs. 12 and 13). One explanation for the low206pb/2θ4pt, v a i u e s of some Kerguelen Plateau tholeiites is thatGondwana continental mantle lithosphere was incorporatedinto the Kerguelen plume source during the formation of theplateau. Contamination by lithospheric components was alsosuggested on the basis of the high La/Ta (Nb) and Th/Ta (Nb)ratios of some plateau tholeiites (Storey et al., 1989). Theoccurrence of Indian Ocean MORBs with low 206Pb/204Pbratios has likewise been considered in terms of contaminationby lithosphere detachment (e.g., Mahoney et al., 1989, inpress). Thermal mobilization of the Gondwana mantle litho-sphere by the KHP is plausible given the small size of theIndian Ocean basin (700 km) at the time of plateau formationand the 1000-2000 -km-diameter heads attributed to mantle

42


CDerO0)Q.

Q.

G3co

m

O

E03

CO

CùGC

o

CL

E03

CO

50

10

1 -

0.1

10 -

0.1

10 —

1 -

0.1

120-747C-11R-1.6-8

120-747C-12R-4, 34-36

120-747C-16R-4, 50-53

120-747C-16R-5, 103-106

~ i 1 1 r

120-749C-15R-5, 125-27

120-749C-16R-7, 69-76B

10050

o 120-750B-15R-2, 88-92

• 120-750B-17R-3, 23-26

Th Nb Ta La Ce Sr~i 1 r~Nd Sm Zr Eu Ti Yb

Figure 6. MORB-normalized incompatible element plots and (insets)chondrite normalized REE plots for (A, B, C) Leg 120 basementtholeiites, (D) Leg 119 basement tholeiite, and (E, F) MD48 dredgesamples. Leg 119 data from Alibert (1991). Normalizing values arefrom Sun and McDonough (1989).

plumes (White and McKenzie, 1989). Although plate recon-structions for Gondwana indicate that the Kerguelen Plateaumust be essentially oceanic (Fig. 1; de Wit et al., 1988; Powellet al., 1988), it should be noted that the extreme isotopiccompositions shown by a Site 738 tholeiite may indicate thatthe southernmost part of the plateau is underlain by continen-tal crust (Alibert, 1991), possibly in a setting analogous to theNorth Atlantic Rockall Plateau (Morton and Taylor, 1987;Merriman et al., 1988) and V0ring Plateau sequences (Vierecket al., 1988).

Among the continental volcanism that accompanied thebreakup of eastern Gondwana and the formation of theKerguelen Plateau, the Rajmahal basalts are the most signifi-

0.1

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o MD48 86DR02-2

• MD48 86DR 05-2

o MD48 86DR06

• MD48 86DR07

a MD48 86DR 08-4

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Figure 6 (continued).

cant in terms of volume. The eruption of both may have beenpartly contemporaneous. Group I Rajmahal tholeiites withlow La/Nb ratios share strong compositional similarities withsome Kerguelen Plateau basalts (Fig. 14), suggesting a directcontribution to Rajmahal volcanism by the KHP, in contrastto the conclusion reached by Mahoney et al. (1983). The Srand Nd isotopic compositions of the Group I tholeiites sup-ports this contention, although, their high ^Pb/-^Pb ratiosalso indicate the presence of continental lithosphere compo-nents; the preferential enrichment of Pb, relative to Sr andNd, in continental lithosphere makes Pb a sensitive indicatorof such contamination. The range in La/Nb and K/Rb ratios ofGroup I tholeiites (Fig. 8) also suggests some contaminationby continental lithosphere or melts derived therefrom. Aprobable example of the latter are the Lower CretaceousIndian lamprophyres, which are characterized by very highLa/Nb ratios and moderate to high K/Rb ratios (up to 1000;

43

M. STOREY ET AL.

Table 2. Major and trace element analyses of samples from the Naturaliste Plateau and the Bunbury Basalt, Australia.

