The Cretaceous to recent history of the southeastern Indian Ocean : the role of mantle plumes examined by geochronology and Sr, Nd, Pb and He isotopic geochemistry

Abstract

Tectonically active for the past ~160 million years, the igneous basement of the Indian Ocean basin has formed due to complex interactions between mid-ocean ridges and mantle plumes. Plateaus, ridges and islands associated with the Kerguelen hotspot, the world's second largest, oceanic igneous province, dominate the bathymetry of the southeastern Indian Ocean. Lavas erupted by the Kerguelen plume range in age from the -119 million year old basalts of the southern Kerguelen Plateau to the recent eruptions at Kerguelen, Heard, and McDonald Islands. This dissertation presents: 1) the Pb geochronology of the oldest rocks recovered from the Kerguelen Plateau, 2) the geochronology and Sr-Nd-Pb-He isotopic characteristics of the olivinerich lavas of the Kerguelen Archipelago, and 3) Sr-Nd-Pb isotopic evidence for contamination of Southeast Indian Ridge basalts with melts from the Amsterdam-St. Paul and Kerguelen plumes. At Elan Bank of the Kerguelen Plateau, Leg 183 of the Ocean Drilling Program recovered basalt, trachyte, and garnet-biotite gneiss clasts from a 26 m thick fluvial conglomerate which is intercalated with Kerguelen Plateau basalts. Zircons and monazites obtained from the gneiss clasts yield a range of Proterozoic Pb-Pb dates when examined by conventional mass spectrometric and electron microprobe techniques. The deposition of the gneiss clasts, slow seismic velocities in the crust of Elan Bank, and anomalous isotopic compositions of basalts above and below the conglomerate suggest that some part of the crust of Elan Bank is a continental fragment formed during the rifting of East Gondwana. Cenozoic eruptions from the Kerguelen plume constructed the primarily basaltic Kerguelen Archipelago (6500 km 2) on the northern Kerguelen Plateau. "Ar/ 39Ar geochronology of 15 basalts from five stratigraphic sections yields isochron ages ranging from 29.26±0.87 to 24.53±0.29 Ma. The oldest dated basalt from the archipelago (-29 Ma) is much younger than the -40 Ma conjunction between the hotspot and the Southeast Indian Ridge. Basalt eruption seems to have ceased shortly after -24 Ma although small volume lava flows and plutonic intrusions continued to form in the archipelago. The basalt age data suggest an average lava accumulation rate of -1.6 0.9 km/my during the Oligocene. The archipelago's volumetric eruption rate (0.009 km3/y) is lower than estimates made for the Cretaceous Kerguelen Plateau (1.7 k 3/y) and the Ninetyeast Ridge hotspot track (0.18 km3 /y), suggesting that the late Cenozoic extrusive activity of the Kerguelen plume is waning. Cenozoic volcanism attributed to the Kerguelen plume occurs over a diffuse area with Quaternary eruptions at Heard and McDonald Islands and within the Kerguelen Archipelago. The lithosphere of the Cretaceous Kerguelen Plateau overriding and insulating the plume. Although the volcanic activity of the Kerguelen plume spans 119 million years, the helium isotopic signature of Kerguelen lavas has never been documented. The isotopically-enriched mantle source of the Kerguelen plume has been attributed to either subducted sediments or delaminated sub-continental lithosphere. * Ar/ 3 9Ar dating of the archipelago samples shows that the three northern samples are ~28 Ma whereas the samples from the southeastern peninsulas fall into two distinct age groups: 10 Ma and 24-25 Ma. The basanite dike is 9 Ma and probably fed the lower Miocene basalts of the southeastern peninsulas. The helium isotopic signature of eight of these basalts was examined by analyzing olivine and pyroxene separates. Multiple analyses of olivine separates reveal 3He/ 4He from 7.43 ±0.17 to 10.40±0.06 R/RA. Analyses of pyroxene separates reveal lower ratios in general (6.53±0.33 to 9.64±0.25 R/RA) although they overlap with the olivine data. The lower 3He/ 4He ratios of the pyroxenes may reflect radiogenic ingrowth of 'He since eruption. Between 36°S and -45°S, two volcanic hotspots, Kerguelen and Amsterdam-St. Paul (ASP), affect the physical structure of the Southeast Indian Ridge which is the longest spreading center in the Indian Ocean. Presently the thickened platform of the Amsterdam-St. Paul hotspot is bisected by the Southeast Indian Ridge and approximately 40 million years ago the ridge intersected the Kerguelen hotspot. Lavas derived from the Kerguelen and Amsterdam-St. Paul hotspots have distinctly higher Sr and Pb isotopic ratios than mid-ocean ridge basalts, and the hotspots differ in that the Kerguelen basalts have higher 87Sr/16Sr and lower Nd and Pb isotopic composition compared to basalts from the ASP hotspot. New Sr, Nd and Pb isotopic data for 46 samples from the Southeast Indian Ridge, nearby islands and seamounts in the region of 77-88'E show that the Amsterdam-St. Paul hotspot (1 7Sr/ 86Sr>0.7035, 1 43Nd / 14 4Nd<0.5128, 206Pb/ 204Pb~19.2, 207 Pb/ 204Pb~15.62, 2 0sPb/ 2 04Pb-39.45) influences basalt compositions of three ridge segments located on the hotspot platform. Additionally, basalts from the ridge segment just north of the hotspot platform have isotopic compositions, that fall on mixing lines between ASP and normal mid-ocean ridge basalts suggesting that material from the Amsterdam-St. Paul hotspot flows northward beneath the ridge. The HIMU-like isotopic and trace element composition of the ASP hotspot requires partial melting of recycled oceanic crust in the mantle source. The low La/Nb of the ASP hotspot basalts suggests that processing during subduction stripped the recycled oceanic crust of highly incompatible elements. In contrast, some basalts from ridge segments have enriched isotopic ratios (8 7Sr / 86Sr>0.7044, 14 3Nd/l"Nd<0.51275, 2 06Pb/ 21Pb-18.1, 207Pb/ 20 Pb-15.54, 20 8Pb/ 20 4Pb~38.6) characteristic of the Kerguelen hotspot. The distribution of these Kerguelen-influenced basalts suggests that metasomatism associated with the early impact of the Kerguelen plume may have contaminated much of the upper mantle beneath the eastern Indian Ocean basin. Additionally, the Kerguelen isotopic signature of basalts from one ridge segment may reflect an active mantle conduit between the Kerguelen plume and the Southeast Indian Ridge.