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Journal of Petrology | Volume 43 | Number 7 | Pages 1105-1108 | 2002
© Oxford University Press 2002
Origin and Evolution of the Kerguelen Plateau, Broken Ridge and Kerguelen Archipelago: Editorial
1DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF OREGON, EUGENE, OR 97403-1272, USA
2DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
3DÉPARTEMENT DES SCIENCES DE LA TERRE ET DE L’ENVIRONNEMENT, UNIVERSITÉ LIBRE DE BRUXELLES CP 160/02, AVENUE F. D. ROOSEVELT 50, B-1050 BRUSSELS, BELGIUM
4OCEAN RESEARCH INSTITUTE, UNIVERSITY OF TOKYO, 1-15-1 MINAMIDAI, NAKANO-KU, TOKYO 164-8639, JAPAN
5INSTITUTE FOR FRONTIER RESEARCH ON EARTH EVOLUTION, JAPAN MARINE SCIENCE AND TECHNOLOGY CENTER, 2-15 NATSUSHIMA-CHO, YOKOSUKA 237-0061, JAPAN
6INSTITUTE FOR GEOPHYSICS, JACKSON SCHOOL OF GEOSCIENCES, UNIVERSITY OF TEXAS AT AUSTIN, 4412 SPICEWOOD SPRINGS ROAD, BUILDING 600, AUSTIN, TX 78759-8500, USA
Received ; Revised typescript accepted
Large igneous provinces (LIPs) are constructed when copious amounts of mantle-derived magma enter the Earth’s crust in localized regions. Although the Kerguelen Plateau and Broken Ridge are now submarine, they formed as a contiguous, largely subaerial LIP during Cretaceous time in the eastern Indian Ocean. Subsequently, they were separated by sea-floor spreading beginning in Middle Eocene time along the Southeast Indian Ridge (SEIR; Fig. 1). The Kerguelen Archipelago is a Cenozoic feature constructed on the Northern Kerguelen Plateau (NKP; Fig. 1). Together with the 
82–38 Ma hotspot track formed by the Ninetyeast Ridge, the Kerguelen Plateau, Broken Ridge and Kerguelen Archipelago represent 119 Myr of volcanism that has been attributed to the Kerguelen mantle plume. Although the igneous basement of the Kerguelen Plateau had been sampled at Sites 738, 747, 749 and 750 (Fig. 1) by Ocean Drilling Program (ODP) Legs 119 and 120, Leg 183 was the first leg dedicated to understanding the origin and evolution of an oceanic LIP. Five sites (1136, 1137, 1138, 1139, 1140) penetrated igneous basement of the Kerguelen Plateau, and at two sites (1141, 1142) the first basement samples were obtained from Broken Ridge (Fig. 1).
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, lamprophyres; BB: Bunbury Basalts.Two objectives of ODP Leg 183 were to sample the submarine LIP in as many locations as possible to: (1) determine the temporal variation in magma production; (2) constrain the origin and evolution of the volcanic rocks forming the LIP. Studies of igneous basement recovered during Leg 183 complement continuing studies of the Cenozoic subaerial lavas that form the Kerguelen Archipelago. This volume includes a collection of papers that address these objectives by using data for lavas forming the Kerguelen Plateau, Broken Ridge and Kerguelen Archipelago.
High-quality radiometric age determinations are critical for understanding rates of LIP formation, their causative dynamic mantle processes, and their temporal relationships to potentially related environmental changes. Duncan employs the 40Ar/39Ar technique to determine ages at all of the ODP Leg 183 igneous basement drill sites (Fig. 1). In general, ages young northwards on the Kerguelen Plateau, from 119 Ma at Site 1136 in the south to 34 Ma at Site 1140 in the north. Emplacement of the conjugate Central Kerguelen Plateau (CKP) and Broken Ridge falls within an 5 Myr window from 100 to 95 Ma. An unanticipated result is 68 Ma lavas on Skiff Bank, west of the significantly younger Kerguelen Archipelago (Fig. 1). On the basis of the ages for Site 1136 basalt, Duncan suggests that rapid submarine construction of the Southern Kerguelen Plateau (SKP) by 118–119 Ma may have contributed to a global environmental crisis recorded by widespread marine black shales (global oceanic anoxic event OAE1).