Sample EL55 12

Major elements:SiO2

TiO2

A12O3

Fe2O3

MnOMgOCaONa2OK2OP2O5Total

56.381.81

17.1311.910.205.043.622.711.010.22

100.03

CIPW norms:QtzCorOrAbAnDiHyOlMtIlmAp

XRF:NbZrYSrRbGaZnNi


15.95.56.0

22.916.5

26.1

2.13.40.5

4.5133.925.7

135.737.524.0

271.4122.7

12.131.014.74.331.270.862.710.290.334.61.401.9

25.9

EL5512-2

59.941.80

15.049.860.124.044.023.252.310.59

100.97

14.41.3

13.727.516.1

20.7

1.73.41.4

8.8203.426.6

131.661.223.5

272.4111.0

28.659.029.96.31.991.364.410.520.625.56.406.7

21.8

EL5512-7

56.631.80

16.9012.060.184.993.522.671.140.22

100.11

16.15.46.8

22.616.0

26.2

2.13.40.5

5.7133.826.5

131.336.524.5

274.4139.0

13.341.020.64.171.290.982.530.290.322.91.623.3

24.7

EL5512-8

54.651.88

18.0213.180.204.983.872.751.010.24

100.78

12.96.06.0

23.317.6

27.6

2.33.60.6

5.0139.528.4

140.737.826.2

306.4127.9

12.436.014.94.761.511.112.870.280.343.71.874.6

27.5

EL5512-9

55.631.85

17.5912.160.195.003.762.731.020.21

100.14

14.75.76.0

23.117.3

26.3

2.13.50.5

5.3139.924.9

138.037.625.0

281.3125.3

9.930.011.73.841.500.881.800.240.323.41.801.7

27.2

EL5512-7*

55.031.69

16.8213.620.165.083.372.881.110.24

100.00

5.0121.034.0

129.023.0

EL5512-8*

54.821.74

17.4713.020.174.803.972.900.880.23

100.00

5.0130.026.0

150.020.0

BB1

51.911.67

16.2011.380.175.80

10.313.280.230.18

101.13

0.4

1.427.828.817.518.8

2.03.20.4

6.618.0

3.771.26

3.070.4

3.60.69

30.5

BB2

52.162.05

14.5512.460.195.719.673.120.380.22

100.51

2.4

2.226.424.618.119.2

2.23.90.5

6.7141.439.2

234.64.3

25.1117.437.5

10.225.6

5.281.45

3.500.46

4.51.602.2

33.4

BB3

53.252.05

14.4312.260.205.819.643.070.480.22

101.41

3.5

2.826.024.218.319.1

2.13.90.5

6.414139.9

229.414.624.5

118.237.8

9.830.0

5.171.39

3.560.41

4.51.204.0

33.4

BB4

51.691.77

15.5711.670.185.71

10.123.090.330.18

100.31

1.3

1.926.227.717.618.8

2.03.40.4

5.1115.135.2

231.25.6

24.1104.669.0

7.221.4

4.151.42

2.860.38

3.81.203.2

30.7

BB5

52.661.96

15.2112.130.195.579.773.220.390.21

101.32

2.2

2.327.325.917.518.8

2.13.70.5

5.9133.437.3

247.35.7

25.3103.041.1

10.026.3

4.981.58

3.210.37

5.01.501.5

31.3

BB6

52.981.31

14.7211.870.206.16

10.462.940.390.13

101.16

2.1

2.324.925.820.819.4

2.12.50.3

3.399.431.3

160.311.320.888.250.1

9.225.1

1.47

2.930.40

3.01.302.7

41.4

BB7

53.111.31

14.7111.800.206.54

10.512.830.400.13

101.53

2.1

2.324.026.320.620.4

2.02.50.3

3.097.732.2

162.68.9

19.583.558.3

8.518.4

3.531.55

2.840.42

3.21.502.2

40.8

Notes: Samples from the Naturalistic Plateau were collected during the Eltanin (EL) cruise; locations are shown in Figure 3. Samples marked with an asterisk (*)are data taken from Coleman et al. (1982).

