Ocean Drilling Program (ODP) Legs 119 and 120 drilling results (Barron, Larsen, et al., 1989; Schlich, Wise, et al., 1989), dredging data (Leclaire et al., 1987; Davies et al., 1989; Duncan, 1991; Weis et al., 1998), and multichannel seismic reflection data (Coffin et al., 1990; Schaming and Rotstein, 1990; Schlich et al., 1993) have shown that igneous basement of the Kerguelen Plateau and conjugate Broken Ridge is basaltic. Numerous dipping intrabasement reflections interpreted as flood basalts have been identified in the crust of the CKP and SKP and on Elan Bank (Könnecke et al., 1997). Wide-angle seismic data from the Kerguelen Archipelago on the NKP show an upper igneous crust 8-9.5 km thick and a lower crust 6-9.5 km thick (Recq and Charvis, 1986; Recq et al., 1990, 1994; Charvis et al., 1995). Wide-angle reflection and refraction experiments employing ocean bottom seismometers have been undertaken recently on both the CKP and SKP (Charvis et al., 1993, 1995; Operto and Charvis, 1995, 1996; Könnecke et al., 1998; Charvis and Operto, 1998). The crustal structure beneath the Kerguelen Archipelago differs significantly from that of the CKP. Igneous crust of the CKP is 19 to 21 km thick and is composed of three layers. The upper layer is 1.2 to 2.3 km thick, and velocities range from 3.8 to 4.9 km/s. It could be composed of either lava flows or interlayered volcanic and sedimentary beds. The second layer is 2.3 to 3.3 km thick, and velocities increase downward from 4.7 to 6.7 km/s. In the ~17-km-thick lower crust, velocities increase from 6.6 km/s at ~8.0 km depth (near the top of the layer) to 7.4 km/s at the base of the crust, with no internal discontinuity. On the southern plateau, the ~22-km-thick igneous crust can be divided into three layers: (1) an upper crustal layer ~5.3 km thick with velocities ranging from 3.8 to 6.5 km/s; (2) a lower crustal layer ~11 km thick with velocities of 6.6 to 6.9 km/s; and (3) a 4- to 6-km-thick transition zone at the base of the crust characterized by velocities of 6.7 to 6.9 km/s (Operto and Charvis, 1995, 1996). This low-velocity, seismically reflective transition zone at the crust/mantle interface has not been imaged on the NKP or CKP; it is the basis for the hypothesis that parts of the SKP contain fragments of continental crust (Operto and Charvis, 1995; 1996).
Previous Sampling of Igneous Basement: Ages and Geochemical
In this section we summarize results of previous sampling (Legs 119 and 120 and dredging) of the Kerguelen Plateau-Broken Ridge LIP. Based in large part on ODP-related studies, there is a consensus that the Kerguelen plume was the major source of magma for constructing the Kerguelen Plateau-Broken Ridge LIP. Although sampling and dating of the entire LIP are grossly insufficient, sampling of the SKP at four spatially diverse locations (Sites 738, 749, and 750, and dredge site MD48-05; see Figs. 3, 4) shows that the uppermost igneous crust of SKP formed over a relatively short interval at ~110 Ma (K/Ar data from Leclaire et al., 1987; Whitechurch et al., 1992; and 40Ar/39Ar data from Pringle et al., 1994; Storey et al., 1996; M.F. Coffin et al., unpubl. data). In contrast, basement basalts from Site 747 on the CKP may be much younger, ~85 Ma (M.F. Coffin et al., unpubl. data). This age is similar to the 83-88 Ma age for lavas from Broken Ridge dredge sites 8 and 10 (40Ar/39Ar data from Duncan, 1991), which coincide spatially with the prebreakup position of Site 747 (Figs. 1, 2). Also, piston coring of sediments on the northeast flank of the CKP between the Kerguelen Archipelago and Heard Island (MD35-510 in Fig. 4) recovered cherts and calcareous oozes of probable Santonian age (Fröhlich and Wicquart, 1989). In summary, we have very few high-quality age data for the 2.3 x 106 km2 (equivalent to approximately eight Icelandic plateaus) of the Kerguelen Plateau-Broken Ridge LIP. These sparse data support the hypothesis that large magma volumes erupted over short time intervals, possibly as two pulses during Cretaceous time--the SKP at ~110 Ma; the CKP, Broken Ridge, and perhaps Elan Bank at ~85 Ma (Fig. 7). In contrast, Cenozoic volcanism (~38 Ma to present) has formed the Kerguelen Archipelago (e.g., K.E. Nicolaysen et al., unpubl. data), Heard and McDonald Islands (Clarke et al., 1983; Quilty et al., 1983), and the bathymetric/gravity highs between the Kerguelen Archipelago and Heard Island (Weis et al., 1998). A major goal of Leg 183 was to drill at other sites throughout the plateau to determine if formation of this LIP was truly episodic or if there was a continuous south to north decrease in age of volcanism.