Temporal variability in magma production attributed to the Kerguelen hotspot is primarily a function of lithospheric thickness and source variability. Coffin et al. present new 40Ar/39Ar age determinations for ODP Site 750 (SKP) basalts (112 Ma), Indian and Antarctic lamprophyres (114 Ma), Rajmahal Traps (118 Ma), and Bunbury Basalts (132 Ma). They use high-quality ages and geophysical results from the Kerguelen Plateau, Broken Ridge, and Ninetyeast Ridge (Fig. 1) to calculate the magma flux from the Kerguelen hotspot since early Cretaceous time. Placing their results in a plate tectonic model for the Indian Ocean shows that the Kerguelen hotspot does not fit standard plume models in two critical areas: (1) peak magma production lagged continental break-up between India and Antarctica by at least 15 Myr; (2) high magma output rates lasted 25 Myr. To account for these observations, Coffin et al. suggest multiple plumes or shearing of a single plume conduit into several diapirs.
The Rajmahal Traps (Fig. 1) represent the first magmatism of continental flood basalt scale attributed to the Kerguelen hotspot. Kent et al. find that eruption of this flood basalt commenced at 118 Ma and continued until 115 Ma. Examining all of the Early Cretaceous age determinations from the Rajmahal Traps and the Kerguelen Plateau in the framework of a plate tectonic model incorporating hotspot motion in a convecting mantle, they suggest that the Kerguelen hotspot supplied magma for both the Rajmahal Traps and the SKP, and that the hotspot is related to the separation of Elan Bank and India (Fig. 1).
The uppermost (ten to hundreds of meters) igneous rocks forming the LIP are primarily tholeiitic basalt, but alkalic lavas dominate at Sites 1139, 1141 and 1142 (Fig. 1), and highly evolved lavas such as trachyte and rhyolite occur at several locations. On the basis of trace element and isotopic characteristics, a component derived from continental lithosphere is very apparent in tholeiitic basalt at Site 738; although the continental signatures differ and are more subtle, Frey et al. argue that tholeiitic basalts from Sites 747 and 750 and perhaps in some units at Site 749 contain components derived from continental crust (Fig. 1). Are these continental components deeply recycled crust that are intrinsic to a mantle plume or do they represent fragments of continents dispersed into the asthenosphere and lithosphere of the Indian Ocean? The occurrence of garnet–biotite gneiss clasts with Neoproterozoic zircons and monazites in a fluvial conglomerate intercalated with tholeiitic basalt at Site 1137 on Elan Bank (Fig. 1) demonstrates that continental crust was dispersed into the Indian Ocean lithosphere during Gondwana break-up. Ingle et al. show that these gneiss clasts have Sr, Nd and Pb isotopic ratios consistent with an origin from the Eastern Ghats Belt of eastern India and that felsic volcanic clasts are not related to the tholeiitic basalt, but were derived by partial melting of continental crust. Although tholeiitic basalt with geochemical evidence for a component derived from continental lithosphere is widely distributed on the Kerguelen Plateau (e.g. unambiguous evidence at Sites 738, 1137 and 747, Fig. 1), Neal et al. find no evidence for such a component in the 144 m of tholeiitic basalt penetration at Site 1138 (CKP) or in the alkalic basalt flows that apparently overlie tholeiitic basalt at Sites 1141 and 1142 on Broken Ridge (Fig. 1).
The NKP had not been drilled before Leg 183. Kieffer et al. conclude that the 68 Ma, 230 m of bimodal alkalic lavas (trachybasalt and trachyte–rhyolite) at Site 1139 on the NKP are part of a shield volcano built on the tholeiitic basalt plateau (Fig. 1). These alkalic lavas show no compelling evidence for a continental component. Farther north on the NKP at Site 1140 (Fig. 1), the uppermost basement is relatively young (34 Ma) tholeiitic pillow basalt. These magmas erupted close to the axis of the SEIR, probably within 50 km of it, and they provide the first evidence of submarine eruptive rocks on the Kerguelen Plateau. Weis & Frey find that the geochemical characteristics of the basalt suite from Site 1140 can be explained by mixing small (1–30%) and variable proportions of a Kerguelen plume-derived magma with SEIR mid-ocean ridge basalt (MORB). Incompatible element analyses of the glassy rims of the pillows by Wallace confirm that the magmas ranged in composition from similar to SEIR MORB to compositions intermediate between MORB and plume-derived magmas. Glasses with a larger proportion of plume component have higher water contents (0·44–0·69%), but their H2O/Ce ratios are lower than in MORB; apparently the Kerguelen plume is not a wetspot.