Middlemost et al., 1988). These probably represent low degreemelts of the continental mantle lithosphere, as suggested bythe presence of mantle phlogopite and high incompatibleelement (Ni and Cr) abundances (Middlemost et al., 1988).

Compared with Group I tholeiites, Group II tholeiites havehigher ^Sr/^Sr ratios, show strong enrichment in highlyincompatible elements, and generally have higher La/Nbratios (Fig. 8). One possibility is that Group II tholeiitesrepresent crustal-contaminated Group I tholeiites. Simplemixing calculations show that contamination by approxi-mately 109^20% from the upper crust mimics quite well theobserved incompatible element abundances of the Group IItholeiites (Fig. 14). Although there may be alternative expla-nations for the geochemical features of the Group II tholeiites,indirect support for the crustal contamination hypothesis isthat the majority of the Rajmahal tholeiites fall on or slightlybelow the one atmosphere olivine-plagioclase-clinopyroxenecotectic, implying that most have been affected by low-pressure, upper crustal fractionation (<l-2 kbar).

The samples from Naturaliste Plateau and the BunburyBasalt show varying degrees of enrichment in the large ionlithophile elements (LILE) over normal MORB. Although iso-topic data were not available at the time of writing, these samplesall appear to have strong continental lithosphere signatures, assuggested by their high La/Nb ratios (Fig. 7) This is not incon-sistent with the suggestion of Heezen and Tharp (1973) that theNaturaliste Plateau may consist of stretched continental litho-sphere, near the ocean-continent divide.

CONCLUSIONS

Plate reconstructions are consistent with a KHP origin forthe Kerguelen Plateau (possibly in a tectonic environmentanalogous to Iceland at the present day) and associatedmagmatism along the continental margin of India, Australia,and Antarctica. Furthermore, the geochemistry of the Ker-guelen Plateau supports the hotspot hypothesis. Some plateausamples are consistent with a KHP source, whereas others

44


CO 1 0 - :

O

CDQ.>

CL

E03

CO

1 -

0.1

CD 10cco

Q.

ECO

0.1

o EL55 12-2

• EL55 12-7

100

o BB2

• BB4

a BB6

Th Nb Ta La Ce Sr P Nd Sm Zr Hf Eu Ti Y Yb

Figure 7. MORB-normalized plots and (insets) chondrite-normalizedREE plots for (A) Eltanin dredge samples from the Naturaliste Plateauand (B) Bunbury Basalt tholeiites.

have compositions that are consistent with mixing betweenMORB and KHP mantle.

Samples from the central and southern parts of the plateauthat have anomalously low 206Pb/204Pb and high La/Nb ratiossuggest contamination of the KHP source by componentsderived from the Gondwana lithosphere. This lithosphericcomponent has its extreme expression in a tholeiite from Site738 and may indicate that the southern end of the plateau isunderlain by continental crust.

Compositional similarities between some Rajmahal tholei-ites (Group I) and Kerguelen Plateau samples suggest a directcontribution to Lower Cretaceous continental volcanism bythe KHP. Other examples (Rajmahal Group II tholeiites,Naturaliste Plateau, and Bunbury Basalt) show evidence forthe substantial involvement of components derived from thecontinental lithosphere, possibly by crustal contamination ofKHP magmas. The Indian lamprophyres may represent small-degree melts of continental mantle lithosphere, with the KHPproviding a possible heat source for this activity.

ACKNOWLEDGMENTSWe are grateful to Y. Bassias and L. Leclaire for supplying

Marion Dufresne samples, J. Sheraton and N.C.N. Stephen-son for Prince Charles Mountains alnoites, and the Lamont-Doherty Geological Observatory Deep-Sea Sample Reposi-tory for Eltanin samples from the Naturaliste Plateau. J. J.Mahoney and two anonymous reviewers are thanked for theirconstructive comments and Eva Barbu for editing the manu-script. Indian Ocean research at the University of Leicester issupported by NERC (Grant No. GST/02/257).