Although the southern and central Kerguelen Plateau formed in a young oceanic basin (Royer and Coffin, 1992; Munschy et al., 1994; Coffin et al., unpubl. data), evidence is equivocal as to whether it formed at a spreading center, like Iceland, or off-ridge, like Hawaii (Coffin and Gahagan, 1995). Before Leg 183, several observations had indicated that much of the uppermost basement of the southern and central Kerguelen Plateau erupted in a subaerial environment-specifically, (1) oxidized flow tops and vesicularity of lava flows at Sites 738 and 747; (2) nonmarine, organic-rich sediments containing up to 5-cm pieces of charcoal overlying the basement at Site 750; and (3) claystone overlain by a basalt cobble conglomerate and glauconitic sediment with wood fragments in the lowermost core at Site 748 (Schlich et al., 1987). Coffin (1992) concluded that the drill sites in the SKP had long (>10 to <=50 m.y.) histories of subaerial volcanism and erosion, followed by subsidence caused by cooling. Zeolite mineralogy of the basaltic basement indicates erosion to deeper levels at Site 749 than at Sites 747 and 750 (Sevigny et al., 1992).
The islands on the Kerguelen Plateau are dominantly formed of <40-Ma transitional and alkaline lavas (Fig. 8) (Weis et al., 1993, 1998; Barling et al., 1994; Yang et al., 1998; K.E. Nicolaysen et al., unpubl. data). Before Leg 183, the only alkaline basalt recovered from the Kerguelen Plateau, Broken Ridge, and Ninetyeast Ridge was a flow ~200 m above basement at Site 748. Tholeiitic basalt of Cretaceous age has been recovered from four dredge and four drill sites on the central and southern Kerguelen Plateau and three dredge sites on Broken Ridge (Figs. 3, 4, 5, 6, 8); seven drill sites on Ninetyeast Ridge have yielded solely tholeiitic basalt ranging from 38 to 82 Ma (Fig. 1). Although the tholeiitic basalts from several of these sites are geochemically distinct, their incompatible element abundances resemble those of ocean-island tholeiitic basalts, rather than typical mid-ocean-ridge basalts (MORBs) (Kerguelen Plateau and Broken Ridge: Davies et al., 1989; Weis et al., 1989; Storey et al., 1992; Mahoney et al., 1995; Ninetyeast Ridge: Frey et al., 1991; Saunders et al., 1991; Frey and Weis, 1995). We infer that tholeiitic basalt was the dominant magma type produced by the Kerguelen plume from ~110 to 38 Ma during formation of the Kerguelen Plateau, Broken Ridge, and Ninetyeast Ridge. The significance of this result is that tholeiitic basalts are derived from relatively high (>5%) extents of partial melting (Kent and McKenzie, 1994), and the inference is that the Kerguelen plume was a high-flux magma source for a long time (Figs. 2, 7). However, the MgO-rich melts expected from large extents of melting of high-temperature plumes (e.g., Storey et al., 1991) have not been recovered from Cretaceous parts of the Kerguelen Plateau. Picritic (i.e., olivine rich) alkaline lavas of Quaternary age are found on Heard Island (Barling et al., 1994), and transitional picritic lavas (14 to 19 Ma) were recently dredged from one of the bathymetric/gravity highs between the Kerguelen Archipelago and Heard Island (Weis et al., 1998) (see Figs. 3, 4). All of these picrites are olivine-rich cumulates rather than crystallized MgO-rich melts.
Most lavas from the Kerguelen Plateau and Broken Ridge have Sr and Nd isotopic ratios that range from the high 87Sr/86Sr-low 143Nd/144Nd end of the field for SEIR MORB to the field proposed for the Kerguelen plume (Fig. 9). In Pb-Pb isotopic plots, Kerguelen Plateau lavas from Sites 747, 749, and 750 define an elongate field subparallel to that for SEIR MORB (Fig. 10); however, like lavas forming the Kerguelen Archipelago, the Kerguelen Plateau lavas are offset from the MORB field to higher 208Pb/204Pb and 207Pb/204Pb at a given 206Pb/204Pb ratio. In addition, submarine Kerguelen Plateau lavas extend to lower 206Pb/204Pb than Kerguelen Archipelago lavas (Fig. 10). These Sr, Nd, and Pb isotopic data have been interpreted as a result of mixing between the Kerguelen plume and entrained depleted (MORB related) asthenosphere (e.g.,Weis et al., 1992).