The Cenozoic Kerguelen Archipelago constructed on the NKP (Fig. 1) has a volcanic record extending from 29 Ma to the Pleistocene. Geochemical studies of the flood basalt forming the archipelago from 29 to 25 Ma show that with decreasing age the basaltic lavas change from tholeiitic–transitional to alkalic. Damasceno et al. use clinopyroxene–liquid thermobarometry to show that the 25 Ma Mt. Crozier alkalic lavas crystallized over a wide range of pressures (1–12 kbar), and they infer that significant fractionation of high-Al clinopyroxene occurred near the base of the crust. They further suggest that thickening lithosphere led to increases in the depth of crustal fractionation because earlier tholeiitic–transitional lavas show no evidence for high-Al clinopyroxene fractionation.
Geochemical studies of sections through the flood basalt of the archipelago document a role for a depleted component whose proportion decreased from 29 to 25 Ma. The isotopic characteristics of the alkalic basalt section at Mt. Crozier have been inferred to represent the Kerguelen plume at 25 Ma, and the Hf–Nd isotopic systematics of archipelago lavas presented by Mattielli et al. require a change in the isotopic characteristics of the plume at 20 Ma. The origin of the depleted component in older archipelago lavas is debated. Frey et al. find near isotopic homogeneity in 26 Ma alkalic and underlying tholeiitic basalt at Mt. Tourmente. The isotopic characteristics of these lavas differ from those of Mt. Crozier alkalic basalt and the differences may reflect intrinsic isotopic heterogeneity within the plume. In contrast, the isotopic heterogeneity of older tholeiitic to transitional basalt sections leads Doucet et al. to argue for two-way communication between the plume and SEIR over distances of a few hundred kilometers. Although the origin of the depleted component is unresolved, a point of agreement is that, like the basaltic portions of drillcore on the NKP (Sites 1139 and 1140), archipelago lavas show no evidence for a continental component. Therefore the role of a continental lithosphere component in basalt related to the Kerguelen plume decreased from Cretaceous to Cenozoic time.
Previously a significant problem in evaluating the similarity of the depleted component in archipelago lavas to SEIR MORB was the scarcity of geochemical data for SEIR MORB. This deficiency is alleviated by the extensive isotopic data set of Mahoney et al. These workers find that from 90°E to 118°E the along-axis variation of He, Sr, Nd and Pb isotopic ratios cannot be explained by eastward flow from the Kerguelen plume, although there may be an influence of the Kerguelen plume on SEIR MORB at 86–90°E (Fig. 1).
In summary, ODP drilling on the Kerguelen Plateau and Broken Ridge LIP has shown that the surface of this LIP: (1) formed over tens of million years, inconsistent with the melting pulse inferred for a large plume head; (2) is dominantly tholeiitic basalt erupted subaerially, but there are significant occurrences of alkalic and explosively erupted, evolved volcanic rocks. In several locations the tholeiitic basalt was contaminated by components derived from continental crust, and fragments of such crust occur on Elan Bank. Cenozoic lavas forming the Kerguelen Archipelago contain no evidence for a continental crust component, but they show that the Hf isotopic ratio of the plume changed at 20 Ma and that the role of a depleted component decreased from 29 to 25 Ma. As we look to the future, advancing our understanding of the origin of this LIP will require improved geophysical, especially seismic, characterization, as well as deep penetrations of igneous basement during the Integrated Ocean Drilling Program.
FOOTNOTES
*Corresponding author. Telephone: (541) 346-5985. Fax: (541) 346-4692. E-mail: pwallace{at}darkwing.uoregon.edu 
Present address: Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC, V6T 174 Canada.
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