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Storey, M., Saunders, A. D., Tarney, J., Leat, P., Thirlwall, M. F.,Thompson, R.N., Menzies, M. A., and Marriner, G. F., 1988.Geochemical evidence for plume-mantle interactions beneathKerguelen and Heard islands, Indian Ocean. Nature, 336:371-374.

Sun, S.-S., and McDonough, W. F., 1989. Chemical and isotopicsystematics of oceanic basalts: implications for mantle composi-tion and processes. In Saunders, A. D., and Norry, M. J. (Eds.),Magmatism in the Ocean Basins. Geol. Soc. Spec. Publ. (Lon-don), 42:313-345.

Talukdar, S. C , 1967. Rhyolites and alkali basalts from the SylhetTrap, Khasi Hills, Assam. Curr. Sci., 36:238-239.

Talukdar, S. C , and Murthy, M.V.N.,1970. The Sylhet Traps, theirtectonic history, and their bearing on problems of Indian floodbasalt provinces. Bull. Volcanol., 35:602-618.

Taylor, S. R., and McLennan, S. M., 1985. The Continental Crust: ItsComposition and Evolution: Oxford (Blackwell Scientific Publications).

Viereck, L. G., Taylor, P. N., Parson, L. M., Morton, A. C ,Hertogen, J., Gibson, I. L., and the ODP Leg 104 Scientific Party,

1988. Origin of the Palaeogene V0ring Plateau volcanic sequence.In Morton, A. C , and Parson, L. M. (Eds.), Early TertiaryVolcanism and the Opening of the NE Atlantic. Geol Soc. Spec.Publ. (London), 39:69-83.

Walker, K. R., and Mond, A., 1971. Mica lamprophyre (alnoite) fromRadok Lake, Prince Charles Mountains, Antarctica. BMR Rec,1971:108.

Weaver, B. L., Marsh, N. G., and Tarney, J., 1983. Trace elementgeochemistry of basaltic rocks recovered at Site 516, Rio GrandeRise, Deep Sea Drilling Project. In Barker, P. F., Carlson, R. L.,Johnson, D. A., et al., Init. Repts. DSDP, 72: Washington (U.S.Govt. Printing Office), 451-456.

Weis, D., Bassias, Y., Gautier, I., and Mennessier, J.-P., 1989.DUPAL anomaly in existence 115 Ma ago: evidence from isotopicstudy of the Kerguelen Plateau (South Indian Ocean). Geochim.Cosmochim. Acta, 53:2125-2131.

White, R. W., and McKenzie, D. P., 1989. Magmatism at rift zones:the generation of volcanic continental margins and flood basalts.J. Geophys. Res., 94:7685-7729.

Date of initial receipt: 2 March 1990Date of acceptance: 6 February 1991Ms 120B-118

47

M. STOREY ET AL.

Table 3. Major and trace element analyses of Rajmahal basalts, India.