In contrast, basalts from Site 738 on the southernmost SKP and dredge 8 from eastern Broken Ridge (Figs. 1, 2, 3, 4, 5) have atypical geochemical characteristics for oceanic lavas. These lavas have very high 87Sr/86Sr, low 143Nd/144Nd, and very high 208Pb/204Pb and 207Pb/204Pb ratios that accompany relatively low 206Pb/204Pb (Figs. 9, 10). They also have relative depletions in abundances of Nb and Ta, and there is a positive correlation between 87Sr/86Sr and the extent of Nb depletion (Fig. 11). Mahoney et al. (1995) concluded that these isotopic characteristics, coupled with depletions of Nb and Ta, arose from a continental lithosphere component that contributed to these basalts, a hypothesis also proposed by Storey et al. (1989) to account for Ta depletion in basalts dredged from the Kerguelen Plateau. Basalts from Site 738 on the SKP and dredged from eastern Broken Ridge are relatively depleted in Nb (Fig. 12A). Significant relative depletion in Nb is also evident in basalts dredged from the 77° graben on the SKP and from Site 747 on the CKP. Trends to anomalously high Th/Nb and La/Nb and a positive correlation between 87Sr/86Sr and La/Nb are also defined by the Bunbury Basalt, southwest Australia, and the Rajmahal Basalt, northeast India (Figs. 11, 12B). These continental basalts, erupted at ~123-130 Ma and 116 Ma, respectively, are contaminated to varying degrees by continental crust (Frey et al., 1996; Kent et al., 1997). The combination of geochemical features in basalts from Site 738 and eastern Broken Ridge (i.e., very high 87Sr/86Sr and low 143Nd/144Nd; high 208Pb/204Pb and 207Pb/204Pb ratios that accompany relatively low 206Pb/204Pb; anomalously high Th/Nb and La/Nb; see Figs. 9, 10, 11, 12) is consistent with continental crust as the continental component. In particular, the low 206Pb/204Pb ratios require aged crust with low U/Pb, such as some types of Archean crust.
In detail, the trend for Site 747 lavas (Fig. 12A) differs from that of other Kerguelen Plateau basalts because Site 747 lavas trend to high La/Nb without elevated Th/Nb. This trend is similar to that for North Atlantic MORB from the lower flow units at Hole 917A (Fig. 12B), which are contaminated by the Archean crust of eastern Greenland, specifically, lower crustal granulite-facies gneiss (Fitton et al., 1998a, 1998b). The effects of continental contamination are very evident in these Hole 917A basalts because their parental magmas were MORB-like with much lower incompatible element abundances than are found in most plume-related lavas. Note that the combination of elevated La/Nb with low Th/Nb is unlike recent estimates of average lower crust composition (e.g., LC in Fig. 12; Rudnick and Fountain, 1995) but is typical of Lewisian granulites (Fig. 12; lower crust estimates are from Weaver and Tarney, 1984). Although not as extreme as some basalts from the lower units of Hole 917A, several geochemical characteristics of Site 747 basalts are consistent with crustal contamination-namely, (1) the trend to high La/Nb without high Th/Nb; (2) the offset to low 143Nd/144Nd from the 87Sr/86Sr-143Nd/144Nd trend defined by Kerguelen Archipelago lavas; and (3) the low 206Pb/204Pb ratios, which are lower than those of all other lavas from the Kerguelen Plateau, Ninetyeast Ridge, Kerguelen Archipelago, and Heard Island (Figs. 9, 10, 11, 12). These characteristics are consistent with ancient continental crust as the contaminant; the high La/Nb-low Th/Nb (Fig. 12A) trend suggests a component similar to Lewisian granulites. Archean granulites are on the conjugate Antarctic and Indian margins (e.g., Black et al., 1992), and it is possible that fragments of such crust were incorporated into the embryonic Indian Ocean and subsequently sampled during formation of the Kerguelen Plateau. In addition, the Os and Pb isotopic ratios of peridotite xenoliths in basalts from the Kerguelen Archipelago are interpreted as reflecting Gondwana lithospheric mantle that was incorporated into the Indian Ocean mantle during rifting (Hassler and Shimizu, 1998; Mattielli et al., 1999).
The geochemical evidence for a continental component in basalts forming the Kerguelen Plateau and Broken Ridge is consistent with a crustal velocity structure suggesting that the SKP contains a stretched continental fragment (Operto and Charvis, 1995; 1996); this geophysical evidence is at ~58°S in the vicinity of the basalts dredged from the 77° graben and cored at Site 750, whereas Site 738 is much farther south at ~63°S and Site 747 is to the north at ~55°S (Figs. 3, 4). These results suggest that continental lithosphere may be widespread in the Kerguelen Plateau.
Leg 183 Scientific Objectives
Leg 183 Table of Contents