Geochemistry and geochronology of ancient southeast Indian and southwest Pacific seafloor

Abstract

Within the Australian‐Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific‐type” and “Indian‐type” mid‐ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge (SEIR). This boundary has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm/yr since 4 Ma; however, its long‐term history remains a matter of speculation. To determine if Pacific‐type upper mantle has migrated westward beneath the eastern Indian Ocean basin as Australia and Antarctica drifted apart during the last 70 m.y., we present new Sr‐Nd‐Pb isotope data, combined with trace element and 40Ar‐39Ar radiometric age determinations, for samples from Legs 28 and 29 of the Deep Sea Drilling Project (DSDP). Basaltic basement at these DSDP sites provides a record of their upper mantle source composition and shows regional variations consistent with upper mantle flow in this region. East of the South Tasman Rise, all DSDP basalts have 87Sr/86Sr (0.7025–0.7029) and 206Pb/204Pb (18.80–19.48) ratios typical of Pacific‐type MORB indicating that Pacific‐type upper mantle existed east of the Australian‐Antarctic continental margin and beneath the Tasman Sea during the early stages of seafloor spreading in this region. Basalts from DSDP sites west of the AAD have high 87Sr/86Sr (0.7030–0.7035), low 206Pb/204Pb (17.99–18.10) and trace element characteristics typical of present day Indian‐type SEIR MORB. Between these two regions, DSDP basalts recovered along the western margin of the South Tasman Rise have isotopic characteristics that are, in one case consistent with an Indian‐type MORB source (Site 280A) and, in the second case, transitional between Pacific‐type and Indian‐type mantle sources. The occurrence of seafloor basalts with transitional or Indian‐type isotopic characteristics well to the east of the present Indian‐Pacific MORB isotopic boundary within the AAD strongly implies that Pacific‐type upper mantle has migrated westward into the region since the South Tasman Rise separated from Antarctica circa 40 Ma.

120 to 0 Ma tectonic evolution of the southwest Pacific and analogous geological evolution of the 600 to 220 Ma Tasman Fold Belt System

“…Although the nature and age of oceanic crust in the Tasman Sea is well demonstrated by magnetic anomalies, basement has only been sampled at DSDP Site 283, southeast of Tasmania (Pyle et al 1995). To our knowledge, there is no information on dredged rocks from the floor of the New Caledonia Basin.…”

Three‐dimensional structure of asthenospheric flow beneath the Southeast Indian Ridge

“…Isostatic a/'guments generally fall within the range of 85 ø to 150øC [Hayes, 1988 [1990,1991] suggested that the upper mantle beneath the axis of the SEIR is currently downwelling beneath the AAD, based upon the The unusual geophysical features of the AAD and surrounding SEIR are associated with a sharp discontinuity in the Sr, Nd, and Pb isotopic signatures of SEIR lavas. This marks the boundary between "Indian" and "Pacific" mid-ocean ridge basalt (MORB) source upper mantle provinces [Klein et al, 1988;Pyle et al, 1992Pyle et al, , 1995. This transition in isotopic signature is presently located near a transform offset of the SEIR between 126.5 ø and 127øE and extends over less than about 70 km along axis [Pyle et al, 1992].…”

“…Off-axis samples dredged from 3-4 Ma seafloor in the easternmost AAD spreading segment record the presence of Indian MORB mantle beneath a segment that is now erupting lavas with Pacific type isotopic characteristics [Pyle et al, 1992]. This change in mantle source signature implies that Pacific mantle has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm yr-1 for the last 3-4 Myr [Pyle et al, 1992[Pyle et al, , 1995 Although the off-axis data described above are not conclusive [Pyle et al, 1995] …”

Testing of multidimensional tectonomagmatic discrimination diagrams on fresh and altered rocks

“…It also indicated an island arc setting but with higher probability of about 63 % followed by 37 % for the continental arc (Table S50). Pyle et al (1995) reported geochemical data from fresh as well as altered seafloor rocks (dredged and drilled) of different ages (0-4 and 15-23 Ma; Table S13). The presence of micaceous alteration minerals, low CaO/Al 2 O 3 reflecting pervasive alteration, unusually high Rb contents, and contamination from seawater alteration were suggested by Pyle et al (1995) as the symptoms of alteration for most samples.…”

“…Pyle et al (1995) reported geochemical data from fresh as well as altered seafloor rocks (dredged and drilled) of different ages (0-4 and 15-23 Ma; Table S13). The presence of micaceous alteration minerals, low CaO/Al 2 O 3 reflecting pervasive alteration, unusually high Rb contents, and contamination from seawater alteration were suggested by Pyle et al (1995) as the symptoms of alteration for most samples. The authors used an intense leaching procedure before sample preparation for geochemical data acquisition.…”