Sample

Major elements:SiO2

TiO2

A12O3

Fe2O3

MnOMgOCaONa2OK2OP2O5

Total

CIPW norms:QtzOrAbAnDiHyMtIlmAp



RB88/2

53.201.70

14.9011.000.176.109.502.300.9400.220

99.90

5.645.56

19.4627.5614.9020.32

1.903.230.51

8.7160.635.3

332.222.622.6

101.531.6

222.3155.4316.4

18.644.623.4

16.438.221.04.691.500.682.660.440]554.402.703.1

30.8

RB88/10

52.301.77

15.0011.900.196.50

10.602.600.2700.190

101.41

3.021.60

22.0028.4718.8220.522.053.360.44

107.430.6

249.17.2

22.8106.769.4

7.921.113.73.871.390.862.860.380.462.970.711.8

35.7

RB88/15

51.802.10

15.5012.100.155.30

10.103.000.2500.230

100.49

2.891.48

25.3928.1016.9718.052.093.990.53

6.4120.830.9

264.42.0

24.8107.651.7

324.0105.0122.7

7.620.515.8

8.822.813.14.111.370.722.800.410.463.500.714.1

40.5

RB88/20

52.901.44

15.709.900.167.109.802.500.5600.200

100.28

3.763.31

21.1629.9714.1422.15

1.712.730.46

5.7118.225.7

328.717.322.591.637.4

208.6295.2201.5

10.831.717.3

11.927.713.83.721.350.562.240.340.403.032.202.3

28.8

RB88/21

50.701.76

14.6011.900.186.60

10.502.300.1900.150

98.92

3.271.12

19.4628.9618.2121.072.053.340.35

6.5106.831.5

245.21.9

22.5100.963.6

271.3229.179.39.0

18.212.3

7.4

11.73.801.500.742.990.370.383.400.453.8

34.2

RB88/29

52.901.66

14.7010.700.166.009.202.100.9500.240

98.57

7.055.61

17.7727.8913.3020.50

1.853.150.56

7.6160.333.8

322.831.721.7

103.634.1

230.5155.6306.5

18.343.222.2

RB88/30

51.401.78

15.1012.000.196.20

10.82.600.1900.190

100.56

2.501.12

22.0028.9819.2519.662.073.380.44

5.799.830.6

227.16.5

22.8103.359.2

265.3207.955.67.0

15.511.1

718.9

3.751.240.762.670.390.342.90.534.9

37.0

RB88/34

57.301.01

15.008.800.128.006.302.900.2500.120

99.74

10.311.48

24.5427.18

2.6729.14

1.521.920.28

49.926.7

148.25.9

22.6138.4147.4130.0672.563.5

5.68.05.5

3.98.3

2.521.020.701.940.270.151.480.492.6

14.5

RB88/35

53.001.82

14.9011.400.185.508.902.601.1700.260

99.53

4.686.91

22.0025.5413.9719.621.973.460.60

10.2170.436.8

324.332.821.8

105.529.6

223.2119.4371.9

19.241.725.3

RB88/42

51.101.78

15.0012.100.186.80

10.802.500.1500.190

100.52

1.920.89

21.1629.2718.%21.452.093.380.44

4.693.929.6

223.34.1

24.0104.765.4

271.8415.044.4

6.514.013.6

BH-1

54.021.75

14.6710.920.175.919.452.791.1770.231

101.09

3.906.96

23.6124.0417.5518.351.883.320.54

8.4159.734.8

325.323.121.694.617.7

BH-6

52.61.65

14.710.70.175.99.22.301.1200.230

98.49

5.366.62

19.4626.4914.5119.681.853.130.53

9.9160.333.9

321.422.121.1

105.029.5

229.4187.1308.6

15.338.121.3

TT-3

52.091.71

1512.010.26.48

10.762.830.1500.185

101.42

1.810.89

23.9527.7920.0720.122.073.250.43

108.331.0

237.90.3

22.293.768.6

7.521.614.13.981.340.803.120.420.473.020.924.1

37.9

Note: See Figure 4 for locations.

48


Table 3 (continued).

