The few geological and geophysical studies of the Lyra Basin at the western margin of the Ontong Java Plateau (OJP; Pacific Ocean) revealed that it is underlain by thicker than normal oceanic crust. The unusually thick oceanic crust is attributed to the emplacement of massive lava flows from the OJP. Dredging was conducted to sample the inferred OJP crust on the Lyra Basin but instead recovered younger extrusives that may have covered the older plateau lavas in the area. The Lyra Basin extrusives are alkalic basalts with ( 87 Sr/ 86 Sr) t = 0.704513–0.705105, ( 143 Nd/ 144 Nd) t = 0.512709–0.512749, ε Nd (t) = +3.0 to +3.8, ( 206 Pb/ 204 Pb) t = 18.488–18.722, ( 207 Pb/ 204 Pb) t = 15.558–15.577, and ( 208 Pb/ 204 Pb) t = 38.467–38.680 that are distinct from those of the OJP tholeiites. They have age-corrected ( 187 Os/ 188 Os) t = 0.1263–0.1838 that overlap with the range of values determined for the Kroenke-type and Kwaimbaita-type OJP basalts, but their ( 176 Hf/ 177 Hf) t = 0.28295–0.28299 and ε Hf (t) = +7.9 to +9.3 values are lower. These isotopic compositions do not match those of any Polynesian ocean island volcanics. Instead, the Lyra Basin basalts have geochemical affinity and isotopic compositions that overlap with those of some alkalic suite and alnöites in the island of Malaita, Solomon Islands. Although not directly related to the main plateau volcanism at 120 Ma, the geochemical data and modeling suggest that the origin of the Lyra Basin alkalic rocks may be genetically linked to the mantle preserved in the OJP thick lithospheric root, with magmatic contribution from the Rarotongan hotspot.

Abstract Sm–Nd data for rocks of granitic composition in an east–west traverse across the Namaqua Sector of the Mesoproterozoic c . 1.2 Ga Namaqua–Natal Province of southern Africa provide new evidence about the timing of crustal extraction from the mantle. Recent ion probe zircon dating has shown that, contrary to previous indications that pre-Namaqua basement had been preserved in parts of the Namaqua Sector, the majority of the magmatic rocks were emplaced during the 1.4–1.0 Ga Namaqua tectogenesis and very little U–Pb evidence of older precursors remains. Sm–Nd model ages show that Mesoproterozoic crustal extraction did occur, but was also strong in the Palaeoproterozoic, with local evidence for the existence of Archaean crust. The distribution of model ages generally corresponds with the established terranes, subdivided using lithostratigraphic and structural criteria. The northern part of the Bushmanland Terrane shows early Palaeoproterozoic to Archaean model ages and is clearly different from the southern part, which has Mesoproterozoic model ages. This supports the previously published results of a north–south traverse and justifies the separation of the Garies Terrane from the Bushmanland Terrane, though further subdivisions are not supported. The continuation of the Richtersveld Province east of Pofadder is supported by one sample, and the terrane boundary between the Bushmanland and Kakamas terranes is marked by an abrupt change from Palaeoproterozoic to Mesoproterozoic model ages. Model ages for the Kakamas and Areachap terranes do not distinguish them well. They suggest a Mesoproterozoic to late Palaeoproterozoic origin for both terranes, neither of which has a purely juvenile character. The influence of Palaeoproterozoic crust-forming events is clear in the Kaaien Terrane to the east, possibly reflecting reworking of the Kaapvaal Craton. The Namaqua Sector of the Province thus has a history of crustal extraction and evolution which reaches back locally to the Archaean, with major Palaeoproterozoic and Mesoproterozoic crust-forming events. This differs from the Natal Sector, which has a largely juvenile character related to a Mesoproterozoic Wilson cycle. Attempts to reconstruct the Mesoproterozoic Supercontinent Rodinia will have to take into account the extensive Nd-isotopic evidence for older crustal events in the Namaqua sector.