Sample

Major elements:SiO2

TiO2

A12O3

Fe2O3

MnOMgOCaONa2OK2OP2O5Total

CIPW norms:QtzOrAbAnDiHyMtIlmAp



LH-1

50.71.55

15.211.30.26

11.12.500.1700.160

98.93

2.471.00

21.1629.7619.9918.25

1.952.940.37

3.995.229.5

233.64.0

22.39576.8

277.0206.382.1

5.117.510.2

GP-1

54.11.74

14.710.90.175.99.42.301.1000.220

100.50

6.646.50

19.4626.5515.3419.401.883.300.51

9.3159.133.6

347.430.223.0

101.720.3

235.8172.2265.3

16.639.121.8

BL-3

6.384.826.8

231.83.9

21.291.866.6

274.4241.697.6

7.211.89.5

KP-4

51.42.21

14.613.40.25.9

10.72.50.2300.210

101.29

3.181.36

21.1627.9419.6819.872.314.200.49

6.5127.035.0

236.34.2

22.5110.267.8

294.4128.489.310.326.114.5

KP-6

52.11.49

15.611.60.26.2

11.82.50.1900.150

101.86

2.281.12

21.1630.7922.0618.232.002.830.35

4.583.826.6

232.75.0

21.188.968.6

267.1220.592.4

5.019.010.4

KP-7

51.32.22

14.713.10.25.8

10.82.60.2500.220

101.14

2.721.48

22.0027.7120.2218.942.264.220.51

7.5131.335.8

240.76.1

25.3111.382.3

305.5132.9102.8

8.723.916.8

NP-1

51.51.76

14.412.40.26.4

112.50.2100.180

100.55

2.631.24

21.1627.4621.3819.72.143.340.42

5.5101.631.7

230.22.9

21.9101.954.8

287.9167.3143.4

5.821.112.9

NP-4

7.0118.136.4

304.48.1

22.0114.260.3

328.2148.6246.0

11.423.716.9

NP-7

52.301.82

15.0011.700.206.60

10.402.500.2700.190

101.04

3.721.60

21.1628.9217.6121.042.023.460.44

4.9102.530.6

233.02.5

23.4105.858.5

265.9237.474.6

7.322.211.9

NP-9

51.801.69

14.4011.600.196.40

10.302.500.3100.190

99.49

3.831.83

21.1627.1618.6720.082.003.210.44

6.7107.330.5

254.36.5

22.897.753.5

255.1230.983.0

7.022.211.3

7.822.013.54.061.390.803.260.400.462.700.753.8

35.7

LA-2

52.602.09

13.7012.300.185.70

10.002.300.3700.230

99.49

7.122.19

19.4625.9718.2918.752.123.970.53

6.7114.533.2

219.35.9

19.2102.662.8

304.0165.380.35.4

18.216.8

LA-6

49.402.18

14.0012.800.205.90

10.602.400.2900.250

98.14

2.341.71

20.3126.5820.1418.892.214.140.58

49

M. STOREY ET AL.

130

120

110tsiµr 100

90

80

70

1

0.8

9i 0.6

0.4

0.2

0

30

25

20

S. 15

10

5

0

1000

_Q 800

600

400

200

2.5

2

1.5

1

0.5

r A

: B

L C

: D

Group I

Group II

1.5 2 2.5 3 3.5 4 4.5 5

Zr/Y

Figure 8. The variation of the Zr/Y ratio in Rajmahal samples vs. (A)Ti/Zr, (B) K2O %, (C) Rb (ppm), (D) K/Rb, and (E) La/Nb.

0.1 —"-Th Nb Ta La Ce Sr P Nd Sm Zr Hf Eu TI Y

Figure 9. MORB-normalized plots and chondrite-normalized (inset)REE plots for (A) Group I Rajmahal tholeiites, (B) Group II Rajmahaltholeiites, and (C) Lower Cretaceous Indian lamprophyres; data fromPaul and Potts (1981).

50


Table 4. Major and trace element analyses ofalnoites from the Prince Charles Mountains,Antarctica.

Sample

Major elements:SiO2

TiO2

A12O3

Fe 2 O 3

MnOMgOCaONa2OK2O

Total

CIPW norms:AnLuNeDiCs01MtIlmAp

XRF:NbZrYSrRbThZnNi


D15

37.142.239.47

12.110.20

16.1116.362.083.290.88

99.87

6.815.39.5

22.739.1

2.14.22.0

118.0235.7

37.51003.9

129.910.685.2

281.2

89.0223.0

99.010.22.991.582.350.219.56.1

14.321.024.6

D18

37.352.42

10.5512.540.22

13.8815.402.463.511.30

99.63

7.416.311.3

1.818.534.0

2.24.63.0

165.1437.7

48.71115.3

148.011.783.5

238.9

96.0225.0106.0

11.03.121.853.280.389.89.2

12.313.530.2

1000 =

=

-

GO 100 —

rr -o =ΦQ.

> 10 -|

"o.

εCO -I -

:

n 1 —'

a

\ ,

o D15

• D18

Ü 120-748C-79R-6, 90-94i i i i i

S •

% 1001

o -

^ 1 0 T,

amp

w 1

La1

^ ^

Eu Lu

—1 1 1 1Th Nb Ta La Ce Sr P Nd Sm Zr Hf Eu Ti Y Yb

Figure 10. MORB-normalized plots and chondrite-normalized (inset)REE plots for Prince Charles Mountains alnoites. Note the strongsimilarity with the Albian alkali basalt from Site 748.

Note: See Figure 1 for location.

51

M. STOREY ET AL.

4 -

2OJ

-4 -

Rajmahaltholeiites (II)

MD48 dredge samples

0.7025 0.7035 0.7045 0.7055 0.7065 0.7075

87Sr/86Sr

Figure 11. ^Sr/^Sr vs. ε N d for Kerguelen Plateau (data from Alibert, in press;Salters et al., this volume; M. Storey, unpubl. data) and Rajmahal basalts (datafrom Mahoney et al., 1983; M. Storey, unpubl. data). Data corrected for T ~ 110m.y., except for Site 748, where T ~ 90 m.y. Site 750 sample acid leached in 6MHC1. Stippled area represents most enriched basalts from Kerguelen Island(Storey et al., 1988, and references cited therein). Data for normal SoutheastIndian Ridge (SEIR) MORB from Dosso et al. (1988), Hamelin et al. (1986), andMichard et al. (1986).

52


15.8

Q_

Q_

o<ΛJ

15.7 -

15.6

15.5

-QQ_

J2CL

COOC\J

39.0 -

38.0 -

37.0

Western Australianlamproites

17.0 18.0206pb/204pb

19.0

Figure 12. 206Pb/204Pb vs. (A) 207Pb/2O4Pb and (B) Pb/^Pb forKerguelen Plateau and Rajmahal Trap tholeiites (data from Weis etal., 1989; Alibert, 1991; Salters et al., this volume; M. Storey, unpubl.data). Lines joining points represent the age correction for samples onwhich U and Pb were determined by isotope dilution. Fields forwestern Australian lamproites and Gaussberg leucitites from Nelsonet al. (1986); field for Kerguelen Island basalts field from Dosso et al.(1979), Storey et al. (1988), and Gautier et al. (1990). NHRL =Northern Hemisphere Reference Line (Hart, 1984).

0.713

0.711

OT ° 7 0 9

0.707CO

0.705

0.703

+10

+5

0

T3

~z.CO -5

-10

-15

-20


O Site 738

Site 747

Site 750

DR02/12

Site 749

Indian Ocean MORB

Site 748

Kerguelen Is.DR08/5

Site 747Kerguelen Is.


= MD48 dredge samples

17 18

206p b / 204 p b

19

Figure 13. ^ P b / ^ P b vs. (A) ^Sr/^Sr and (B) ε N d for KerguelenPlateau tholeiites. Sources of data are the same as for Figures 11 and12.

100 ~3

10 -=

1 ~

0.1

O120-749C-15R-5.125-27

o RB88/21

• RB88/2

• Upper Crust

RB88/21 (80%) + Upper Crust (20%)—i 1 1 1 i rTh Nb Ta La Ce Sr P Nd Sm Zr Hf Eu Ti Y Yb

Figure 14. Comparison between MORB-normalized incompatibleelement plots for Group I Rajmahal tholeiite (RB 88/21) and Site 749Kerguelen Plateau tholeiite. The addition of 20% of material withupper-crust composition (from Taylor and McLennan, 1985) to GroupI Rajmahal tholeiite mimics incompatible element abundances of theGroup II Rajmahal tholeiite illustrated (RB 88/2).

53

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